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Digitized  by  the  Internet  Archive 

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DEPARTMENT    OE   THE    INTElilOR 


MONOGRAPHS 


OP  THE 


United  States  Geological  Survey 


VOLUME    XXXVI 


WASHINGTON 

GOVERNMENT    PRINTING    OFFICE 

1899 


UNITED   STATES   GEOLOGICAL   SURVEY 

CHARLES   I).  WALCOTT,  DIRECTOR 


THE 


CMSTAL  FALLS  IRON-IARING  DISTRICT  OF  MICHIGAN 

liY 

J.  MORGAN  CLEMENTS  and  HENRY  LLOYD  SMYTH 

WITH 

A  CHAPTER  ON  THE  STURGEON  RIVER  TONGUE 


WILLIAM  SHIRLEY  BAYLEY 

AND 

AN  INTRODUCTION 

BY 

CHARLES   RICHARD    VAN    HISE 


WASHINGTON 

GOVERNMENT    PRINTING    OFFICE 
1899 


"A^o»). 


CONTENTS. 


Page. 

Letter  of  trans.mittal ,. xv 

Introduction,  iiY  C'h.\rle8  Riciiaku  Van  Hise xvii 

Outline  of  monograph xxix 

Pakt  I. — The  western  part  tiF  the  Crystal  Falls  district,  isy  J.  Morgan  Clements.  11 

Chapter  I. — Introdiction 11 

Previous  work  in  the  district 13 

Mode  of  work 22 

Magnetic  observations 24 

Chapter  II.— Geographical  limits,  structure  and  stisatigraphy,  and  physiography 25 

Geographical  limits 25 

Structure  and  stratigraphy 25 

Physiography 29 

Topography 29 

Drainage 31 

Timber  and  soil 36 

Chapter  III. — The  Archean 38 

Distribution,  exposures,  and  topography - 38 

ReLations  to  overlying  formations 39 

Petrographical  characters 10 

Biotite-granite  (granitite) 40 

Gneissoid  biotite-granite,  border  facies  of  granite 43 

Acid  dikes  in  Archean 45 

Basic  dikes  in  Archean 46 

Schistose  dikes 46 

Massive  dikes 48 

R^sum6 49 

Chapter  IV. — The  Lower  Hdronian  series 50 

Section  1.— The  Randville  dolomite 50 

Distribution,  exposures,  and  topography 50 

Petrographical  characters 51 

Relations  to  underlying  and  overlying  formations 53 

Thickness 53 

Section  2.— The  Mansfield  slate 54 

Distribution,  exposures,  and  topography 54 

Possible  continuation  of  the  Mansfield  slate 55 

Petrographical  characters 56 

Gray  wacke 56 

Clay  slate  .and  phyllite ■■ 57 

Origin  of  clay  slate  .and  phyllite 58 

Present  composition  necessarily  dift'erent  from  that  of  rock  from  which  derived 58 

Analysis  of  Mausfield  slate 59 

Comments  on  analysis 59 

Comparison  of  analysis  of  Mansfield  clay  slate  with  analyses  of  clays 60 

Comparison  of  analysis  of  Mansfield  clay  slate  with  analyses  of  other  clay  slates. ..  61 

Siderite-slate,  chert,  ferruginous  chert,  and  iron  ores 62 

Rjlations  of  siderite-slate,  ferruginous  chert,  and  ore  bodies  to  clay  slate 63 

Relations  of  Mansfield  slate  to  adjacent  formations 63 

Relations  to  intrusives 63 

Relations  to  volcanics 64 

V 


VI  CONTENTS. 

Chapter  IV. — The  Lower  Huroniax  series — Continued. 

Section  2. — The  Mansfielil  slate — Continued.  Page. 

Structure  of  the  Mansfield  area 64 

Thickness 64 

Ore  deposits 65 

General  description  of  Mansfield  mine  deposit 66 

Relations  to  surrounding  beds 68 

Comijosition  of  ore 68 

Microscoiiical  character  of  the  ores  and  associated  chert  bands 69 

Origin  of  the  ore  deposits 70 

Conditions  favorable  for  ore  concentration 72 

Exploration 73 

Section  3. — The  Hemlock  formation 73 

Distribution,  exposures,  and  topography 73 

Thickness 74 

Eelatious  to  adjacent  formations 75 

Relations  to  intrusi ves 77 

Volcanic  origin 78 

Classification 79 

Acid  Tolcanica 80 

Acid  lavas 80 

Khyolite-porphy  ry 81 

Texture 83 

Aporhy  olite-porphy  ry 87 

Schistose  acid  lavas 87 

Acid  pyroclastics 94 

Basic  volcanics 95 

Basic  lavas 9.5 

General  characters 95 

Nomenclature 93 

Metabasalts 98 

Nonporphyritic  metabasalt 98 

Petrographical  characters 98 

Chemical  composition 103 

Porphyritic  metabasalt 103 

Petrographical  characters 104 

Chemical  composition 105 

.   '               Variolitic  metabasalts 108 

EUip.soidal  structure 112 

Origin  of  ellipsoidal  structure 118 

Amygdaloidal  structure 124 

Flattening  of  amygdaloidal  cavities 12l) 

Alterati  on  of  the  basalts 126 

Description  of  some  phases  of  alteration 127 

Pyroclastics 135 

Eruptive  breccia 135 

Volcanic  sedimentary  rocks 136 

Coarse  tuffs 137 

Fine  tuffs  or  ash  (dust)  beds 142 

Relations  of  tuffs  and  ash  (dust)  beds 143 

Volcanic  conglomerates 143 

Schistose  pyroclastics 145 

The  Bone  Lake  crystalline  schists , 148 

Distribution 148 

Field  evidence  of  connection  with  the  volcanics 149 

Petrographical  characters 150 


CONTENTS.  VII 

CnAPTKR  IV. — Thk  I.owkk  Hukonian  skriks — Continuod. 

Section  3. — The  llciulock  t'onnation — Continued.  I'agn. 

Normal  scdiinontaiit^s  (if  the  Hemlock  formation 152 

Economic  products 1,")3 

Building  and  ornamental  stones 153 

Road  materials 154 

CHAPTEU  V. — TllK  UlTKI!  HURHNIAX  SICHIISS 155 

Distril)Ution,  oxposurea,  and  topograpbj- 1.55 

Magnetic  lines 1.56 

Thickness 1.57 

Folding 158 

Crystal  Falls  syncline 158 

Time  of  folding  of  tbo  Upper  Huronian 161 

Relations  to  other  series 162 

Relations  to  intrusives 16i 

Correlation 164 

Petrograpliical  characters 165 

Sedimentary  rocks 165 

Microscopical  description  of  certain  of  the  sedimentaries 169 

Igneons  rocks 174 

Ore  deposits 175 

History  of  opening  of  the  district 175 

Distribution 175 

Western  half  of  sec.  34,  T.  46  N.,  R.  33  W 176 

Sec.  20,  T.  45  N.,  R.  33  AV , 176 

The  Amasa  area 177 

The  Crystal  Falls  area 178 

Character  of  the  ore 180 

Relations  to  adjacent  rocks 182 

Origin 183 

Size  of  the  ore  bodies 184 

Methods  of  mining 184 

Prospecting 185 

Production  of  ore  from  the  Crystal  Falls  area 186 

Chapter  VI. — The  Intrusives 187 

Order  of  treatment 188 

Age  of  the  iutrusives 188 

Relations  of  folding  and  the  distribution  of  the  intrusives 189 

Section  1. — Unrelated  intrusives 190 

Classification 190 

Acid  iutrusives 190 

Geographical  distribution  and  exposures  of  granites 190 

Biotite-granite 191 

Micropegmatites 192 

Muscovite-biotite-granite 193 

Relations  of  granites  to  other  intrusives 194 

Dynamic  action  iu  granites 194 

Contact  of  granites  and  sedimentaries 194 

Evidence  of  intrusion 195 

Basic  intrusives 198 

Metadolerite 199 

Geograiihical  distribution 199 

Petrographical  characters 199 

Maeroscoiiical 199 

Microscopical 200 


VIII  CONTENTS. 

Chapter  VI. — The  Intrusives— Continued. 
.Section  I. — Unrelated  intrusives — Continued. 
Basic  intrusives — Continued. 

Metadolerite — Continual.  Page. 

Relations  to  adjacent  rocks 20S 

Relations  to  Lower  Huronian  Mansfield  slates 203 

Relations  to  Lower  Huronian  Hemlock  volcanics 204 

Relations  to  Upper  Huronian 204 

Relations  to  other  intrusives 204 

Contact  metaniorpbism  of  Mansfield  slates  by  the  dolerite 204 

Spilosites 206 

Analyses  of  spilosites 207 

Desiuosites 207 

Adinoles 208 

Analyses  of  adinoles 208 

Comparison  of  analyses  of  rtormal  Mansfield  clay  slates  and  the  contact  prod- 
ucts   209 

No  endomoriiliic  effects  of  dolerite  intrusion 211 

Metabasalt 211 

Ultrabasic  intrusives .' 212 

Picrite-porphyry  (porphyritic  limburgite) 212 

Geographical  distribution  and  exposures 212 

Petrographical  characters 212 

Gray  tremolitized  picrite-porphyry 213 

Dark  serpentinized  picrite-porphyry 217 

Classification 220 

Section  II. — A  study  of  a  rock  series  ranging  from  rocks  of  intermediate  acidity  through 

those  of  basic  composition  to  ultrabasic  kinds 221 

Diorite 222 

Nomenclature 222 

Distribution  and  exposures 223 

Petrographical  characters 223 

Description  of  interesting  variations 226 

Sec.  15,  T.  42  N.,  R.31  W 226 

Across  river  from  Crystal  Falls 227 

Southeiistof  Crystal  Falls 227 

Analysis  of  diorite 231 

Gabbro  and  norite 233 

Petrographiciil  characters 233 

Description  of  interesting  kinds  of  gabbro 240 

Hornblende-gabbro  in  sec.  15,  T.  42  N.,  R.  31  W 240 

Sees.  15,  22,  28,  and  29,  T.  42  N.,  R.  31  W 241 

Hornblende-gabbro  dikes 243 

Bronzite-norite  dike 244 

Sec.29,T.42N.,  R.31  W.,  1200  N., 200  W 245 

Dynamically  altered  gabbro 247 

Relative  ages  of  gabbros 249 

Peridotites ^249 

Distribution,  exposures,  and  relations 249 

Petrographical  characters 249 

Peridotite  varieties 252 

Wehrlite 2.53 

Amphibole-peridotite 253 

Gradations  of  amphibole-peridotite  to  wehrlite  and  olivine-gabbro 254 

Process  of  crystallization 257 

Analysis  of  peridotite 259 


CONTENTS.  IX 

CiiAriKK  \'I. — TiiK  iNTUfsivES — Coutiiiued. 
Si'ction  11. — A  study  of  a  rock  series,  etc. — Continued. 
Peridotites — Continued. 

I'eridotito  varieties — Continued.  Page. 

Pcridotito  from  sec.  22,  T.  42N.,  R.31  W.,  I'JSION.,  loOW 26() 

Relations  of  peridotites  to  other  rocks 261 

Ago  of  peridotites 262 

General  observations  ou  the  above  series 262 

Textural  characters  of  the  series 262 

Chemical  composition  of  the  series 263 

Relative  ages  of  rocks  of  the  series 265 

Part    II. — Thk    kastern    part    of   the  Crystal  Fall.s  district,   including   the   P'elch 
Mountain  range,  by  Henry  Lloyd  Smyth. 

Chapter  I. — CiEOORAPHicAL  limits  and  physiography 32& 

Introduction 329 

Preliminary  sketch  of  geology 331 

Character  of  surface 331 

Drainage - 334 

Chapter  II. — Magnetic  observations  in  geological  mapping 336 

Section  I. — Introduction 336 

Section  II. — Description  of  the  magnetic  rocks 338 

Section  III. — Distribution  of  magnetism  in  the  magnetic  rocks 339 

Section  IV. — Instruments  and  methods  of  work .' 341 

Section  V. — Facts  of  oliservatiou  and  general  principles 344 

(1)  Observed  deticctions  when  the  strike  is  north  and  south  and  the  dip  vertical 344 

(2)  Detleetious  of  the  horizontal  needle 345 

(3)  Deflections  of  the  dip  needle.  , 347 

(4)  Horizontal  and  vertical  components  when  the  magnetic  rock  dips  vertically 349 

(5)  Horizontal  and  vertical  components  when  the  magnetic  rock  dips  at  any  angle 350 

(6)  Determination  of  depth 854 

(7)  Summary 356 

Section  VI. — Applications  to  special  cases 3.56 

(1)  The  magnetic  rock  strikes  east  or  west  of  north  and  dips  vertically. 357 

(2)  .The  magnetic  rock  strikes  east  and  west 359 

(3)  Two  parallel  magnetic  formations 361 

Section  VII. — The  interpretation  of  more  complex  structures 366 

(1)  Pitching  synclines 367 

(2)  Pitching  anticlines 370 

(3)  Formations  split  by  intrusives 371 

(4)  Summary 372  ■ 

Chapter  III. — The  Felch  Mountain  range 374 

Section  I. — Position,  extent,  and  previous  work 374 

Section  II. — General  sketch  of  the  geology 383 

Section  III. — The  Archean 385 

Topography 386 

Petrographical  characters 387 

Section  IV. — The  Sturgeon  quartzite 398 

Distribution,  exjiosures,  and  topography 398 

Folding  and  thickness 399 

Petrographical  characters 401 

Section  V. — The  Randville  dolomite 406 

Distribution,  exposures,  and  topography 406 

Petrographical  characters 408 


X  CONTENTS. 

Chapter  III. — The  Felcu  Mountain  range — Continued.  Page. 

Section  VI. — The  Manisfield  scliists 411 

Distribution,  exposures,  and  topography 411 

Petrographical  characters 412 

Section  A'll. — The  Groveland  formation 415 

Distribution,  exposures,  and  topography 415 

Petrographical  characters 417 

Section  VIII. — The  mica-schists  and  quartzites  of  the  Upper  Huronian  series 423 

Petrographical  characters 425 

Section  IX. — The  iutrusives 426 

Chapter  IV. — The  Michigamme  Mountain  and  Fence  River  areas 427 

Section  I. — The  Archean 428 

Section  II. — The  Sturgeon  formation .' 430 

Section  III.— The  Raudville  dolomite 431 

Distribution  and  exposures 431 

Folding  and  thicliness 432 

Petrograpliical  characters 434 

Section  IV. — The  Mansliold  formation 437 

Distribution,  exposures,  and  topography 438 

Folding  and  thickness 438 

Petrographical  characters 439 

Section  V. — The  Hemlock  formation 440 

Distribution,  exposures,  and  topography 440 

Folding  and  thickness 441 

Petrographical  characters 442 

Section  VI.  — The  Groveland  formation 446 

Distribution,  exposures,  and  topography 446 

Folding  and  thicliness 448 

Petrographical  characters 448 

Chapter  V. — The  northea.stkrn  area  and  the   relations  between  the  Lower  Mar- 

(jtette  and  the  Lower  Menominee  series 451 

Chapter  VI. — The  Sturgeon  River  tongue,  by  William  Shirley  Bayley 458 

Description  and  boundary  of  area 4.58 

Literature 459 

Relations  between  the  sedimentary  rocks  and  the  granite-schist  complex 461 

The  IJasement  Complex 463 

The  gneissoid  granites 463 

The  amphibole-sihists 465 

Origin  of  the  amphibole-schists 466 

The  biotitc-schists 467 

The  intrusive  rocks 469 

Comparison  of  the  Sturgeon  River  and  the  Marquette  crystalline  series 470 

The  Algoukian  trough 471 

Relations  between  the  conglomerate  and  the  dolomite  series 472 

Relations  between  the  dolomites  and  conglomerates  and  the  overlying  sandstones 473 

The  conglomerate  formation 473 

Important  exposures 474 

Petrographical  descriptions 477 

The  dolomite  formation  479 

Important  exposures 480 

Petrographical  descriptions 481 

Slates  and  sandstones  on  the  Sturgeon  River 481 

The  igneous  rocks 482 

The  intrusive  greenstones 482 

Petrographical  description 482 

The  banded  greenstones 485 

Petrographical  description 486 


ILLUSTRATIONS. 


Pago. 
Plate     I.  Colored  map  ahowiug  the  distribution  of  pre-C';imbriau  and  other  rocks  in  the  Lake 
Superior  region,  and  tlie  goographical  relations  of  the  Crystal  Falls  district  of 

Michigan  to  the  adjoining  Marquette  and  Menominee  districts  of  Michigan 11 

II.  Topographical  map  of  the  Crj-stal  Falls  district  of  Michigan,  including  a  portion  of 

the  Marquette  district  of  Michigan In  pocket. 

III.  Geological  map  of  the  Crystal  Falls  district  of  Michigan,  including  a  portion  of  the 

Mar([uette  district  of  Michigan In  pocket. 

IV.  Portion  of  geological  map  of  the  Menominee  iron  region,  by  T.  B.  Brooks  and  C.  E. 

Wright 18 

V.  Generalized  sections  to  illustrate  the  stratigraphy  and  structure  of  the  northwestern 

part  of  the  Crystal  Falls  district  of  Michigan "       28 

VI.  Generalized  sections  to  illustrate  the  stratigraphy  and  structure  of  the  southern  part 

of  the  Crystal  Falls  di,striet  of  Jlichigan 28 

VII.  Generalized  columnar  section 30 

VIII.  Map  of  a  portion  of  the  Crystal  Falls  district,  showing  iu  detail  the  glacial  topog- 
raphy and  illustrating  the  development  of  the  Deer  Kiver 32 

IX.  Sketch  of  the  Mansfield  mine  as  it  was  before  it  caved  iu,  iu  1893 66 

X.  A,  Reproduction  of  the  weathered  surface  of  a  variolite;   B,  Reproduction  of  the 

polished  surface  of  a  variolite 110 

XI.  Colored  reproduction  of  an  ellipsoid,  with  matrix,  from  <in  ellipsoidal  basalt 116 

XII.  Mount  Giorgios,  viewed  from  its  west  flank,  in  April,  1866,  illustrating  the  charac- 
teristic block  lavas,  from  Fouque's  Santorin  et  ses  Eruptions,  PI.  VIII 120 

XIII.  Reproduction  iu  colors  of  a  basalt  tuflf 140 

XIV.  Idealized  structural  map  and  detail  geological  map,  with  sections,  to  show  the  dis- 

tribution and  structure  of  the  Huronian  rocks  in  the  vicinity  of  Crystal  Falls, 

Michigan 160 

XV.  Portion  of  Brooks's  PI.  IX,  Vol.  Ill,  Wisconsin  Geological  Survey 172 

XVI.  Detail  geological  map  of  the  vicinity  of  Amasa,  Michigan 176 

XVII.  Detail  geological  map  of  the  vicinity  of  Crystal  Falls  and  Mansfield,  Sheet  1 178 

XVIII.  Detail  geological  map  of  the  vicinity  of  Crystal  Falls  and  Mansfield,  Sheet  II 178 

XIX.  J,  Inclusions  iu  a  fractured  quartz  pheuocryst;  />',  Quartz  phenocryst  with  rhombo- 

hedral  parting 268 

XX.  yl,  Micropoikilitic  rhyolite-porphyry;   /?,  Micropoikilitic  quartz-porphyry 270 

XXI.  J,  Very   fine-grained   micropoikilitic   rhyolite-porphyry  viewed  without  analyzer; 

B,  Very  fine-grained  micropoikilitic  rhyolite-porphyry  viewed  with  analyzer 272 

XXII.  A,  Perlitic  parting  in  aporhyolite;  B,  Perlitic  parting  in  aporhyolite 274 

XXIII.  A,  Schistose  rhyolite-porphyry;  B,  Aporhyolite  breccia 276 

XXIV.  .4,  Schistose  rhyolite-porphyry ;  /?,  The  same  viewed  between  crossed  nicols 278 

XXV.  ^,Amygdaloidal  texture  of  basalt;  i>,  Amygdaloidal  vitreous  basalt 280 

XXVI.  A,  Amygdaloidal  vitreous  basalt ;  B,  Amygdaloidal  vitreous  basalt  showing  sheaf-like 

aggregates  of  feldspar 282 

XXVII.  ^,  Reproduction  in  colors  of  amygdaloidal  basalt;  i>,  Pseudo-amygdaloidal  matrix 

of  ellipsoidal  basalt ;  C, Water-deposited  pyroclastic 284 

XXVIII.  A,  Fine-grained  basalt  with  well-developed  igneous  texture;  B,  Illustration  of  the 
obliteration  of  the  igneous  texture  of  a  basalt  by  secondary  products  when  viewed 
between  crossed  nicols  286 

XI 


XII  ILLUSTRATIONS. 


XXIX.  J,  Basalt  showing  characteristic   texture   in    ordinary  light;    B,    Basalt  showing 

obliteration  of  texture  between  crossed  nicols 288 

XXX.  A,  Basalt  showing  iu  ordinary  light  a  distinctly  amygdaloidal  texture;  B,  The  same 
basalt  with  its  amygdaloidal  texture  obliterated  when  viewed  between  crossed 

nicols 290 

XXXI.  A,  Basalt  affected  by  calcification  process;  B,  Basalt  affected  by  calcification  process 

viewed  between  crossed  nicols 292 

XXXII.  A,  Illustration  of  perlitic  parting  in  a  fragment  from  a  basaltic  tuff;  B,  Sickle- 
shaped  bodies  in  volcanic  tuff 294 

XXXIII.  A,  Water-deposited  sand;  B,  Gradation  iu  water- deposited  volcanic  sediment 296 

XXXIV.  A,  Contact  product  of  granite ;  B,  Brecciated  matrix  between  ellipsoids 298 

XXXV.  .4,  Contact  between  granite  and  a  metamorphosed  sedimentary ;  /?,  Contact  between 

granite  and  a  metamorphosed  sedimentary  viewed  between  crossed  nicols 300 

XXXVI.  A,  A  variety  of  spilosite  with  white  spots ;  B,  A  variety  of  spilosite  with  white  spots 

viewed  between  crossed  nicols 302 

XXXVII.  -J,  Normal  spilosite  or  spotted  contact  product;  .B,  Normal  spilosite  of  somewhat 

different  character 304 

XXXVIII.  J,  Passage  of  spilosite  into  demosite;  i?,  Occurrence  and  alteration  of  bronzite  in 

bronzite-norite 306 

XXXIX.  A,  Biotite-granite  viewed  between  crossed  nicols ;  B,  Mica-diorite  viewed  between 

crossed  nicols 308 

XL.  A,  Quartz-mica-diorite-porphyry ;  B,  Quartz-mica-diorite-porphyry  viewed  between 

crossed  nicols 310 

XLI.  J,  Porjihyritic  poikilitic  hornblende  gabbro;  i?,  Poikilitic  hornblende  gabbro 312 

XLII.  ll,  Moderately  fine-grained   hornblende  gabbro   showing  parallel  texture;  £,  Mod- 
erately fine-grained  hornblende  gabbro  showing  parallel  texture  viewed  between 

crossed  nicols  314 

XLIII.  A,  Normal  granular  hornblende  gabbro;     B,  Schistose  hornblende  gabbro  viewed 

between  crossed  nicols 316 

XLIV.  J,  Moderately  fine-grained  hornblende  gabbro;  iJ,  bronzite-norite 318 

XLV.  J,  Bronzite-norite-porphyry ;  i>,  Feldspathic  wehrlite 320 

XLVI.  J,  Feldspathic  wehrlite  viewed  between  crossed  nicols;  Bj  Feldspathic  wehrlite 322 

XLVII.  Relations  of  magnetic  beds  to  variation  and  dip 352 

XLVIII.  Relations  of  magnetic  beds  to  variation  and  dip 362 

XLIX.  Geological  map  of  the  Felch  Mountain  Range 374 

L.  Geological  map  of  a  portion  of  the  Crystal  Falls  district 450 

LI.  Geological  map  of  the  Sturgeon  River  tongue 458 

LII.  Map  of  exposures  in  sec.  7  and  portions  of  sees.  8,  17,  and  18,  T.  42  N.,  R.  28  W 474 

LIII.  Schist  conglomerate  from  dam  of  Sturgeon  River 476 

Fig.  1.  Reproduction  of  a  portion  of  the  geological  map  of  the  Upper  Peninsula  of  Michigan, 

by  William  A.  Burt.  1846 15 

2.  Enlarged  reproduction  of  a  portion  of  a  map  of  the  Lake  Superior  land  district,  by  Foster 

and  Whitney 17 

3.  Enlarged  reproduction  of  a  portion  of  a   geological  map  of  the  Upper  Peninsula  of 

Michigan,  by  Rominger,  Brooks,  and  Pumpelly,  1873 18 

4.  Granite-porphyry  with  inclusions  of  gneissoid  granite 45 

5.  Illustration  of  the  effect  on  the  topography  of  the  differential  erosion  of  basic  dikes 

and  granite 46 

6.  Concentric  cracks  formed  by  the  caving  in  of  the  Mansfield  mine 65 

7.  Sketch  of  the  surface  of  the  outcrop  of  an  ellipsoidal  basalt,  showing  the  general  char- 

acter of  the  ellipsoids  and  matrix 112 

8.  Sketch  showing  the  concentration  of  the  amygdaloidal  cavities  on  one  side  of  an  ellipsoid, 

this  side  prob.ably  representing  the  side  nearest  the  surface  of  the  flow 113 

9.  Ellipsoids  with  sets  of  parallel  lines  cutting  each  other  at  an  angle 114 

10.  Reproduction  of  illustration  of  aa  lava,  after  Dana 120 

11.  Profile  section  illustrating  results  of  diamond-drill  work 177 


ILLUSTRATIONS.  XIII 

Page. 

Fi(i.l2.  Sketch  illustrating  CDiitortion  of  Uppor  Huron  I  an  strata 179 

13.  Skotili  show i mi;  chaiigo  of  strike  of  Upper  lliironiaii  beds,  <lue  to  the  folds 179 

14.  Sketch  to  illustrate  the  occnrrance  of  ore  bodies 182 

15.  Magnetic  cross  section  in  T.  45  N.,  R.  31  W 345 

16.  Circles  of  attraction 346 

17.  The  forces  acting  on  the  dip  needle 347 

18.  Curves  showing  the  relations  between  the  horizontal  components  at  the  points  of  maxi- 

mum deflection,  for  rocks  dipping  at  various  angles  and  buried  to  various  depths 354 

19.  Truncated  anticlinal  fold  with  gently  dipping. limbs 364 

20.  Truncated  anticlinal  fold  with  steeply  dipping  limbs 365 

21.  Plan  and  cross  sections  of  a  pitching  syucline 367 

22.  Magnetic  map  of  the  Groveland  Basin 370 

23.  Plan  and  cross  sections  of  a  pitching  anticline '. 371 

24.  Magnetic  map  of  a  single  formation  split  by  au  intruded  sheet 372 


LETTER    OF    TRANSMITTAL. 


Department  of  the  Interior, 

United  States  Geological  Survey, 

Washington,  D.  C,  April  23,  1898. 

Sir:  I  transmit  herewith  the  manuscript  and  iUustrations  of  a  mono- 
graph upon  the  Crystal  Falls  Iron-bearing  District  of  Michigan,  by 
J.  Morgan  Clements  and  H.  L.  Smyth.  The  district  is  thus  called  from  its 
principal  mining  town.  The  area  reported  upon  connects  the  Marquette 
district  on  the  north  and  the  Menominee  district  on  the  south. 

This  monograph  is  one  of  the  series  which  is  to  treat  of  the  iron-bearing 
districts  of  the  Lake  Superior  region.  The  first  of  the  series  was  that  on 
the  Penokee  district  (Monograph  XIX);  the  second  of  the  series  was  that 
on  the  Marquette  district  (Monograph  XXVIII).  The  present  report  is  the 
third  of  the  series,  and  the  report  upon  the  Menominee  district,  now  being 
prepared  by  W.  S.  Bayley,  will  be  the  fourth. 

No  previous  detailed  report  has  been  issued  upon  the  Crystal  Falls 
district,  although  the  area  has  been  touched  upon  by  Messrs.  T.  B.  Brooks 
and  Carl  Rominger.  The  present  report  is  the  first  which  contains 
geological  maps  and  sections  of  the  disti'ict. 

The  field  work  upon  which  the  present  report  is  based  began  about  five 
years  ago.  The  work  of  the  first  season  was  a  topographical  survey  and  a 
reconnaissance  geological  survey.  The  work  of  the  second  season  was 
detail  geological  work  by  H.  L.  Smyth,  W.  N.  Merriam,  and  their  assist- 
ants. The  work  of  these  two  seasons  was  done  for  private  parties.  These 
parties  turned  over  to  me  the  original  specimens,  notes,  and  maps  to  assist 
in  the  preparation  of  this  report.  The  detail  geological  mapping  of  the 
second  year  had  covered  only  a  part  of  the  area  included  within  the  present 
report,  and  in  the  remainder  of  the  area  in  the  following  years  detail 
surveys  were  made  by  J.  Morgan  Clements  and  W.  S.  Bayley. 


XVI  LETTER  OF  TEANSMITTAL. 

In  the  vicinity  of  the  mines  and  the  iron-bearing  formations  the  exami- 
nation of  the  district  has  been  of  the  most  detailed  character,  practically  all 
of  the  ledges  having  been  visited  and  advantage  having  been  taken  of 
underground  workings  and  borings.  Moreover,  in  order  to  determine  the 
succession,  for  large  areas,  it  was  necessary  to  make  a  close  magnetic 
survey  with  the  dial  compass  and  dip  needle.  In  the  areas  more  remote 
from  the  mines  the  work  was  of  a  less  detailed  character. 

The  western  half  of  the  district  is  treated  by  J.  Morgan  Clements  in 
Part  I  of  this  monograph.  The  eastern  half  of  the  district,  with  the  excep- 
tion of  the  Sturgeon  River  tongue,  is  treated  by  H.  L.  Smyth  in  Part  II. 
The  chapter  upon  the  Sturgeon  River  tongue  was  prepared  by  W.  S.  Bayley. 
My  own  part  of  the  work  has  been  a  general  supervision  of  the  entire  surve}', 
with  frequent  trips  into  the  region  to  assist  in  solving  the  general  structural 
problems. 

To  the  gentlemen  who  furnished  the  complete  results  of  their  surveys 
for  the  first  two  seasons,  we  are  deeply  indebted.  The  drawing  for  the 
maps  was  done  by  E.  C.  Bebb.  The  colored  plates  were  prepared  b}- 
J.  L.  Ridgway. 

Very  respectfully,  your  obedient  servant, 

C.  R.  Van  Hise, 

Geologist  in  Charge. 

Hon.  Charles  D.  Walcott, 

Director  United  States  Geological  Survey. 


INTRODUCTION. 


By  C.  R.  Van  Hise 


This  report  is  a  full  account  of  the  Crystal  Falls  iron-bearing-  district 
of  Michigan. 

The  rocks  of  the  district  comprise  two  groups,  separated  by  uncon- 
formities. These  are  the  Archean  and  the  Algonkian.  The  Algoukian 
includes  both  the  Lower  Huronian  and  the  Upper  Huronian  series,  and 
these  are  also  separated  by  unconformities.  The  terms  Lower  Huroniau 
and  Upper  Huronian  are  applied  to  the  series  which  occur  in  this  district 
because  they  are  believed  to  belong  to  the  same  geological  province  as  the 
Huronian  rocks  of  the  north  shore  of  Lake  Huron,  and  to  be  equivalent  to 
the  Lower  Huronian  and  Upper  Huronian  series  which  there  occur.  The 
reasons  for  this  belief  are  fully  given  in  Bulletin  86.' 

The  Archean  is  believed  to  be  wholly  an  igneous  group,  and  there- 
fore no  estimate  of  its  thickness  can  be  given.  It  covers  a  broad  area  in 
tlie  eastern  part  of  the  district,  and  from  this  several  arms  project  west. 
West  of  the  main  area  there  are  two  large  oval  areas  of  Archean. 

The  Lower  Huronian  series,  from  the  base  upward,  comprises  the  Stur- 
geon quartzite,  from  100  feet  to  more  than  1,000  feet  thick;  the  Randville 
dolomite,  from  500  feet  to  1,500  feet  thick;  the  Mansfield  slate,  from  100 
feet  to  1,1(00  feet  thick;  the  Hemlock  volcanic  formation,  from  1,000  feet  to 
10,000  or  more  feet  thick;  and  the  Groveland  fonnation,  about  500  feet 
thick.  We  tlms  have  a  minimum  thickness  for  the  series  of  about  2,200 
feet,  and  a  possible  maximum  thickness  of  more  than  16,000  feet.     However, 

^  Correlation  papers,  Archeau  and  Algonkian,  by  C.  R.  Van  Hise :  Bull.  U.  S.  Geol.  Survey  No.  86, 
1892,  pp.  156-199. 

MON  XXXVI IX  xvn 


XVIII  INTRODUCTION. 

a  large  part  of  the  latter  is  cjmpowed  of  volcanic  material.  It  is  not  likely 
tliat  the  sediments  at  any  one  place  are  as  much  as  5,000  feet  thick. 

The  Upper  Huronian  is  a  great  slate  and  schist  series,  which  it  is  not 
possible  to  separate  on  the  maps  into  individual  formations,  and  it  is  impos- 
sible to  give  even  an  approximate  estimate  of  the  thickness  of  this  series. 

Various  igneous  i-ocks  intrude  in  an  intricate  manner  both  the  Upper 
Huronian  and  the  Lower  Huronian  series. 

The  aim  of  the  following  paragraphs  is  to  sketch  very  briefly  the 
history  of  the  district. 

THE  ARCHEAN. 

The  Archean  consists  mainly  of  massive  and  schistose  granites  and  of 
gneisses.  Nowhere  in  the  Archean  have  any  rocks  of  sedimentarj^  origin 
been  discovered.  The  Archean  has  been  cut  by  various  igneous  rocks, 
both  basic  and  acid,  at  different  epochs.  These  occur  in  the  form  both  of 
bosses  and  of  dikes,  the  latter  sometimes  cutting,  but  more  ordinarily  show- 
ing a  parallelism  to,  the  foliation  of  the  schistose  granites.  The  granites 
must  have  formed  far  below  the  surface,  and  therefore  nmst  have  been 
deeply  denuded  before  the  transgression  of  the  Lower  Huronian  sea.  The 
Archean  granites  and  gneisses  and  the  earlier  intrusives  alike  have  been 
profoundly  metamorphosed,  and  at  various  places  have  been  completely 
recrystallized. 

THE   LOWER  HUROXIAI^   SERIES. 

The  Sturgeon  quartzite,  the  first  deposit  of  the  advancing  sea,  when 
formed  consisted  mainly  of  sandstone,  but  in  places  at  the  base  of  coarse 
conglomerate.  The  conglomerate  is  best  seen  in  the  Sturgeon  River 
tongue.  Elsewhere  e\ddence  of  conglomeratic  character  at  the  base  of 
the  formation  is  seen,  but  the  metamorphism  has  been  so  great  as  nearl}^  to 
destroy  the  pebbles.  However,  in  the  Sturgeon  River  tongue  is  a  great 
schistose  conglomerate,  which,  while  profoundly  metamorphosed,  still  gives 
evidence  of  the  derivation  of  its  material  from  the  older  Archean  rocks. 
The  sandstone  has  been  changed  to  a  vitreous,  largely  recrystallized 
quartzite,  which  now  shows  only  here  and  there  vague  evidence  of  its 
clastic  chai-acter. 

The  Sturgeon  formation  varies  from  probably  more  than  1,0()0  feet  in 


INTHODUCTION.  XIX 

tliickiu'ss  in  tlic  Sturiieoii  River  tono'uo  to  less  tliaii  100  fec^t  in  tliickness  at 
])lii(.T's  in  the  Felcli  Mountain  ran<^'e,  and  is  altogether  absent  in  the  north- 
eastern j)art  of  the  district. 

In  tlu'  southeastern  j)art  of  the  district  the  Sturgeon  quartzite  is  over- 
lain l)\-  the  Handville  dolomite.  In  the  central  part  of  the  district  the 
([uartzite  Ix^tween  the  Archeau  and  the  Randville  is  so  thin  that  it  can  not 
be  reijresented  on  the  maps  as  a  separate  formation.  In  the  northeastern 
part  of  the  <listrict  a  quartzite  resting  on  the  Archean,  l)ut  occupying  a 
higher  position  stratigraphically  than  the  Randville  dolomite,  is  overlain  bv 
an  iron-bearing  formation.  It  appears,  therefore,  that  the  Sturgeon  sea 
gradually  overrode  the  district,  and  that  at  the  time  the  Sturgeon  quartzite 
was  deposited  in  the  southeastern  part  of  the  area,  the  Archean  was  not  yet 
sulimerged  in  the  central  and  northeastern  parts  of  the  district.  However, 
since  the  quartzite  resting  on  the  Archean  in  the  latter  area  can  not  be  sepa- 
rated lithologically  from  the  Sturgeon  quartzite,  both  are  given  the  same 
formation  color,  but  the  later  quartzite  is  given  a  separate  letter  symbol. 
The  quartzite  color  therefore  represents  a  transgression  deposit  of  the 
same  general  lithological  character,  rather  than  a  formation,  all  parts  of 
which  have  exactly  the  same  age.  While  nowhere  in  the  district  is  there 
an}'  marked  discordance  between  the  schistosity  of  the  Archean  and  the 
Sturgeon  quartzite,  the  conglomerates  at  the  base  of  the  latter  formation  in 
the  Sturgeon  River  tongue  are  believed  to  indicate  a  great  un<"onfoi-mity 
between  the  Archean  and  the  Lower  Huronian  series.  The  change  from 
the  Sturgeon  deposits  to  those  of  the  Randville  was  a  transition. 

The  Randville  dolomite  is  a  nonclastic  sediment,  and  is  believed  to 
mark  a  periftd  of  su.bsidence  and  transgression  of  the  sea  to  the  northeast, 
resulting  in  deeper  waters  for  much  of  the  district.  Since  the  Randville 
dolomite  has  its  full  thickness  on  the  Fence  River  just  east  of  the  western 
Archean  oval,  and  does  not  appear  at  all  about  the  Archean  oval  a  short 
distance  to  the  northeast,  it  is  probable  that  the  shore  line,  during  Rand- 
ville time,  was  between  these  two  areas  and  that  tlie  land  arose  somewhat 
abruptly  toward  the  northeast.  As  the  Randville  formation  has  a  thick- 
ness of  1,500  feet,  it  probably  represents  a  considerable  part  of  Lower 
Huronian  time. 

Following  the  de^^osition  of  the  Randville  dolomite,  deposits  of  very 
different  character  occur  in  different  parts  of  the  district.     These  deposits 


XX  INTRODUCTION. 

are:  (1)  The  Mansfield  formation,  (2)  the  Hemlock  volcanic  formation,  and 
(3)  the  Groveland  formation. 

The  Mansfield  formation  was  a  mudstone,  which  has  subsequently  been 
transformed  into  a  slate  or  schist.  The  Hemlock  formation  is  mainly  a 
great  volcanic  mass,  including  both  basic  and  acid  rocks,  lavas,  and  tuff's, 
but  it  contains  also  subordinate  interbedded  sedimentary  rocks.  This  for- 
mation occupies  a  larger  area  than  any  other  of  the  Lower  Huronian  forma- 
tions and  is  perhaps  the  most  characteristic  feature  of  the  Crystal  Falls 
district.  The  Groveland  is  the  iron-bearing  formation.  It  includes  sideritic 
rocks,  cherts,  jaspilites,  iron  ores,  and  other  varieties  characteristic  of  the 
iron-bearing  formations  of  the  Lake  Superior  region.  In  all  important 
respects  these  rocks  are  similar  to  those  of  the  Negaunee  formation  of  the 
Marquette  district,  with  the  exception  that  in  the  southeastern  part  of  the 
Crystal  Falls  district,  associated  with  the  nonclastic  material,  there  is  a 
considerable  proportion  of  clastic  deposits.  The  Groveland  formation 
contains  iron  carbonate  and  possibly  glauconite,  from  which  its  other 
characteristic  rocks  were  derived. 

The  variability  in  the  character  of  the  deposits  overlying  the  Randville 
formation  is  probably  caused  by  the  great  volcanic  outbreaks  in  the  western 
part  of  the  district.  In  the  southern  and  southeastern  parts  of  the  area  the 
deposit  overlying  the  Randville  formation  is  the  Mansfield  slate  and  schist. 
North  of  Michigamme  Mountain  and  of  the  Mansfield  area  the  Mansfield 
formation  is  replaced  along  the  strike  by  the  Hemlock  volcanic  formation, 
which  directly  overlies  the  limestone  for  most  of  the  way  about  the  western 
Archean  oval.  The  effect  of  the  volcanic  outbreak  apparently  did  not 
reach  so  far  as  the  northeastern  part  of  the  district. 

Overlying  the  Mansfield  formation  in  the  southeastern  part  of  the  dis- 
trict and  the  Randville  formation  in  the  central  part  of  the  district  is  the 
Groveland  iron-bearing  formation.  In  the  Mansfield  slate  area  the  iron- 
bearing  rocks  appear  near  the  top  of  the  Mansfield  formation  intercalated 
with  the  slates.  The  Groveland  formation  can  not  be  certainly  traced 
farther  north  than  the  northeastern  portion  of  the  western  Archean  oval. 
It  is  apparently  replaced  along  the  strike  by  the  Hemlock  volcanics. 

In  the  northeastern  part  of  the  district  the  Groveland  formation, 
equivalent  to  the  Negaunee  formation  of  the  Marquette  district  of  Michigan, 


INTRODUCTION.  ■  XXI 

is  found  above  the  Ajibik  formation.  The  occupation,  in  the  western  part 
of  tlie  district,  by  the  Hemlock  volcanics  of  the  same  part  of  the  geological 
column  as  the  Hemlock  volcanics  east  of  the  western  Archean  oval,  the 
Mansfield  slate,  and  the  Groveland  formation,  is  explained  by  the  fact 
that  in  the  western  part  of  the  district  the  volcanoes  first  broke  out  and 
there  continued  their  activity  longest.  While  north  of  Crystal  Falls  the 
volcanic  rocks  were  being  laid  down,  the  Mansfield  formation  was  being 
deposited  in  the  southeastern  part  of  the  district.  This  activity  continued 
there  through  the  time  that  the  Groveland  formation  was  being  deposited 
in  other  parts  of  the  disti'ict. 

From  the  foregoing  it  appears  that  the  Hemlock  formation  in  the 
western  part  of  the  district  is  equivalent: 

(1)  East  of  the  western  Archean  oval,  to  the  Hemlock  volcanics  found 
there  and  the  overlying  Groveland  formation ; 

(2)  At  Michigamme  Mountain,  to  the  Mansfield  slates  and  the  Groveland 
formation ; 

(3)  In  the  Mansfield  area,  to  the  Mansfield  slates  and  the  Hemlock 
volcanics  occurring  there;  and 

(4)  In  the  southeastern  part  of  the  district,  to  the  Mansfield  and 
Groveland  formations. 

The  replacement  of  an  iron-bearing  formation  by  the  great  volcanic 
fonnation  just  described  is  exactly  paralleled  in  the  Upper  Huronian  rocks 
of  the  Penokee  iron-bearing  series,  where  the  pure  iron-bearing  formation 
is  replaced  at  the  east  end  of  the  district  by  a  great  volume  of  volcanic 
rocks  intercalated  with  slates  and  containing  bunches  of  iron-formation 
material.^ 

Following  the  deposition  of  the  Lower  Huronian  series  the  region  was 
raised  above  the  sea  and  eroded  to  different  depths  in  difterent  places.  In 
the  Felch  Mountain  range  the  only  formations  above  the  Raudville  dolomite 
are  a  thiu  bed  of  slate  and  the  Groveland  iron  formation.  In  the  north- 
eastern part  of  the  district  only  a  thin  belt  of  iron-formation  rocks  remains. 
In  the  central  and  western  parts  of  the  district  there  is  a  great  thickness  of 
volcanics.     Tliis,  however,  does  not  imply  a  difference  of  erosion  equal  to 

'  The  Penokee  iron-beariug  district  of  Michigan  and  Wisconsin,  by  R.  D.  Irving  and  C.  R.  Van 
Hise:  Men.  U.  S.  Geol.  Survey,  Vol.  XIX,  1892,  pp.  428-433. 


XXII  INTRODUCTION. 

the  difference  iu  thickness  of  these  rocks,  for  doubtless  when  the  volcanics 
were  built  up  there  was  contemporaneous  subsidence,  so  that  at  the  end 
of  Lower  Huronian  time  there  may  have  been  little  variation  in  the 
elevation  of  the  upper  surface  of  the  series,  but  very  great  difference  in  its 
thickness. 

THE   UPPER   HURONIAIV. 

After  the  Lower  Huronian  series  was  deposited  the  district  was  raised 
above  the  sea,  may  have  been  gently  folded,  and  was  eroded  to  different 
depths  in  different  parts  of  the  district. 

Following  the  earth  movements  and  erosion  the  waters  for  some  reason 
advanced  over  the  district  and  the  Upper  Huronian  series  was  deposited. 
The  basal  horizon  was  a  conglomerate,  which  has,  however,  very  different 
characters  in  different  parts  of  the  district. 

In  the  eastern  half  were  Archean  rocks,  the  Sturgeon  quartzite,  the 
Mansfield  slate,  and  the  Clroveland  iron  formation.  Upon  these  was  depos- 
ited a  sandstone  which  locally  was  very  ferruginous.  This  has  subsequently 
been  changed  into  a  ferruginous  quartzite.  The  typical  occurrence  of  this 
quartzite  is  at  the  east  end  of  the  Felch  Mountain  range.  It  also  appears 
between  the  Archean  ovals  in  the  northeastern  part  of  the  district.  If 
distinct  conglomerates  were  formed  at  the  bottom  of  this  quartzite,  they  are 
buried  under  glacial  deposits  or  have  disappeared  as  the  resiilt  of  meta- 
moi'phism. 

In  the  western  part  of  the  district  the  rocks  of  the  Lower  Huronian  at  the 
surface  are  the  great  Hemlock  formation,  and  here  the  basal  horizon  of  the 
Upper  Huronian  is  a  slate  or  slate  conglomerate,  the  fragments  of  which  are 
derived  mainly  from  the  underlying  Hemlock  formation.  The  sandstones 
and  conglomerates  varied  upward  into  shales  and  grits,  which  have  been 
subsequently  altered  into  mica-slates  and  mica-schists.  After  a  considerable 
thickness  of  mudstone  and  grit  was  deposited,  there  followed  a  layer  of 
combined  clastic  and  nonclastic  sediments,  the  latter  including  iron-bearing 
carbonates.  These  appear  to  be  at  a  somewhat  persistent  horizon,  and  in 
this  belt  are  found  the  iron-formation  rocks,  and  iron  ores  in  the  Upper 
Huronian  in  the  vicinity  of  Crystal  Falls.  Above  these  ferruginous  rocks 
there  was  deposited  a  great  thickness  of  shales  and  grits,  which  have  been 
transformed  into  mica-slates  and  mica- schists. 


INTRODUCTION,  XXIII 

JX>T.DI>r<;    OF  TIIK  ARCHKAN  AND   IIURONIAN  SERIES. 

The  Crystal  F.iUs  district  liad  now  been  an  area  of  deposition  for  a 
very  lonj;-  time,  and  a  great  tliickness  of  sediments  had  accumnlated.  A 
profound  pliysical  revohition  next  occurred,  the  greatest  since  Archean 
time.  The  region  was  raised  above  the  sea  and  was  folded  in  a  most  com- 
plex manner.  As  a  consecpience,  the  more  conspicuous  folds  vary  from  a 
north-south  to  an  east-west  direction.  The  closer  folds  in  the  northeastern 
piu-t  of  the  area  are  nearly  north-south.  In  the  centi-al  part  of  the  area  the 
closer  folds  strike  northwest-southeast.  In  the  eastern  and  southeastern 
parts  of  the  district  the  closer  folds  are  nearly  east-west.  All  of  these 
folds,  however,  have  steep  pitches.  It  therefore  follows  that  the  region 
was  subjected  to  great  compressive  stresses  in  all  directions  tangential  to 
the  surface  of  the  earth,  and  that  the  yielding  was  mainly  in  one  direction  * 
here  and  in  another  there,  although  on  every  fold  there  is  evidence  of  yield- 
ing in  two  directions  at  right  angles  to  each  other.  Some  of  the  folds  are 
very  close,  as  in  the  case  of  the  Huronian  area  between  the  two  Archean 
ovals  in  the  northeastern  part  of  the  district,  and  in  the  Felcli  Mountain 
range.  In  other  areas — as,  for  instance,  in  the  Crystal  Falls  syncline — - 
the  major  fold  is  somewhat  open.  However,  upon  the  open  folds  are  super- 
imposed folds  of  a  higher  order,  so  that  the  detail  structure  is  very  compli- 
cated.    So  far  as  known,  the  district  has  nowhere  been  faulted. 

Subsequent  to  or  during  the  late  stage  of  this  time  of  folding  there 
was  a  period  of  great  igneous  activity,  probably  contemporaneous  with  the 
Keweenawan.  At  this  tin:ie  there  were  introduced  into  both  the  Lower  and 
the  Upper  Huronian  rocks  vast  bosses  and  numerous  dikes.  The  intrusives 
vary  from  those  of  an  ultrabasic  character,  such  as  peridotites,  tlii'ough  those 
of  a  basic  character,  such  as  gabbros  and  dolerites,  to  those  of  an  acid  char- 
acter, such  as  granites.  These  intrusives,  while  altered  by  metasomatic 
changes,  do  not  show  marked  evidence  of  dynamic  metamorphism — there- 
fore the  conclusion  that  they  were  introduced  later  than  the  period  of  intense 
folding,  already  described. 

A  few  illustrations  are  mentioned.  The  Archean  and  other  great 
massifs  are  less  profoundly  altered  than  are  the  softer  and  weaker  deposits 
of  the  Huronian.  In  these  more  rigid  formations,  such  as  the  granites  and 
quartzites,  all  phases  of  alteration  by  granulation  and  recrystallization  are 


XXIV  INTRODUCTION. 

beautifully  exliibited.  The  Sturgeon  River  area  affords  one  of  the  best- 
known  illustrations  of  a  schistose  conglomerate  the  matrix  of  which  has  com-- 
pletely  recrystallized  and,  therefore,  can  not  be  discriminated  from  a  gneiss 
of  igneous  origin,  but  contains  numerous  pebbles  and  bowlders  flattened  in 
the  plane  of  schistosity. 

The  great  Hemlock  volcanic  formation  varies  from  rocks  which  ai"e 
altered  chiefly  by  metasomatic  change  to  those  which  have  become  com- 
plete crystalline  schists  containing  no  vestige,  either  macroscopically  or 
microscopically,  of   a  texture  or  structure  which  may  be  interpreted  as 


Igneous. 


SUBSEQUENT  HISTORY. 


After  the  introduction  of  the  intrusives  the  region  was  subjected  to  vast 
denudation,  which  reduced  it  approximately  to  its  present  configuration 
This  period  of  erosion  continued  until  late  Cambrian  time,  when  the  sea 
again  overrode  the  district  and  deposited  upon  the  older  rocks  Upper  Cam- 
brian sediments.  Long  after  the  deposition  of  the  Cambrian,  and  perhaps 
later  Paleozoic  rocks,  the  district  was  again  raised  above  the  sea,  and  the 
major  part  of  the  Cambrian  deposits  have  been  removed,  although  they  are 
found  in  patches  throughout  much  of  the  district,  and  occur  as  a  continu- 
ous sheet  just  east  of  the  area  discussed. 

The  district  niay  have  again  been  submerged  in  Cretaceous  time ;  but 
if  so,  the  deposits  formed  were  removed  after  the  area  finall}''  emerged  from 
the  sea.  Since  Cretaceous  time  the  region  seems  to  have  been  one  of 
erosion.  During  the  Pleistocene  period  a  thick  mantle  of  glacial  deposits 
was  spread  over  the  entire  district.  Since  Pleistocene  time  erosion  has 
advanced  far  enough  to  uncover  the  rocks  here  and  there. 

METAMORPHISM. 

The  folding  varied  in  its  closeness  in  different  parts  of  the  district. 
Moreover,  the  formations  are  of  very  variable  character,  including  a  great 
variety  of  sediments  and  of  igneous  rocks.  The  formations,  therefore,  xarj 
greatly  in  their  capacity  to  resist  stresses.  It  thus  follows  that  during  the 
folding  process  certain  formations  yielded  to  a  much  greater  degree  than 
others.  The  amount  of  contained  water  and  other  conditions  were  also 
variable.  As  a  result  of  these  many  variable  factors,  it  is  one  of  the  most 
characteristic  features  of  the  district  that  there  are  to  be  found  neai'ly  all 


INTRODUCTION. 


XXV 


varieties  <^f  niotamorphism  in  various  stages  of  advanceineiit.  The  working 
out  of  the  details  of  the  transformations  of  the  different  kinds  of  rocks 
during  their  processes  of  inetamorphism  is  one  of  the  chief  scientific  results 
which  has  come  from  a  study  of  the  district. 

CORRELATIOX. 

In  order  to  compare  the  succession  in  the  Crystal  Falls  district  with 
that  in  the  adjacent  Marquette  and  Menominee  districts,  the  descending  pre- 
Cambrian  succession  in  each  of  the  tlii'ee  districts  is  here  given  in  parallel 
columns,  the  formations  which  are  thought  to  be  equivalent  being  placed 
opposite  one  another : 

Descending  succession  of  formations  in  the  Marquettef  Crystal  Falls,  and  Menominee  districts. 


MARQUETTE  DISTRICT. 

Upper  Marquette. 

(1)  Micbigamme  formation,  bearing  a  sbort 

distanre  above  its  base  an  iron-bearing 
horizon,  and  being  replaced  in  much 
of  the  districtbj  the  Clarksburg  vol- 
canic formation. 

(2)  Ishpeming  formation,  being  composed  of 

the  Goodrich  quartzite  in  the  eastern 
part  of  the  district  and  of  the  Goodrich 
quartzite  and  the  Bijiki  schists  in  the 
western  part  of  the  district. 

ITjicon/ormity. 
Lower  Marquette. 
<1)  Negaunee  iron  formation.  1,000  to  1,500 
feet. 

(2)  Siamo    slate,   in  places  including  inter- 

stratified  amygdaloida,  200  to  625  feet 
thick. 

(3)  Ajibik  (luartzite,  7l)0  to  900  feet. 

(4)  "Wewe  slate,  550  to  1,050  feet. 

(5)  Kona  dolomite,  550  to  1,375  feet. 

(6)  Meanard  quartzite,  100  to  670  feet. 


CRYSTAL  FALLS  DISTRICT. 

Upper  Huronian. 

(1)  Michigamme  formation,  bearing^ 
a  short  distance  above  its  base 
an  iron-bearing  horizon. 


(2)  Quartzite  in  eastern  part  of  dis- 
trict. 


Unconformity. 
Lower  Huronian. 

(1)  The  Groveland formation,  about 

500  feet  thick. 

(2)  Hemlock    volcanic     formation, 

1,000  to  10,000  feet  thick. 
In  western  part  of  district  also 
occupies  the  place  of  (1)  and 
f3). 

(3)  Mansfield     formation,    100     to 

1,900  feet  thick. 

(4)  Kandville  dolomite,  500  to  1,500 
feet  thick. 
(5)  Sturgeon  quartzite,  100  to  l.UOO 
feet  thick. 


MENOMINEE   DISTRICT. 

Upper  Menominee. 
(1)  Great  Slate  formation 


Unconformity . 

Lower  Menominee. 

(1)  Vulcan   iron  formation  con- 
taining slates. 


(2)  Antoine  dolomite. 


(3)  Sturgeon  quartzite. 


Unconformity. 
Archeaii. 


Unconformity. 
Archean. 


Unconformity. 
A  rchean. 


From  the  tliree  columns  it  appears  that  the  equivalents  in  the  different 
districts  can  be  made  out  with  a  considerable  degree  of  certainty.  There 
are,  however,  various  differences,  due  to  several  causes. 

For  Upper  Hui'onian  time,  omitting  the  Clarksburg  formation,  the  sue- 


XXVI  INTEODUCTION. 

cession  in  the  Marquette,  Crystal  Falls,  and  Menominee  districts  is  substan- 
tially the  same.  The  Clarsksburg  formation  in  the  Marquette  district  may 
be  omitted  from  consideration,  because  it  is  volcanic  and  replaces  in  jjart 
the  Michigamme  and  Ishpeming  formations.  The  Upper  Huroniau  was  a 
great  period  of  slate  and  grit  deposition.  The  chief  difference  which  appears 
between  the  Menominee  district  and  the  other  two  districts  is  that  in  the 
former  no  iron  ores  have  been  found  in  the  Ujjper  Huronian  within  the 
district  proper,  although  such  rocks  occur  a  short  distance  to  the  west,  at 
Commonwealth  and  Florence,  in  Wisconsin. 

The  succession  for  the  Lower  Huronian  in  the  three  districts  can  be 
paralleled  with  a  high  degree  of  probability.  The  chief  differences  are 
due  to  the  disturbance  of  the  great  volcanic  outburst  in  the  western  part 
of  the  Crystal  Falls  district  and  to  the  uneven  surface  of  the  Archean  land 
at  the  beginning  of  Lower  Huronian  time.  As  a  consequence  of  the  latter, 
the  waters  did  not  reach  the  western  part  of  the  Marquette  district  and  the 
northeastern  part  of  the  Crystal  Falls  district  as  early  as  the  eastern  part  of 
the  Marquette  district,  the  central  part  of  the  Crystal  Falls  district,  and  the 
Menominee  district.  The  transgression  of  the  Lower  Huronian  sea  for  the 
region  covered  in  these  three  districts  was  therefore  from  the  southeast 
toward  the  northwest. 

The  Negaunee  iron  formation  of  the  Marquette  district  is  equivalent  to 
the  Grdveland  iron  formation  of  the  Crystal  Falls  district  and  the  Vulcan 
iron  formation  of  the  Menominee  district. 

The  Siamo  slate  and  the  Ajibik  quartzite  of  the  Marquette  district  are 
approximately  equivalent  to  the  Hemlock  volcanic  formation  in  much  of 
the  Crystal  Falls  district,  but  in  places  where  the  latter  formation  displaces 
the  j\Iansfield  formation  they  are  equivalent  to  only  a  part  of  the  Hemlock 
volcanic  formation.  The  Wewe  slate  of  the  Marquette  district  is  equivalent 
in  the  western  part  of  the  Crystal  Falls  distinct  to  a  part  of  the  Hemlock 
volcanic  formation,  and  in  the  southeastern  part  of  the  district  is  probably 
equivalent  to  a  part  of  the  Randville  dolomite.  It  appears  that  the  Siamo 
slate,  Ajibik  quartzite,  and  Wewe  slate  of  the  Marquette  district,  and  the 
Mansfield  and  Hemlock  formation  of  the  Crystal  Falls  district,  are  equiv- 
alent to  a  part  of  the  Antoine  dolomite  of  the  Menominee  district. 

The  great  dolomite  formation  occurring  in  all  of  the  districts  is  sup- 
posed to  be  equivalent,  except  that,  as  just  explained,  the  de^iosition  of 


INTKODUGTION.  XXVII 

liiiu'stone  contiimod  longer  in  the  southejistern  part  of  the  Crystal  Falls 
district  and  in  the  Menominee  district  than  in  the  remainder  of  the  region. 
The  absence  of  the  limestone  and  lower  formations  in  the*  western  two- 
thirds  of  the  Marquette  district  and  the  northeastern  part  of  the  Crystal 
Falls  district  is  explained  by  the  fact  that  during  early  Algonkian  time  this 
part  of  the  region  was  not  submerged.  The  Mesnard  quartzite  of  the  Mar- 
quette district  and  the  Sturgeon  quartzite  of  the  Crystal  Falls  and  Menom- 
inee districts  stand  opposite  each  other. 

From  the  foregoing  it  is  apparent  that  the  tlu-ee  districts  together  pre- 
sent a  most  interesting  and  complex  structural  problem.  While  there  is 
sufficient  similarity  in  the  formations  for  one  to  feel  considerable  assurance 
of  their  general  equivalence  in  the  different  districts,  it  is  certain  that  the 
formations  of  similar  kind  did  not  begin  and  end  at  the  same  time.  Slore- 
over,  there  are  remarkable  lateral  transitions  in  sedimentation,  as  a  result  of 
the  uneven  surface  of  the  Archean  at  the  beginning  of  Algonkian  time  and 
because  of  volcanic  outbursts.  As  a  resitlt  of  the  first  of  these  conditions,  it 
is  necessary  to  equate  fragmental  formations  which  occur  in  the  central  and 
western  parts  of  the  Marquette  district  and  the  northeastern  part  of  the 
Crystal  Falls  district,  with  nonfragmental  limestones  in  the  area  to  the  east 
and  south.  Consequent  upon  the  Upper  Huronian  volcanic  outbursts  in  the 
Marquette  district,  the  Michigamme  and  Ishpeming  formations  are  largely 
replaced  by  the  Clarksburg  volcanics.  Similar  outbursts  in  the  western  part 
of  the  Crystal  Falls  district  in  Lower  Huronian  time  placed  volcanic  rocks 
for  this  part  of  the  district  opposite  the  Mansfield  slate  and  the  Groveland 
iron  formation. 

The  foregoing  relations,  combined  with  the  great  variety  and  complexity 
of  the  sediments  of  the  district,  the  presence  of  many  forms  of  contempora- 
neous volcanic  deposits,  the  intrusion  of  the  widest  ^•ariety  of  igneous  rocks 
of  various  ages  from  Archean  to  later  Algonkian  time,  and  the  complicated 
folding  and  metamorphism  to  which  the  district  has  been  subjected,  will 
readily  convince  one  that  the  working  c)ut  of  the  detail  structure  of  the 
district  by  Messrs.  Clements,  Sn^yth,  Bayle}',  ]\Ierriam,  and  others  has  not 
been  accomplished  without  most  painstaking  and  laborious  work,  especially 
as  the  region  is  covered  by  timber  or  brush  and  is  overspread  by  a  mantle 
of  glacial  deposits. 


OUTLINE  OF  THIS  MONOGRAPH, 


Part  I. 


Chapter  I.  The  Crystal  Falls  district  is  situated  on  the  Upper  Peninsula  of  Michigan,  and 
foniis  a  connecting  link  between  the  Marquette  and  Menominee  districts  of  Michigan.  A  history  of 
the  previous  work  in  the  district,  accompanied  by  a  summary  of  the  literature,  is  given,  and  there  is 
reproduced  a  series  of  maps  which  indicate  the  development  of  knowledge  concerning  the  distribution  . 
of  the  rocks  and  their  structural  relations.  As  explanatory  of  the  locations  given,  the  mode  of  work 
is  described  and  the  object  and  method  of  taking  magnetic  observations  is  briefly  outlined. 

Chapter  II  treats  of  the  geographical  limits,  structure,  stratigraphy,  and  physiography.  The 
portion  of  the  district  here  described  includes  approximately  540  square  miles.  Structurally  it  is 
closely  related  to  the  Maniuette  district;  the  essential  features  being  a  northwest-southeast  set  of 
folds,  with  a  superimposed  series  trending  northeast-southwest.  The  oldest  rocks  belong  to  the 
Archean.  They  cover  an  oval  area  which  is  surrounded  by  the  Algonkian  rocks  represented  by  the 
Lower  and  Upper  Huroniau  series.  The  Archean  and  Algonkian  are  overlain  with  strong  uncon- 
formity by  rocks  of  the  Cambrian  division  of  the  Paleozoic.  The  drift  deposits  of  the  Quaternary 
are  everywhere  present.  Only  the  pre-Paleozoic  rocks,  however,  are  discussed.  The  most  noticeable 
topography  is  that  of  the  drift,  which  in  places  is  seen  to  be  superimposed  upon  pre-Pleistocene 
topography.  The  maximum  elevation  is  1,900  feet  above  sea  level,  and  the  minimum  1,250  feet.  The 
conclusion  is  reached  that  this  portion  of  Michigan  before  Glacial  times  had  been  reduced  to  the 
condition  of  an  approximate  peneplain.  The  drainage  is  chiefly  by  a  few  large  streams  which  flow 
into  Green  Bay  of  Lake  Michigan.  A  small  part  is  drained  by  streams  flowing  into  Lake  Superior. 
In  portions  of  the  area  the  drainage  has  reached  an  advanced  stage,  in  other  portions  it  is  very 
youthful.  The  development  of  the  drainage  is  illustrated  in  the  ease  of  the  Deer  River.  The  timber 
and  soil  vary  much  in  character. 

Chapter  III  treats  of  the  Archean.  The  rocks  of  this  nge  form  an  elliptical  core,  following  the 
axis  of  a  northwest-southeast  trending  anticline.  Exposures  are  few  because  of  the  superimposed 
drift.  The  Archean  is  overlain  uncouformably  by  Algonkian  sediments  derived  from  the  granite,  and 
there  is  absence  of  contact  action.  These  facts  indicate  that  it  was  the  floor  upon  which  the  over- 
lying sediments  were  deposited.  Petrographically  it  consists  chiefly  of  biotite-granite.  On  the 
periphery  of  the  area  a  biotite  gneissoid  granite  is  very  well  developed.  Some  of  this  at  least  is 
of  dynamic  origin.  The  Archean  is  cut  by  acid  and  basic  dikes  which  are  now  both  schistose  and 
massive. 

Chapter  IV  treats  of  the  Lower  Huroniau  series.  This  series  is  subdivided  into  the  following 
formations,  from  the  base  upward:  The  Raudville  dolomite,  the  MausHeld  slate,  and  the  Hemlock 
volcanics. 

Section  I.  The  Randville  dolomite  is  poorly  exposed.  It  consists  of  quartzose  dolomite,  grading 
down  into  quartz-schist  and  recomposed  granite.  It  has  evidently  been  derived  partly  from  the 
granite,  and  is  cousiderably  younger  than  that.  Its  relations  to  the  overlying  formations  were  not 
observed.  The  thickness  could  not  here  be  determined,  but,  in  the  area  studied  by  Smyth,  its  maxi- 
mum is  about  1,500  feet. 


XXX  OUTLmE  OF  THIS  MONOGRAPH. 

Section  II.  The  Mansfield  slate  is  best  exposed  in  the  vicinity  of  the  town  of  the  same  name. 
It  here  occupies  a  valley,  through  which  flows  the  Michigamme  River.  Petrographically  this 
formation  includes  graywatkes,  clay  slate,  phyllite,  siderite-slate,  chert,  ferruginous  chert,  and  iron 
ores,  with  various  metamorphic  products  derived  from  them.  The  slate  predominates.  The  Maualield 
slates  are  intruded  by  basic  igneous  rocks,  which  underlie  them.  They  are  overlain  by  volcanics, 
which  contain  fragments  of  the  slates,  and  are  hence  younger  than  they.  The  Mans6eld  slates  strike 
north  and  south  and  dip  on  an  average  80^  to  the  west.  They  represent  the  limb  of  a  westward- 
dipping  monocline.  The  maximum  thickness  of  the  belt  is  1,900  feet.  Followed  south  away  from 
the  point  of  maximum  thickness  it  rapidly  thins  out  and  disappears.  In  the  slate  but  a  single  ore 
body  of  commercial  importance  has  been  found.  This  is  exploited  by  the  one  Bessemer  ore-producing 
mine  of  the  district.  The  ore  body  is  presumed  to  have  resulted  from  the  alteration  of  a  siderite, 
and  the  concentration  in  a  favorable  position  of  the  iron  from  the  portions  of  the  ferruginous  beds 
removed  by  erosion.  A  possible  continuation  of  the  Mansfield  slate  is  suggested  by  the  occurrence  of 
small  outcrops  of  somewhat  similar  slates  about  5  miles  slightly  to  the  west  of  north. 

Section  III.  The  Hemlock  formation  consists  almost  exclusively  of  volcanic  rocks,  both  b.asic 
and  acid,  with  crystalline  schists  derived  from  them.  Sedimentary  rocks  play  a  very  unimportant 
rule.  Exposures  are  numerous  west  of  the  belts  of  previously  described  rocks,  and  where  erosion 
has  removed  the  drift  the  formation  has  a  marked  influence  ou  the  topography.  The  thickness  is 
estimated  from  the  dip  to  reach  23,000  feet,  but  this  is  probably  illusory  because  of  reduplication  due 
to  folding.  In  the  northern  portion  of  the  district  the  formation  overlies  the  Kona  dolomite.  In  the 
southern  portion  it  overlies  conformably  the  Mansfield  slate.  It  is  probable  that  volcanic  activity 
began  in  the  north  and  moved  south,  and  that  some  of  the  volcanics  to  the  north  are  contemporaneous 
with  the  Mansfield  slates.  The  volcanics  are  cut  by  a  few  acid  dikes.  Basic  dikes  forming  enormous 
bosses  of  basic  rock  are  of  freiiuent  occurrence.  The  volcanic  origin  of  the  major  portion  of  this 
formation  is  perfectly  clear.  Some  of  the  volcanics  are  submarine.  The  greater  proportion,  however, 
were  derived  from  volcanic  vents,  which  could  not  be  located,  but  were  probably  situated  near  the 
Huronian  shore  line.  The  Hemlock  volcanics  are  divided  into  igneous  and  sedimentary  rocks.  Under 
the  igneous  rocks  there  are  described  both  acid  and  basic  lavas  and  pyroclastics.  Under  the  sedimen- 
tary rocks  there  are  described  volcanic  sediments,  both  of  eolian  and  water-deposited  character.  By 
extreme  metamorphism  crystalline  schists  have  been  produced  from  l)Oth  igneous  and  sedimentary 
rocks.  The  acid  volcanics  include  rhyoliteporphyries  and  aporhyolite-porphyry.  The  rhyolite- 
porphyry  shows  interesting  micropoikilitic  textural  characters.  Some  of  the  porphyries  have  been 
rendered  schistose  by  pressure.  Acid  pyroclastics  are  scarce  and  were  derived  from  the  aporhyolite. 
The  basic  lavas  correspond  to  the  modern  basalts.  They  are  much  altered.  To  indicate  these  facts 
and  at  the  same  time  show  their  correspondence  to  the  Tertiary  aud  recent  basalts,  they  are  called 
"  metabasalts."  The  basic  lavas  include  noni^orphyritic,  porphyritic,  aud  variolitie  types.  A  columnar 
structure  was  not  observed,  but  an  ellipsoidal  structure  is  very  common.  This  structure  is  described 
ill  detail  and  the  couclusiou  reached  that  basalts  possessing  this  structure  were  originally  very 
viscous  and  correspond  to  the  modern  aa  lavas.  The  amygdaloidal  structure,  which  is  almost  univer- 
sally jiresent  in  the  volcanics,  is  described  aud  illustrated.  The  alteration  of  the  basalts  is  discussed 
aud  speci.al  cases  described.  As  result  of  this  alteration  the  textural  as  well  as  the  mineralogical 
characters  may  be  completely  changed,  and  the  volcanic  origin  of  the  resulting  rocks  could  not  be 
determined  but  for  their  association.  In  the  zone  of  weathering,  calcification  is  the  controlling 
alteration  jirocess.  In  rocks  more  deeply  buried,  silicification  is  the  process  which  predominates. 
The  pyroclastics  comprise  eruptive  breccia,  including  thereunder  friction  breccias  and  flow  breccias, 
and  volcanic  sedimentary  rocks.  The  eolian  deposits,  which  arc  described  as  tuft's,  grade  from  fine 
dust  deposits  up  into  tliose  in  which  the  fragments  are  bowlders.  The  water-deposited  volcanic 
fragmentals  are  known  as  volcanic  conglomerates,  and  likewise  grade  from  those  of  which  the  particles 
are  of  minute  size  into  those  of  which  the  fragments  are  of  very  large  size.  At  various  places 
clastic  rocks  occur  which  are  now  schistose,  and  whose  exact  mode  of  origin — that  is,  whether  eolian 
or  water  deposited — could  not  be  determined.     Normal  sediments  consisting  of  slate  with  limestone 


OUTLINE  OF  THIS  MONOGRAPH.  XXXI 

liMises  form  u  loiitiiulai'  deposit  in  the  volcuiiics  uiiar  tlie  top  of  tin;  formation.  'I'licy  are  inHijjiiiH- 
cant  in  quantity. 

Under  the  Uouo  Lalvu  crystalline  schists  there'  arc  included  rocks  of  completely  crystalline 
character,  but  which  by  field  and  microscopical  study  have  beeu  connected  with  the  volcanics  and 
are  considered  to  have  been  derived  from  rocks  similar  in  nature  to  them. 

The  rocks  composiun'  the  Hemlock  formation  arc  little  likely,  owing  to  their  somber  color,  to 
be  much  used  for  building  or  ornamental  purposes.  They  offer,  however,  an  inexhaustible  supply  of 
the  best  cjuality  of  road-Unildiug  material. 

Chapter  V  treats  of  the  I'pper  Hurouian  series.  This  series  is  connected  in  the  northern  part 
of  the  area  described  with  the  Upper  Maniuette  series  of  the  ad.joiniug  Marquette  district,  and  is 
considered  to  correspond  stratigraphically  to  the  Upper  Maniuette.  Owing  lo  lack  of  exposures  and 
to  the  intricacy  of  folding,  the  series  could  not  be  subdivided.  It  covers  a  great  area  surrounding 
the  Hemlock  formation,  and  extends  beyond  the  limits  of  the  map.  The  exposures  are  scanty.  It 
inlluences  the  topography  only  in  a  very  general  way,  being  for  the  most  part  heavily  drift  covered. 
Its  thickness  could  not  be  estimated.  The  rocks  of  this  series  wrap  around  the  subjacent  Lower 
Huronian  series.  The  line  between  them  is  uudulatory.  The  indentations  in  the  Lower  Huronian 
represent  minor  cross  eyuclines,  and  the  jirotuberauees  represent  minor  cross  anticlines.  The  most 
prominent  fold  of  the  series  is  known  as  the  Crystal  Falls  syncline.  The  strike  of  the  axis  of  this 
syncline  is  in  general  to  the  south  of  west,  and  pitches  in  the  same  direction.  The  syncline  is  not 
simple,  but  has  minor  rolls,  as  shown  in  various  exposures.  This  folding  has  beeu  productive 
of  extensive  reibungsbreccias.  The  folding  occurred  immediately  preceding  the  deposit  of  the 
Keweenawau  series  iu  other  parts  of  the  Lake  Superior  region.  The  Upper  Huroniau  is  penetrated 
by  iutrusive  rocks  of  acid,  iutermediate,  and  basic  composition.  The  rocks  constituting  the  series 
may  be  divided  into  those  of  sedimentary  and  those  of  igneous  origin.  The  sedimentary  rocks  are 
graywackes,  ferruginous  graywackes,  micaceous,  carbonaceous,  and  ferruginous  clay  slates  and 
their  crystalline  derivatives,  and  thinly  laminated  cherty  sideriite-slate,  ferruginous  chert,  aud  iron 
ores.  In  two  places  rocks  of  conglomeratic  nature  occur.  The  extensive  folding  which  the  series  has 
undergone,  coupled  with  the  intrusions  of  the  igneous  rocks,  has  produced  crystalline  schists  from  the 
muds  and  grits.  These  are  extensively  developed  iu  the  southern  portion  of  the  district  in  the  vicinity 
of  the  Paint  and  Jlichigamme  rivers.  The  igneous  rocks  which  have  penetrated  the  Upper  Hurouian 
subsequent  to  the  folding  which  affected  it  are  not  described  under  the  series.  Interlaminated  with 
the  crystalline  schists  there  are,  however,  certain  rocks,  now  perfectly  crystalline  hornblende- 
gneisses,  which  are  presumed  to  have  resulted  from  the  metamorphism  of  either  basic  intrusive  sheets 
or  interljedded  flows. 

The  economic  development  of  the  district  followed  that  of  the  adjoining  Menominee  district. 
The  exploited  ore  deposits  occur  near  Amasa,  and  iu  the  vicinity  of  Crystal  Falls.  The  ore  is  hematite 
and  limonite.  The  grade  is  non-Bessemer.  The  ore  is  associated  with  white  and  reddish  chert,  and 
this  formation  lies  between  carbonaceous  slates.  The  ore  bodies  in  general  pitch  to  the  west  at 
varying  angles,  which  correspond  to  the  pitch  of  the  axes  of  the  syncliues  iu  which  they  occur.  These 
minor  folds  in  turn  correspond  to  the  western  pitch  of  the  main  Crystal  Falls  synclinorium.  The  ore 
bodies  are  concentrates  in  synclinal  troughs,  as  described  by  Van  Hise  for  other  Huronian  districts. 
The  mining  is  now  for  the  most  part  undergouud,  and  is  carried  on  iu  open  stopes.  The  greatest 
shipment  of  ore  from  the  area,  including  the  Lower  Hurouian  Mansfield  mine,  was  .586,970  tons  in 
1892.     The  total  shipment  for  1898  reached  325,814  long  tons. 

Chapter  VI  treats  of  the  intrusives.  There  is  here  included  a  varied  assortment  of  rocks, 
exhibiting  iu  common  intrusive  relations  to  the  sedimentary  and  igneous  rocks.  The  term  "  intrusive" 
is  not  to  be  interpreted  as  synonymous  with  the  "dike  rocks"  of  some  authors.  These  rocks  are 
never  found  to  penetrate  the  Cambrian  rocks,  and  have  not  been  affected  by  the  folding  which  meta- 
morphosed the  Upper  Huroniau  sediments.  They  are  presumed  to  be  of  Keweenawau  age.  The  intru- 
sives have  in  some  cases  beeu  injected  along  the  axes  of  the  folds,  these  representing  the  lines  of 
greatest  shattering,  and  hence  least  resistance. 


XXXII  OUTLINE  OF  THIS  MONOGRAPH. 

In  Section  I  there  are  described  a  number  of  intrusive  roclis  which  can  not  be  connected  genet- 
ically with  one  another.  These  comprise  ordinary  biotite-granites,  with  micropegmatitic  varieties, 
muscovite-biotite-granite,  metadolerite,  metabasalt,  and  picrite-porphyry.  AVhere  the  granites  have 
intruded  the  Upper  Huronian  series  they  have  contorted  the  strata,  and  include  and  metamorphose 
the  rocks,  producing  muscovite-biotite-gneiss,  and  staurolitiferous  and  garnetiferous  mica-schists. 
The  metadolerites  possess  no  special  points  of  interest  in  themselves,  but  where  they  have  intruded 
the  Mansfield  slates  they  have  caused  interesting  exomorphism.  The  slates  are  converted  into 
adinoles,  spilosite8,and  desraosites.  Chemical  analyses  indicate  the  chief  change  which  has  taken 
place  in  the  production  of  these  rocks  from  the  clay  slate  to  have  been  in  the  increase  of  silica  and 
soda  as  the  contact  is  approached.  There  seems  thus  to  have  been  a  direct  transference  of  sodiuip 
and  silicon  from  the  igneous  rock  to  the  sedimentary.  The  metabasalt  dikes  are  of  little  interest. 
The  ultrabasic  picrite-porphyries  are  extremely  altered.  This  alteration  has  produced  in  one  case 
chiefly  tremolite  and  in  another  serpentine.  One  of  these  serpentine  picrite-porphyries  is  polar-mag- 
netic. The  picrite-porphyries  .ire  presumed  to  have  contained  a  vitreous  base,  and  correspond  to  the 
modern  Tertiary  limburgite. 

In  Section  II  there  is  given  a  study  of  a  series  of  rocks  varying  from  those  of  intermediate 
acidity  through  those  of  basic  composition  to  ultrabasic  kinds.  The  exposures  of  these  rocks  are 
found  iu  an  area  underlain  by  the  Upper  Huronian  series,  extending  from  Crystal  Falls  southeast  to 
and  a  short  distance  beyond  the  Michigamme  River.  The  prevailing  rocks  are,  on  the  one  hand, 
diorites  of  intermediate  acidity,  ranging  to  more  acid  rocks,  tonalites,  quartz-mica  diorites,  and 
granite.  On  the  other  hand,  we  have  hornblende-gabbros,  gabbros,  norites,  and,  lastly,  peridotites 
of  varying  luineralogical  character.  Only  those  kinds  of  rocks  of  which  analyses  have  been  obtained — 
mica-diorite,  hornblende-gabbro,  norite,  and  wehrlite — are  discussed.  The  diorites  are  holocry- 
stalline  rocks  of  medium  to  coarse  grain.  In  texture  they  show  some  variation  from  those  which  are 
hypidiomorphic  granular  to  those  in  which  the  texture  is  imi)erfectly  ophitic.  As  facies  of  the  dioritic 
magma  there  are  described  diorite,  mica-diorite,  quartz-niica-diorite,  tonalite,  and  plagioclase-bearing 
granite.  A  quartz-mica-diorite-porphyry  occurs  in  narrow  dikes  cutting  the  mica-diorite.  Analysis 
of  the  mica-diorite  shows  it  to  stand  upon  the  border  between  the  lime-soda  feldspar  rocks  and  the 
orthoclase  rocks.  Thegabbros  and  norites  arehulocrystalline  rocks  of  moderately  iine  to  coarse  grain. 
They  show  considerable  v.ariation  in  texture.  The  hypidiomorphic  granular  texture  predominates, 
but  some  few  show  a  good  parallel  texture.  Others  are  noticeably  porphyritic,  a  few  have  poikilitic 
textures,  and  less  commonly  there  is  an  approach  to  the  ophitic  texture.  Hornblende  and  labradorite 
is  the  must  common  mineral  association,  giving  typical  hornblende-gabbro.  A  monoclinic  pyroxene  at 
times  becomes  .abundant,  giving  a  transition  to  the  normal  gabbro.  Bronzite  at  times  is  the  promi- 
nent bisilicate  constituent  of  these  rocks,  giving  bronzite-norite.  A  bronzite-norite-porphyry  also 
occurs.  Locally  the  hornblende-gabbro  has  been  crushed,  and  there  is  produced  therefrom  a  schistose 
rock  which  represents  a  transition  to  a  hornblendo-gueiss.  Of  these  rocks  the  hornblende-gabbro  was 
first  formed.  It  was  intruded  by  normal  gabbro,  and  both  of  these  types  were  then  cut  by  dikes  of 
bronzite-norite  and  bronzite-norite-porphyry.  The  rocks  included  under  the  peridotites  show  con- 
siderable mineralogical  variation.  There  is  produced  an  amphibole-peridotite,  which,  when  augite 
becomes  predominant,  grades  to  wehrlite.  This  in  its  turn  becomes  feldspathic,  and  indicates  a  tran- 
sition to  olivine-gabbro.  The  amphibole-peridotite  also  becomes  feldspathic  and  quartzitic,  indicat- 
ing a  transition  toward  diorite.  When  the  order  of  crystallization  of  the  minerals  composing  the 
granular  rocks  of  the  entire  series  described  above  is  considered,  it  is  seen  to  have  been  as  follows: 
Bronzite  is  apparently  the  oldest.  The  olivine  and  monoclinic  pyroxene  come  next  and  are  of  essen- 
tially the  same  age.  Mica  and  hornblende  follow,  and  are  contemporaneous.  Then  comes  plagio- 
clase,  orthoclase,  and  qu.artz.  A  consideration  of  the  chemical  analyses  of  the  rocks  above  described 
shows  them  to  belong  to  a  series  ranging  from  a  diorite,  on  the  one  hand,  to  hornblende-gabbro  and 
norite  and  to  peridotite  on  the  other.  On  the  acid  side  of  the  series  variations  are  shown  microscopic- 
ally, but  of  these  rocks  chemical  analyses  have  not  been  obtained.  It  is  not  possible  to  state  which 
of  these  rocks  most  nearly  resembles  in  its  composition  the  original  magma  of  which  the  di£ferent 


OUTLINE  OF  THIS  MONOGRAPH.  XXXIII 

types  ropreseut  tlm  differentiation  products.  Tlie  lioruMende-jjiilibro  i«  that  typn  whicli  apparently 
first  loaclied  its  present  geological  position.  It  was  followed  in  the  acid  part  of  the  series  liy  the 
diorite,  which  in  its  turn  was  succeeded  by  the  diorite-porphyry.  Along  the  basic  scries  hornbleude- 
gabbro  was  succeeded  by  gabbro,  followed  by  the  bronzite-norite  and  the  peridotite.  In  general  the 
forces  of  differentiation  have  been  toward  increasing  acidity  and  increasing  basicity. 

Pakt  II. 

Chapter  I  treats  of  geographical  limits  and  physiography.  The  geographical  limits  of  the  area 
described  arc  given  and  a  brief  statement  made  concerning  the  eouditious  under  which  the  work  was 
done.  In  the  preliminary  sketch  of  the  geology  the  rocks  represented  are  stated  to  range  in  ago  from 
Archeau  to  early  Paleozoic.  In  that  part  of  the  district  north  and  west  of  the  Michigamme  River  the 
Archean  is  exposed  in  several  regularly  outlined  oval  areas  from  10  to  12  miles  long  and  from  2  to  6 
miles  wide.  The  intervjils  Ijetweeu  these  ovals  are  occupied  by  highly  tilted  metamorphosed  sedimen- 
tary and  igneous  rocks  of  Algonkian  age.  In  the  southern  and  eastern  portions  of  the  district  the  edges 
of  the  tilted  rocks  are  covered  by  the  gently  dipping  Cambrian  sandstone.  Xevertheless,  field  work 
shows  that  the  distribution  of  the  Archeau  in  ovals,  which  is  so  characteristic  for  the  areas  north, 
also  holds  here.  The  chief  surface  feature  is  a  rolling  plain,  which  slopes  gently  to  the  southeast, 
and  upon  which  is  superimposed  the  glacial  drift  with  its  characteristic  topograjihical  features, 
multitudinous  in  variety  and  detail,  but  insignificant  in  relief.  While  the  details  of  the  topography 
are  mainly  glacial,  the  broader  features  have  often  clearly  been  determined  by  the  presence  of  the 
more  resistant  Archeau  and  Algonkian  rocks.  The  drainage  is  to  the  southeast,  mainly  into  Lake 
Michigan,  through  the  Michigamme  aud  the  Sturgeon  rivers.  Details  of  the  drainage  have  been 
determined  by  the  distribution  of  the  rocks.  It  is  interesting  to  note  that  the  Michigamme  flows 
along  the  eastern  edge  of  its  drainage  basin,  having  no  eastern  tributaries. 

Chapter  II  treats  of  magnetic  observations  in  geological  mapping.  Certain  of  the  rocks 
occurring  in  the  Crystal  Falls  district  contain  magnetite  in  such  quantity  that  they  have  a  marked 
influence  on  the  magnetic  needle.  Advantage  is  taken  of  this  fact  in  the  mapping  of  the  rocks  where 
exposures  are  w.auting.  The  instruments  and  methods  of  work  used  in  making  magnetic  oI)Servatious 
are  described.  Facts  of  observation  are  mentioned,  aud  general  principles  are  laid  down.  Applica- 
tion of  these  principles  to  special  cases  is  then  considered,  and  finally  a  description  of  the  method  used 
in  the  interpretation  of  complex  structures  follows. 

Chapter  III.  In  Section  I  the  position,  extent,  aud  previous  work  done  in  the  Feloh  Mountain 
range  is  described.     An  abstract  of  the  literature  covering  the  area  is  given. 

Section  II  contains  a  general  sketch  of  the  geology  of  this  range.  The  rocks  range  from  Archean 
to  early  Paleozoic.  These  last  are  not  considered  for  the  jiresent.  The  Archeau  is  distributed  in  areas 
which  represent  the  cores  of  large  arches  formed  over  the  whole  region  by  mountain  building  energy, 
and  subsequently  truncated  by  deep  Cambrian  denudation.  The  rocks,  chiefly  of  sedimentary  origin 
intermediate  between  the  Archean  and  Paleozoic,  to  which  the  name  Algonkian  is  applied,  occupy  a 
nari-ow  strip  ranging  from  a  mile  aud  a  half  to  less  than  a  mile  in  width,  and  extending  east  and  west 
for  a  distance  of  over  13  miles.  This  strip  constitutes  the  Felch  Mountain  range.  It  is  bordered  on 
the  north  and  south  by  the  Archean.  The  lowest  part  of  the  Algonkian  occupies  parallel  zones  nest 
to  the  Archean  lioth  on  the  north  aud  on  the  south,  aud  is  succeeded  toward  the  interior  of  the  strip 
by  the  younger  members.  The  general  structure  therefore  is  synclinal,  Iiut  is  not  simple.  The  strip 
contains  two  or  more  synclines  separated  by  anticlines.  They  have  likewise  been  affected  by  cross 
folds,  which  give  a  different  pitch  to  the  axes  of  the  east  and  west  folds.  The  structure  is  also  compli- 
cated by  faulting.  The  Algonkian  is  divided  into  two  series  separated  by  an  unconformity.  In  the 
first  occur,  from  the  base  upward,  the  .Sturgeon  quartzite,  the  Eaudville  dolomite,  the  Mansfield 
schist,  and  the  Groveland  iron  formations.     Above  these  follows  a  youuger  series  which  is  undivided. 

Section  III  treats  of  the  Archean.  This  limits  the  Algonkian  rocks  on  the  north  and  south,  and 
is  very  well  exposed.     The  topography  is  very  rough.     Usually,  but  not  always,  a  topographical 

MON   XXXVI III 


XXXIV  OUTLINE  OF  THIS  MONOGllAPH, 

(lepressiou,  occupied  by  a  swamp  or  by  a  stream,  exists  along  the  coutact  betweeu  the  Archeau  and 
the  Algonkian.  Petrographically  the  Archean  consists  of  (1)  granites  or  granitic  gneisses,  (2) 
gneisses,  (3)  mica-schists,  (4)  hornblende-gneisses,  or  amphibolites.  The  granites  possess  the  usual 
characters  of  such  rooks.  The  gneissoid  members  of  this  division  are  merely  crushed  granites, 
and  are  connected  with  the  massive  rocks  by  indistinguishable  gradations.  The  gneisses  are  banded 
laminated  rocks,  the  minerals  of  which  have  crystallized  in  parallel  elongated  forms.  Subsequent 
to  crystallization  they  have  been  acted  on  by  great  stresses.  The  mica-schists  are  even,  medium- 
grained  rocks,  with  generally  well-developed  schistosity.  The  original  character  of  these  schists 
is  wholly  indeterminable.  Their  relationship  with  the  granites  and  gneisses  is  perhaps  a  reason 
for  regarding  them  as  derived  from  originally  massive  granites  by  dynamic  metamorphism.  The  ' 
hornblende-gneisses,  or  amphibolites,  are  black  or  dark-green  rocks,  which  are  universally  foliated. 
They  occur  in  narrow  bands  in  the  granites  and  gneisses.  Their  boundaries  are  sharp  and  frequently 
cut  the  foliation  of  the  amphibolites  and  of  the  gneisses.  The  field  relation  as  well  as  the  composi- 
tion of  the  amphibolites  leads  to  the  conclusion  that  they  are  old  dikes  of  basic  rocks  which  have 
been  metamorphosed  and  recrystallized. 

Section  IV  treats  of  the  Sturgeon  quartzite.  The  Sturgeon  formation  is  the  most  widespread 
member  of  the  Algonkian  series  in  the  Felch  Mountain  range.  It  occurs  in  two  parallel  zones  of 
varying  width,  immediately  adjoining  the  Archeau  to  the  north  and  to  the  south,  except  when 
displaced  from  thi.s  position  by  faults.  It  is  fairly  well  exposed.  It  frequently  forms  distinct  lineal 
ridges,  which,  with  but  few  exceptions,  seldom  rise  to  the  mean  altitude  of  the  adjoining  Archean. 
Owing  to  the  comjileteness  of  recrystallizatiou,  the  original  sedimentary  features  have  almost  been 
obliterated,  so  that  it  is  difiScnlt  to  find  places  suitable  for  dip  observations.  SufiScient  dips  have 
been  found  to  show  that  subordinate  folds  occur  within  the  quartzite.  The  average  thickness  is 
probably  not  less  than  450  feet,  and  may  bo  more.  Petrographically  the  formation  includes  massive 
quartzites  and  mashed  quartzites  or  micaceous  quartz-schists,  in  some  of  which  the  relations  of  the 
quartz  present  unusual  features. 

Section  V.  The  Randville  dolomite,  consisting  of  crystalline  dolomitio  rocks,  overlies  the 
Sturgeon  quartzite.  The  Randville  dolomite  covers  a  larger  share  of  the  surface  in  the  Felch  Moun- 
tain range  than  any  other  member  of  the  Algonkian.  Natural  exposures  are  fairly  numerous  and 
very  evenly  distributed.  Moreover,  test  pits  and  diamond-drill  borings  have  shown  the  pi-esence  of 
the  formation  in  the  covered  areas.  Relatively  the  dolomite  is  a  weak  rock,  and  occupies  relatively 
low  ground.  An  average  thickness  of  700  feet  is  estimated  for  the  Randville  dolomite  within  the 
Felch  Mountain  range.  Petrographically  the  formation  consists  of  a  rather  coarse-grained,  thoroughly 
crystalline  dolomite,  with  more  or  less  abundant  crystals  of  tremolite  and  a  number  of  other  minerals 
of  minor  importance. 

Section  VI.  The  Mansfield  schist  is  only  exposed  in  certain  test  pits.  Its  presence  has  also 
been  determined  by  diamond-drill  borings.  The  thickness  is  so  small — not  more  than  200  feet — that, 
though  it  weathers  readily,  it  produces  no  noticeable  eft'ects  on  the  general  topography.  Petro- 
graphically it  consi.sts  of  fine-grained  mica-muscovite  or  mica-biotite-schists,  probably  derived  from 
the  metamorphism  of  a  clasfc.     It  shows  nothing  of  especial  interest. 

Section  VII.  The  Groveland  formation  is  magnetic  and  has  been  traced  by  means  of  compass  and 
dip  needle.  Excellent  natural  as  well  as  numerous  artificial  exposures  render  the  data  concerning  the 
distribution  of  the  formation  very  satisfactory.  The  most  prominent  hills  in  the  Algonkian  belt  owe 
their  relief  to  the  fact  that  they  are  underlain  by  the  Groveland  formation.  Petrographically  we  may 
recognize  two  main  kinds  of  rock.  The  usual  kind  consists  of  quartz  and  the  anhydrous  oxides  of 
iron,  while  the  other  and  much  rarer  consists  essentially  of  an  iron  amphibole  with  quartz  and  the 
iron  oxides  as  associates.  Both  of  these  kinds  are  clearly  of  detrital  origin.  The  conclusion  is 
reached,  based  on  certain  microscopical  structures,  that  iron  and  silica  were  originally  present 
largely  in  the  form  of  glauconite. 

Section  VIII.  Mica-schist  aud  ferruginous  quartzites  of  the  Upper  Huronian  series  occur  in 
the  eastern  part  of  the  Felch  Mountain  range.     The  rocks  constituting   the   series  are  soft  iron- 


OUTLINE  OF  THIS  MONOGRAPH.  XXXV 

stained  mica-schists,  with  thin,  interbandod  beds  of  ferrugiuous  aud  miciiccous  (juartziti!.  Neither 
kind  shows  traces  of  chistic  origin.  V'rom  their  structures  and  general  relations  they  are  believed  to 
have  been  derived  from  sedimentary  rooks  by  metamorphism. 

Section  IX.  The  Algonkian  rocks  are  cut  by  iutrusives,  among  which  both  acid  and  basic  rocks 
are  reiiresented.  The  acid  rocks  are  granites  occurring  in  narrow  dikes.  No  dikes  of  granite  .ire 
known  to  cut  the  Kaiidville  or  Manstield  formations. 

Chapter  IV  treats  of  the  Miehigamme  Mountain  and  Fence  River  areas.  These  areas  occur  in 
the  central  part  of  the  district.  In  the  Fence  River  area  the  structure!  is  very  simple.  In  the  Miehi- 
gamme Mountain  area  the  structure  is  comple.x. 

Section  I  treats  of  the  Archean.     The  prev.alent  rock  is  granite,  cut  by  acid  and  basic  dikes. 

Section  II  treats  of  the  Sturgeon  form.-itiou.  This  is  scarcely  known  as  a  distinct  Algonkian 
member  in  this  area  .apart  from  the  Randville  formation.  In  one  section  purely  clastic  sediments 
were  observed,  for  which  it  is  convenient  to  retain  the  name.  These  exposures  cousist  of  slates  iind 
gray  wackes,  with  some  layers  of  a  coarser  texture. 

Section  III  treats  of  the  Randville  dolomite.  In  the  Fence  River  area  the  dolomite  lies  on  the 
east  side  of  the  Archean  and  occupies  a  belt  about  one-half  mile  in  width,  aud  extending  from  the 
mouth  of  the  Fence  River  about  10  miles  to  the  north  and  west,  where  it  leaves  the  portion  of 
the  district  studied.  In  the  Miehigamme  Mountain  area  the  dolomite  tops  the  low  arcli  in  a  broad 
crumpled  sheet.  The  form<atiou  in  the  Fence  River  area  occurs  in  an  eastward-dipping  monocline 
with  a  number  of  minor  plications.  An  average  thickness  of  about  1,500  feet  is  estimated  for  the 
formation  in  this  area.  In  the  scattered  outcrops  of  the  Miehigamme  Mountain  area  the  dolomite 
strikes  .and  dips  toward  all  points  of  the  compass,  caused  by  the  gentle  archiug  from  the  general 
northwest-southeast  axis,  combined  with  sharp  local  folds  which  run  nearly  east  and  west.  Petro- 
o-raphically  the  formation  ranges  from  coarse  saccharoidal  marbles,  sometimes  very  pure  but  usually 
filled  with  secondary  silicates,  to  fine-grained,  little-altered  limestones,  which  are  occasionally  so 
impure  as  to  be  calcareous  or  dolomitic  sandstones  and  shales.  The  prevalent  colors  are  white,  but 
various  shades  of  pink,  light  .and  deep  blue,  aud  pale  green  occur.  Some  of  the  varieties  are  oolitic. 
This  structure  does  not  seem  to  have  been  noted  previous  to  this  in  limestones  of  pre-C'ambri,an  age. 

Section  IV  treats  of  the  Mansfield  formation.  The  typical  locality  of  this  formation  is  in  the 
vicinity  of  the  Mansfield  mine,  which  lies  to  the  west  of  the  district  studied.  Where  it  occurs  in  the 
Miehigamme  Mountain  area,  the  formation  consists  of  phyllites  or  mica-slates  of  various  colors.  The 
structure  of  this  area  is  so  complex  and  the  outcrops  so  few  as  to  forbid  any  but  an  approximate 
outlining  of  the  general  boundaries  of  the  formation.  The  geological  position  of  the  form.ation  is 
free  from  doubt.  It  overlies  and  passes  downward  into  the  Randville  dolomite.  The  formation  does 
not  seem  to  influence  the  topography.  Like  the  preceding  one,  it  has  been  extensively  folded. 
The  average  thickness  is  probably  not  less  than  400  feet.  The  mica-slates  or  phyllites  possess  no 
especial  petrographieal  interest. 

Section  V  treats  of  the  Hemlock  formation.  The  Mansfield  formation  of  the  Miehigamme 
Mountain  area  changes  along  the  strike  into  rocks  of  a  ditiferent  character,  to  which  the  above  name 
is  given.  In  the  Fence  River  area  it  occupies  a  belt  between  2,000  and  3,000  feet  in  width,  between 
the  Randville  dolomite  on  the  west  and  the  Groveland  formation  on  the  east.  The  l>est  exposures 
occur  on  the  sections  made  by  the  Fence  River.  No  folds  have  been  observed  within  this  formation. 
The  thickness  probably  varies  from  0  to  2,300  feet  as  a  maximum.  The  rocks  of  the  formation  are 
chiefly  chloritic  and  ophitic  schists,  with  which  are  associated  schists  bearing  biotite,  ilmenite,  and 
ottrelite ;  greenstone,  conglomerates  or  agglomerates,  .and  amygdaloids.  The  general  characters  of  the 
schists  are  (1)  a  groundmass  composed  of  chlorite,  quartz,  m.agnetite,  epidote,  and  in  some  cases 
plagioclase  microlites,  and  (2)  the  presence  in  this  groundmass  of  much  larger  porphyritic  individuals 
of  several  secondary  minerals.  As  evidence  of  the  origin  of  these  schists,  first,  there  is  the  absence 
of  rocks  possessing  any  sedimentary  characters;  next,  lavas  and  also  greenstone-conglomerates  or 
agglomerates  are  undoubtedly  present  in  the  series ;  furthermore,  the  minerals  which  compose  the 
schist  are  those  which  would  result  from  the  alteration  in  connection  with  dynamic  metamorphism  of 


XXXVI  OUTLINE  OF  THIS  MONOGRAPH. 

igneous  rocks  of  basic  or  intermediate  chemical  composition ;  and  finally,  the  grain  and  character 
of  the  groundmass,  and  in  some  slides  tlie  jireaence  of  plagioclase  microlites  disposed  iu  oval  lines 
point  directly  to  an  igneous  origin  and  to  consolidation  at  the  surface.  The  conclusion  is  reached  that 
the  Hemlock  formation  of  the  Fence  River  area  is  composed  of  a  series  of  old  lava  flows  varying  in 
composition  from  acid  to  basic. 

Section  VI  treats  of  the  Groveland  formation.  This  is  of  wide  extent  throughout  this  part  of 
the  Crystal  Falls  district,  but  its  outcrops  are  limited  to  three  localities.  Its  distribution  has  been 
determined  by  means  of  its  magnetic  properties.  It  is  not  topographically  prominent,  except  in  the 
Michiganime  Mountain  area,  where  it  forms  part  of  Michigamme  Mountain.  In  the  Fence  River  area' 
it  is  probably  not  folded.  It  there  dips  to  the  east.  At  Michigamme  Mountain  it  is  found  iu  several 
well-marked  folds.  The  thickness  of  the  formation  is  estimated  to  be  approximately  500  feet.  The 
rocks  are  iuterbauded  ierruginous  quartzite  and  actiuolite  and  griinerite  schists,  which  still  contain 
evidence  of  detrital  origin. 

Chapter  V  treats  of  the  Northeastern  area  and  the  relations  between  the  Lower  Marquette  and 
the  Lower  Menominee.  The  territory  included  in  the  Northeastern  area  extends  from  the  northern- 
most outcrops  of  the  Fence  River  area  to  the  northern  end  of  the  Republic  trough,  a  distance  of  about 
11  miles.  Outcrops  are  scarce  throughout  this  area,  and  the  main  conclusions  are  drawn  from  the 
magnetic  work.  Through  the  structural  and  lithological  results  of  the  magnetic  work  the  gap 
between  the  Marquette  and  the  Crystal  Falls  district  is  bridged,  and  it  is  shown  with  a  high  degree 
of  probability  that  the  Negaunee  iron  formation  of  the  Marquette  range  is  identical  with  the  Groveland 
iron  formation  of  the  Felch  Mountain  range. 

Chapter  VI  treats  of  the  Sturgeon  River  tongue.  In  the  southeastern  part  of  the  Crystal 
Falls  district  and  just  north  of  the  Felch  Mountain  range  a  tongue  of  fragmental  rocks  extending 
eastward  has  been  studied.  The  extreme  leugth  is  12  or  13  miles.  Its  width  at  its  eastern  end  is  li 
miles;  to  the  west  it  widens  rapidly.  It  is  bounded  both  to  the  north  and  to  the  south  by  Archean 
granites  and  schists ;  to  the  east  it  is  overlain  by  Paleozoic  sandstones  and  limestones ;  and  at  its  west 
end  it  is  covered  by  glacial  deposits.  Within  the  tongue  two  small  granite  islands  occur.  The 
Archean  or  Basement  Complex  rocks  comprise  gneissoid  granites,  hornldeude-schists,  and  biotite- 
schists,  which  are  cut  by  dikes  of  greenstone  and  granite,  and  veins  of  quartz.  The  sedimentary  rocks 
comprise  conglomerates,  arkoscs,  quartzites,  sericite-schists,  clay  slates,  rocks  that  are  probably 
tufaceous,  dolomitic  limestones,  and  calcareous  sandstones  and  slates.  They  may  be  divided  into  a 
conglomerate  series  and  a  dolomite  series.  From  the  distribution  of  the  exposures  of  the  two  series 
it  is  concluded  that  the  conglomerate  series  is  the  older,  and  that  conformably  above  it  follows  the 
dolomite  series.  The  two  form  a  westward-pitching  syncline.  The  conglomerate  series  and  the 
dolomite  series  are  correlated  respectively  with  the  Sturgeon  quartzite  and  the  Randville  dolomite  of 
the  adjoining  Felch  Mountain  range. 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT  OF  MICHIGAN 


Part  I.— THE  ^A^ESTERN    PART  OF  THE  DISTRICT 


MON    XXXVI 1 


CONTENTS. 


Page. 

ChAPTKR  I. — INTKOIIUCTION ; ^^ 

Previous  work  in  the  district 13 

Mode  of  work ^2 

Maj^uetic  observations 24 

Chapteh  II— Gkograpiucal  limits,  structure  and  stratigraphy,  and  physiography 25 

Geograiihical  limits 2o 

Structure  and  stratigraphy 25 

Physiography -" 

Topography 29 

Drainage 31 

Timber  and  soil 36 

Chapter  III.— The  Archean 38 

Distribution,  exposures,  and  topography 38 

Relations  to  overlying  formation,s - 39 

Petrograpliical  characters '10 

Biotite-granite  (grauitite) 10 

Gneissoid  biotite-granite,  border  facies  of  granite 13 

Acid  dikes  iu  Archuan • 15 

Basic  dikes  in  Archeau '1'^ 

Schistose  dikes 1'^ 

Massive  dikes ■1° 


Re8umi5  . 


49 


Chapter  IV. — The  Lower  Huronian  series •'^O 

Section  1.— The  Rand ville  dolomite  50 

Distribution,  exposures,  and  topography 50 

Petrographical  characters - - °1 

Relatious  to  underlying  and  overlying  formations 53 

Thickness 53 

Section  2. — The  Mansfield  slate 54 

Distribution,  exposures,  and  topography s^l 

Possible  continuation  of  the  Mansfield  slate ^>3 

Petrographical  characters 51) 

Graywacke ^'^ 

Clay  slate  and  phyllite 57 

Origin  of  clay  slate  and  phyllite 58 

Present  composition  necessarily  different  from  that  of  rock  from  which  derived 58 

Analysis  of  Mausfield  slate 59 

Comments  on  analysis ''" 

Comparison  of  analysis  of  Mansfield  clay  slate  with  analyses  of  clays 60 

Comparison  of  analysis  of  Mansfield  clay  slate  Tvith  analyses  of  other  clay  slates..  61 

Siderite-slate,  chert,  ferruginous  chert,  and  iron  ores 62 

Relations  of  siderite-slate,  ferruginous  chert,  and  ore  bodies  to  clay  slate 63 


4  CONTENTS, 

Chapter  IV.— The  Lower  Huronian  series— Contiuned. 

Section  2. — The  Mansfield  slate— Continued.  P?ge. 

Relations  of  Manstield  slate  to  adjacent  formations ., 63 

Relations  to  intnisives 63 

Relations  to  volcanics 6l 

Structure  of  the  Mansfield  area 64 

Thickness 64 

Ore  deposits - 65 

General  description  of  Mansfield  mine  deposit 68 

Relations  to  surrounding  beds 68    ' 

Composition  of  ore 68 

Microscopical  character  of  the  ores  and  associated  chert  bauds 69 

Origin  of  the  ore  deposits 70 

Conditions  favorable  for  ore  concentration 72 

Exploration - 73 

Section  3.— The  Hemlock  formation .- 73 

Distribution,  exposures,  and  topography 73 

Thickness 74 

Relations  to  adjacent  formations 75 

Relations  to  intrusives 77 

Volcanic  origin 78 

Classi  fication 79 

Acid  volcanics • 80 

Acid  lavas 80 

Rhyolite-porphyry 81 

Texture 83 

Aporliyolite-porphyry 87 

Schistose  acid  lavas 87 

Acid  py roclastics - ^ 

Basic  volcanics 95 

Basic  lavas "5 

General  characters 95 

Nomenclature 95 

Metabasalts 98 

Nonporphy ritic  metabasalts 98 

Petrographical  characters 98 

Chemical  composition 103 

Porphyritic  metabasalt 103 

Petrographi  cal  characters 104 

Chemical  composition 105 

Variolitic  metabasalts 108 

Ellipsoidal  structure 112 

Origin  of  ellipsoidal  structure 118 

Amygdaloidal  structure 124 

Flattening  of  amygdaloidal  cavities 126 

Alteration  of  the  basalts 126 

Description  of  some  phases  of  alteration 127 

Pyroclastics 135 

Eruptive  breccia 135 

Volcanic  sedimentary  rocks 136 

Coarse  tuffs 137 

Fine  tufi's  or  ash  (dust)  beds 142 

Relations  of  tuffs  and  ash  (dust)  beds 143 

Volcanic  conglomerates 143 

Schistose  pyroclastics WS 


CONTENTS.  5 

ClIArTKK  IV. — TllK    LoWKR   HURONIA.N   SKUiKS — C'outiiimjd. 

Section  3. — The  Hemlock  formation— Continued.  Page. 

Tl)<«  Bono  Lake  crystallini^  scliists 148 

Uistriliution 148 

Field  evidence  of  connection  witli  tlie  voleanics 149 

I'etrogiiipliical  iharacter.s 150 

Normal  8edimentarie.s  of  the  Hemlock  formation  152 

Economic  products 153 

Building  and  ornamental  stones 153 

Road  materials 154 

Chapter  V. — The  Upper  Hukonian  series 155 

Distribution,  e.xposures,  and  topography 155 

M.agnetic  lines 156 

.    Thickness 157 

Folding w 1.58 

Crystal  Falls  syncline 158 

Time  of  folding  of  the  Upper  Huronian 161 

Relations  to  other  series 162 

Relations  to  iutrusives 164 

Correlation 164 

Petrographical  characters 165 

Sedimentary  rocks 165 

Microscopical  description  of  certain  of  the  sedimeutaries 169 

Igneous  rocks I74 

Ore  deposits j75 

History  of  opening  of  the  district I75 

Distribution I75 

Western  half  of  sec.  34,  T.  46  N.,R.  33  W 176 

Sec.  20,  T.  45  N.,  R.  33  W 176 

The  Amasa  area I77 

The  Crystal  Falls  area 178 

Character  of  the  ore 180 

Relations  to  adj  acent  rocks 182 

Origin I83 

Size  of  the  ore  bodies 184 

Methods  of  mining Igj. 

Prospecting I85 

Production  of  ore  from  the  Crystal  Falls  area 186 

Chapter  VI. — The  Intrusives 187 

Order  of  treatment 188 

Age  of  the  intrusives 188 

Relations  of  folding  and  the  distribution  of  the  intrusives 189 

Section  I. — Unrelated  intrusives _ igo 

Classification 190 

Acid  intrusives igO 

Geographical  distribution  and  exposures  of  granites 190 

Biotite-granite 191 

Mieropegmatites 192 

Muscovite-biotite-granite I93 

Relations  of  granites  to  other  intrusives I94 

Dynamic  action  in  granites I94 

Contact  of  granites  and  sedimeutaries I94 

Evidence  of  intrusion I95 


(3  CONTENTS. 

Chapter  VI.— The  Intrusives — Continued. 

Section  I. — Unrelated  intrusives — Continued.  "P^ge. 

Basic  intrusives 198 

Metadolerite 199 

Geographical  distribution 199 

Petrographical  characters 199 

JIacroscopical 199 

Microscopical 200 

Relations  to  adjacent  roclis 203 

Relations  to  Lower  Huroniau  Mansfield  slates 203 

Relations  to  Lower  Huroniau  Hemlock  rolcanics 204 

Relations  to  Upper  Huroniau 20+ 

Relations  to  other  intrusives 204 

Contact  metamorphism  of  Mansfield  slates  by  the  dolerite 204 

.Spilosites  206 

Analyses  of  spilosites 207 

Desmositcs 207 

Adiuoles 208 

Analyses  of  adinoles 208 

Comparison  of  analyses  of  normal  Mansfield  clay  slates  and  the  contact  prod- 
ucts  - 209 

No  endomorphic  effects  of  dolerite  intrusion 211 

Metabasalt 211 

Ultrabasic  intriisives 212 

Picrite-porphyry  (porphyritic  limburgite) 212 

Geographical  distribution  and  exposures 212 

Petrographical  characters 212 

Gray  treniolitized  picrite-porphyry 213 

Dark  serpentinizod  picrite-porphyry 217 

Classification  220 

Section  11. — A  study  of  a  rock  series  ranging  from  rocks  of  intermediate  acidity  through 

those  of  basic  composition  to  ultrabasic  kinds 221 

Diorite , 222 

Nomenclature 222 

Distribution  and  exposures 223 

Petrographical  characters 223 

Description  of  interesting  variations 226 

Sec.  l.^T.  42N.,R.  31W 226 

Across  river  from  Crystal  Falls 227 

Southeast  of  Crystal  Falls 227 

Analysis  of  diorite 231 

Gabbro  and  norite 233 

Petrographical  characters 233 

Description  of  interesting  kinds  of  gabbro -    240 

Hornblende-gabbro  in  sec.  1.5,  T.  42  N.,  R.  31  W 240 

Sees.  15,22,28,  and  29,  T.  42  N.,R.  31  W 241 

Hornblende-gabbro  dikes 243 

Bronzite-norite  dike 244 

Sec.  29,  T.  42  N.,  R.31  W.,  1200  N.,  200  W 245 

Dynamically  altered  gabbro 247 

Relative  ages  of  gabbros 249 

Peridotites 249 

Distribution,  exposures,  and  relations 249 

Petrographical  characters 249 


CONTENTS.  7 

CiiArTKK  \I. — TiiK  iNiiiUsivBS — Coiitinuefl. 

Sootioii  II. — A  study  ot':i  rock  series,  etc. — Continued. 

Peridotite — t'outiuucd.  Paje. 

Periilotite  varieties 252 

Wehrlito 253 

Amiiliiliole-puridotite 233 

Gr.adatious  of  ampbiboK'-peridotite  to  wehrlite  and  olivine-gabbro 254 

Process  of  crystallization 257 

Analysis  of  peridotite 259 

Peridotite  from  sec.  22,  T.  42N.,  R.31  W.,  1990  N.,  150  W 260 

Relations  of  peridotites  to  other  rocks ., 261 

Age  of  peridotites 262 

General  observations  on  the  above  series 262 

Textiiral  characters  of  the  series 262 

Chemicil  composition  of  the  series 263 

Relative  ages  of  rocks  of  the  series 265 


ILLUSTRATIONS 


Page. 
Pl-ATE     I.  Colored  insii)  showing  the  distribution  of  pre-Cambriiin  and  otber  rocks  in  the  Lake 
Superior  region,  and  the  geographical  relations  of  the  Crystal  Falls  district  of 

Michigan  to  the  adjoining  Marquette  and  Meuominee  districts  of  Michigan 11 

II.  Topographical  map  of  the  Crystal  Falls  district  of  Michigan,  includiug  a  portion  of 

the  Marquette  district  of  Michigan In  pocket. 

III.  Geological  map  of  the  Crystal  Falls  district  cf  Michigan,  including  a  portion  of  the 

Marquette  district  of  Michigan In  pocket. 

IV.  Portion  of  a  geological  map  of  the  Menominee  iron  region,  by  T.  B.  Brooks  and  C.  E. 

Wright 18 

V.  Generalized  sections  to  illustrate  the  stratigraphy  and  structure  of  the  northwestern 

part  of  the  Crystal  Falls  district  of  Michigan 28 

VI.  Generalized  sections  to  illustrate  the  stratigraphy  and  structure  of  the  southern  part 

of  the  Crystal  Falls  district  of  Michigan 28 

VII.  Generalized  columnar  section 30 

VIII.  Map  of  a  portion  of  the  Crystal  Falls  district,  showing  in  detail  the  glacial  topog- 
raphy and  illustrating  the  development  of  the  Deer  River 32 

IX.  Sketch  of  the  Mansfield  mine  as  it  was  before  it  caved  in,  in  1893 66 

X.  A,  Reproduction  of  the  weathered  surface  of  a  variolite;   B,  Reproduction  of  the 

polished  surface  of  a  variolite 110 

XI.  Colored  reproduction  of  an  ellipsoid,  with  matrix,  from  an  ellipsoidal  basalt 116 

XII.  Mount  Giorgios,  viewed  from  its  west  flank,  in  April,  1866,  illustrating  the  charac- 
teristic block  lavas,  from  Fouque's  Santorin  et  ses  Eruptions,  PI.  VIII 120 

XIII.  Reproduction  in  colors  of  a  basalt  tuft' 140 

XIV.  Idealized  structural  map  and  detail  geological  map,  with  sections,  to  show  the  dis- 

tribution and  structure  of  the  Huronian  rocks  in  the  vicinity  of  Crystal   Falls, 

Michigan 160 

XV.  Portion  of  Brooks's  PI.  IX,  Vol.  Ill,  Wisconsin  Geological  Survey 172 

XVI.  Detail  geological  map  of  the  vicinity  of  Amasa,  Michigan 176 

XVII.  Detail  geological  map  of  the  vicinity  of  Crystal  Falls  and  Mansfield,  Sheet  1 178 

XVIII.  Detail  geological  map  of  the  vicinity  of  Crystal  Falls  and  Mansfield,  Sheet  II 178 

XIX.  ^,  Inclusions  in  a  fractured  (juartz  phenocryst;  7i,  Quartz  phenocryst  with  rhombo- 

hedral  parting 368 

XX.  ^,  Micropoikilitic  rhyolite-porphyry;  5,  Micropoikilitic  quartz-porphyry 270 

XXI.  ^,  Very  fine-grained  micropoikilitic  ryholite-porphyry  viewed  without  analyzer;  B, 

Very  fine-grained  micropoikilitic  rhyolite-porphyry  viewed  with  analyzer 272 

XXII.  j4,  Perlitic  parting  in  aporhyolite;  £,  Perlitic  iiarting  iu  aporhyolite 274 

XXIII.  J,  Schistose  rhyolite-porphyry ;  B,  Aporhyolite  breccia 276 

XXIV.  J,  Schistose  rhyolite-porphyry;  i?,  The  same  viewed  between  crossed  nicols 278 

XXV.  ^,  Amygdaloidal  texture  of  basalt;  i},  Amygdaloidal  vitreous  basalt 280 

XXVI.  J,  Amygdaloidal  vitreous  basalt;  B,  Amygdaloidal  vitreous  basalt  showing  sheaf-like 

aggregates  of  feldspar 282 

XXVII.  A,  Reproduction  in  colors  of  amygdaloidal  basalt;  B,  Pseudo-amygdaloidal  matrix  of 

ellipsoidal  basalt ;  C,  Water-deposited  pyroclastic 284 

XXVIII.  J,  Fine-grained  basalt  with  well-developed  igneous  texture:  i?,  Illustration  of  the 
obliteration  of  the  igneous  texture  of  a  basalt  by  secondary  products  when  viewed 

between  crossed  nicols 286 

9 


10  ILLUSTKATIONS. 

Page. 
Plate  XXIX.  ^,  Basalt  showiDg  characteristic  texture  in  ordinary  light;  B,  Basalt  showing 

obliteration  of  texture  between  ci'ossed  nicols 288 

XXX.  A,  Basalt  showing  in  ordinary  light  ii  distinctly  amygdaloidal  texture;  B,  The 
same  basalt  with  its  amygdaloidal  texture  obliterated  when  viewed  between 

crossed  nicols 290 

XXXI.  A,  Basalt  aft'ected  by  calcification  process;  B,  Basalt  affected  by  calcification 

process  viewed  between  crossed  nicols 292 

XXXII.  A,  Illustration  of  perlitic  parting  in  a  fragment  from  a  basaltic  tuff;  B,  Sickle- 
shaped  bodies  in  volcanic  tuft' 294 

XXXIII.  ./,  Water-deposited  sand ;  B,  Gradation  in  water-deposited  volcanic  sediment . ..       296 

XXXIV.  .1,  Contact  product  of  granite;  /?,  Brecciated  matrix  between  ellipsoids 298 

XXXV.  A,  Contact   between   granite  and   a  metamorphosed  sedimentary;  B,  Contact 

between  granite  and  a  metamorphosed  sedimentary  viewed  between  crossed 

nicols 300 

XXXVI.  A,  A  variety  of  spilosite  with  white  spots;  B,  A  variety  of  spilosite  with  white 

spots  viewed  between  crossed  nicols 302 

XXXVII.  A,  Normal  spilosite  or  spotted  contact  product;  i?,  Normal  spilosite  of  some- 
what dift'erent  character 304 

XXXVTII.  .1,  Passage  of  spilosite  into  demosite;  B,  Occurrence  and  alteration  of  bronzite 

in  bronzite-norite 306 

XXXIX.  A,  Biotite-granite  viewed  between  crossed  nicols ;  B,  Mica-diorite  viewed  between 

crossed  nicols 308 

XL.  A,    Quartz-niioa-iliorite-porphyry ;  B,  Quartz-mica-diorite-porphyry  viewed  be- 
tween crossed  nicols 310 

XLI.  J,  Porphyritic  poikilitic  hornblende  gabbro;  /?,  Poikilitic  hornblende  gabbro.. .       312 
XLII.   .-(,  Moderately  fine-grained  hornblende  gabbro showing  parallel  texture;  B,  Mod- 
erately fine-grained    hornblende    gabbro    showing   parallel    texture   viewed 

between  crossed  nicols. ..'- 314 

XLIII.  J,  Normal  granular  hornblende  gabbro;  B,  Schistose  hornblende  gabbro  viewed 

between  crossed  nicols 316 

XLIV.  ,1,  Moderately  fine-grained  hornblende  gabbro;  i?,  bronzite-norite 318 

XLV.  J,  Brouzite-norite-porpbyry ;  /?,  Feldspathic  wehrlite 320 

XLVI.  J,  Feldspathic  wehrlite  viewed  between  crossed  nicols;  /i,  Feldspathic  wehrlite. .       322 

Fig.  1.  Reproduction  of  a  portion  of  the  geological  map  of  the  Upper  Peninsula  of  Michigan, 

by  William  A.  Burt,  1846 , 15 

2.  Enlarged  reproduction  of  a  portion  of  a  map  of  the  Lake  Superior  land  district,  by 

Foster  .nnd  Whi  tney 17 

3.  Enlarged  reproduction  of  a  portion  of  a  geological  map  of  the  Upper  Peninsjila  of  Michi- 

gan, by  K'ominger,  Brooks,  and  Pumpelly,  1873 18 

4.  Grauite-porpbyry  with  inclusions  of  gneissoid  granite 45 

5.  Illustration  of  the  effect  on  the  topograjjhy  of  the  differential  erosion  of  basic  dikes  and 

granite 46 

6.  Concentric  cracks  formed  by  the  caving  in  of  the  Mansfield  mine 65 

7.  Sketch  of  the  surface  of  the  outcrop  of  an  ellipsoidal  basalt,  showing  the  general  char- 

acter of  the  ellipsoids  and  matrix 112 

8.  Sketch  showing  tlie  concentration  of  the  amygdaloidal  cavities  on  one  side  of  an  ellip- 

soid, this  side  probablj' representing  the  side  nearest  the  surface  of  the  flow 113 

9.  Ellipsoids  with  sets  of  parallel  lines  cutting  each  other  at  an  angle 114 

10.  Reproduction  of  illustration  of  aa  lava,  after  Dana 120 

11.  Profile  section  illustrating  results  of  diamond-drill  work 177 

12.  Sketch  illustrating  contortion  of  Upper  Huronian  strata 179 

13.  Sketch  showing  change  of  strike  of  Upper  Huronian  beds,  due  to  the  folds 179 

14.  Sketch  to  illustrate  the  occurrence  of  ore  bodies 182 


us  GEOLOGICAL  SURVEY 


MONOGRAPH    XXXVI  PL 


GEOLOGIC  MAP  OF  FART  Ol'^  THl.;  LAKE  HUI'KRIOH  REGKW 

Compilftd  from  Official  maps  of  U.  S.MiniiPsoUi.aiid  Caiiadian  Sui^'eys 
Scah.- 


Ah*  The  Afrnamuirr  IrotvBrarirtfiSent^ 

.     Ah5  Thf  n'usconj>ui  f'niliry  Stntes 

Hi    KONlA^   ■,     Ah6  T/tf  fmokeelran-Bttifinff  Series 

Ah  7  Hi/-  St  Louis  Sfrifj 

Ahe  The  Chippewa  VitUey  QuartMitei: 

Ah  9  The  Blaek  River  TrowBearin^  .Srfiwfj 

Ah  10  The  Aiumikie  Series 

Ah  1 1  TTie  Fhlded  SrJUata  afOinadfU 


urn  STAT  Ml 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT 

OF  MICHIGAN. 


PART  1.  THE  WESTERN  PART  OF  THE  DISTRICT. 


By  J.  Morgan  Clements. 


CHAPTER    I. 

INTRODUCTION. 

The  present  report  is  an  account  of  a  portion  of  the  Crystal  Falls  dis- 
trict of  Michigan,  so  called  from  the  most  important  town,  Crystal  Falls, 
the  comity  seat  of  Iron  County.  The  iron-bearing  district  along  the  Paint 
River,  near  the  site  of  the  town  of  Crystal  Falls,  was  first  called  in  literature 
the  Paint  River  district  by  Brooks.^  As  soon  as  the  town  was  begun,  about 
1880,  the  name  of  the  town  was  applied  to  the  district.^  It  is  situated 
on  the  Upper  Peninsula  of  Michigan,  adjoining  the  northeastern  border  of 
Wisconsin,  and  serves  as  a  link  connecting  the  two  well-known  iron-ore- 
producing  districts  of  Michigan,  the  Marquette,  and  the  Menominee.  The 
Crystal  Falls  district  is  of  itself  of  considerable  economic  importance,  as 
will  be  seen,  though  not  deserving  to  be  ranked  with  either  of  the  two 
above-mentioned  iron  districts.     Since  the  geological  relations  of  the  rocks 

'  The  irou-bearing  rocks  (economic),  by  T.  B.  Brooks:  Geol.  Survey  of  Michigan,  Vol.1,  Part  I, 
1873,  p.  182. 

"Rept.  Com.  Miu.  Statistics  Mich,  for  1881,  p.  222. 

11 


12  THE  CRYSTAL  FALLS  IRON- BEARING  DISTRICT. 

of  the  Marquette  district  have  now  been  ascertained/  it  is  hoped,  by  means 
of  the  determination  of  the  succession  in  the  intermediate  Crystal  Falls 
district,  that  the  Menominee  rocks  may  be  closely  correlated  with  those  of 
the  Marquette  district. 

The  accurate  delimitation  of  the  iron-bearing  or  coal-bearing  forma- 
tions, or  any  other  formations  containing  valuable  mineral  products,  is  of 
inestimable  value  to  miners  and  investors.  In  the  iron  districts  of  Michigan 
alone  innumerable  test  pits  have  been  sunk  in  areas  of  solid  granite,  and  at 
great  distances  outside  of  the  possible  iron  formations,  thus  wasting  large 
sums  of  money.  Although  the  investigations  carried  on  in  the  Crystal  Falls 
district,  the  results  of  which  are  here  recorded,  do  not  enable  us  to  point  out 
definitely  the  places  where  the  prospector  will  find  iron  deposits,  they  have 
enabled  us  to  delimit  in  a  broad  way  the  various  formations,  and  warrant 
the  statement  that  iron  deposits  may  occur  in  certain  areas  and  that  the 
prospector  will  assuredly  not  find  iron  dejjosits  in  certain  others. 

The  opportunity  of  studying  the  Crystal  Falls  district  was  given  me 
tlu-ough  Prof.  C.  R.  Van  Hise.  In  the  prosecution  of  the  field  studies  and 
in  the  preparation  of  the  report  I  have  availed  myself  of  his  advice  and 
suggestions,  which  have  been  generously  ofi"ered  and  which  have  been 
found  of  greatest  value.     To  him  I  am  most  deeply  indebted. 

The  report  is  based  not  only  on  my  own  field  work,  but  also  on  the 
field  work  done  by  a  number  of  other  geologists,  whose  notebooks  have 
been  placed  at  my  disposal.  The  names  of  these  geologists  may  be  found  on 
page  22.  Among  them,  the  notes  of  Mr.  W.  N.  Merriam  and  Dr.  W.  S. 
Bayley  have  been  found  especially  valuable.  Mr.  Merriam,  assisted  by  Dr. 
Bayley,  spent  a  season  in  doing  very  detailed  work  on  the  area  shown  on  the 
sketch  map  at  the  bottom  of  PI.  Ill,  between  Crystal  Falls  and  Mansfield, 
and  from  this  point  northwest  to  some  distance  beyond  Amasa.  The  mag- 
netic lines  represented  in  this  part  were  traced  by  Mr.  Merriam,  and  the 
geology  in  general  is  the  same  that  he  outlined  on  his  final  field  map 

I  wish  to  thank  Mr.  C.  K.  Leith,  who  has  been  of  the  greatest  clerical 
assistance,  and  Mr.  E.  C.  Bebb,  by  whom  the  maps  were  drawn;  also  Mr. 


•  The  Marquette  iron-bearing  district  of  Michigan  (preliminary),  by  C.  R.  Van  Hise  and  W.  S. 
Bayley;  with  a  chapter  on  the  Republic  Trough,  by  H.  L.  Smyth:  Fifteenth  Ann.  Rept.  U.  S.  Geol. 
Survey,  1895,  pp.  477-650.     Ibid,  (final),  Mon.  U.  S.  Geol.  Survey,  Vol.  XXVllI,  1897. 


PKEVIOUS  WORK.  13 

J.  L.  Ividg-wiiy,  by  wlioin  tlie  colored  plates  of  natural  size  specimens  w(;re 
j)repure(l. 

PREVIOUS  WORK  IN  THE  DISTRICT. 

( )u  account  of  its  comparatively  slight  economic  importance,  and  also 
on  account  of  its  isolation,  very  little  work  of  whicli  the  results  have  been 
published  was  done  in  this  district  prior  to  that  on  which  this  monograph  is 
based.  As  a  rule,  the  earlier  observers  began  the  season's  work  either  in 
the  Marquette  or  in  the  Menominee  range,  and  working  westward  the 
Crystal  Falls  district  was  reached  only  as  the  season  neared  its  close,  or  as 
the  appropriation  was  nearly  exhausted.  The  published  work  upon  this 
district  is  given  below  in  chronological  order. 

X850. 

Burt,  Wm.  A.  Report  of  linear  surveys  with  reference  to  mines  and  minerals, 
in  the  Northern  Peninsula  of  Michigan  in  the  years  1845  and  184C.  Dated  March  20, 
1847.  Thirty-tirst  Congress,  first  session,  1850;  Senate  documents,  Vol.  Ill,  No.  1, 
pp.  842-882,  with  map. 

Dui'ing  the  year  1846  a  linear  survey  was  made  of  that  part  of  the 
Upper  Peninsula  of  Michigan  described  as  being  bounded  on  the  north  by 
the  fifth  correction  line,  on  the  south  by  the  fourth  correction  line  and  the 
Brule  River,  on  the  east  by  ranges  23  and  26  W.,  and  on  the  west  l)y 
range  37  W.  This  includes  in  its  limits  the  district  under  discussion.  In 
the  course  of  the  survey,  geological  observations  were  made  by  William  A. 
Burt,  the  deputy  surveyor  in  charge  of  the  work.  The  report  and  accom- 
panying geological  map  embodying  the  results  of  these  observations  are 
concealed  among  the  Senate  documents  of  the  Thirty-first  Congress.  The 
following  quotations  from  this  report  give  all  the  observations  on  the  part 
of  the  territory  surveyed  in  which  we  are  at  jjresent  interested: 

Topography. — Wcst  of  range  31  west,  and  north  of  the  Brule  River  to  the  fifth 
correction  line,  is  a  tract  of  about  43  townships  in  which  the  rock  is  mostly  greenstone 
and  hornblende  slates.  This  part  of  the  surveyed  district  is  less  broken  than  that 
above  described,  and  a  large  proportion  of  it  may  be  denominated  rolling  lauds. 
There  are,  however,  many  ridges  and  conical  hills  of  various  heights  upon  this  part  of 
the  survey,  and  also  deep  valleys  of  streams,  many  of  which  have  ledges  upon  their 
sides.  These  general  characteristics  are  often  changed  for  cedar,  spruce,  or  tamarack 
swamps,  which  are  most  numerous  in  townships  46,  47,  and  48  N.  [This  includes  the 
part  of  the  district  supposed  to  be  the  continuation  of  the  northern  Wisconsin 
peneplain  (p.  31).] 


14  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

Granite  (and  syenite). — These  Tocks  occupy  au  area  of  about  22  townships  on  tlie 
northeast  part  of  the  survey,  between  the  fifth  correction  line  and  the  south  boundary 
of  township  45  N.,  and  east  of  range  32  W.,  in  a  series  of  irregular  uplifts,  frequently 
forming  high  cliffs  and  sloping  ledges  on  the  most  elevated  portion  of  this  district. 
[This  covers  a  part  of  the  Archeau  granite  oval  of  the  Crystal  Falls  district,  as  well 
as  the  large  Archeau  areas  northeast  of  it.) 

Argillaceous  slates. — The  argillaccous  slates  alluded  to  in  townships  42  and  43  N.  are 
generally  overlaid  by  deep  drift;  their  boundaries,  therefore,  could  not  be  satisfactorily 
defined.  West  of  the  Peshaknmine  River  these  slates  appeared  to  have  undergone 
considerable  chauge  by  igneous  action,  aud  were  often  associated  with  an  oxide  of 
iron;  but  east  of  the  Peshakumine  no  change  by  igneous  action  in  the  slates  was 
observed,  and  on  this  part  they  have  generally  a  reddish  color 

They  dip  variously  at  a  high  angle,  and  are  supposed  to  conform  to  the  greenstone 
on  the  north  and  west,  and  to  overlie  or  pass  into  the  mica-slates  on  the  south ;  and 
in  their  middle  portion  they  di|)  about  90°,  with  strike  nearly  east  and  west.  (These 
slates  correspond  to  our  least  metamorphosed  phases  of  the  Upper  Huronian.] 

Greenstone  and  hornblende  slate. — Thcsc  rocks  occupy  a  larger  area  lu  the  district 
surveyed  than  any  other  class  of  rocks.  They  extend  from  the  granitic  and  other 
rocks  east  of  them  westward  beyond  the  survey.  [See  their  outline  on  map, 
tig.  1.] 

The  greenstone  and  hornblende  slates  form  a  less  broken  surface  than  the 
granitic  range;  and  next  to  it  is  the  most  elevated  range  in  this  district,  having  an 
estimated  altitude,  in  many  places,  of  from  1,001)  to  1,100  feet  above  Lake  Superior. 

These  rocks  are  frequently  seen  in  the  beds  and  bauks  of  streams  aud  in  ridges 
aud  conical  hills  of  various  heights,  often  forming  precipitous  ledges  upon  their 
sides 

The  greenstone  of  this  region  is  generally  more  or  less  granular  and  syenitic, 
with  a  dark  green  color  when  moist;  its  composition  is  hornblende,  feldspar,  and 
quartz — the  former  mineral  greatly  predominating.  In  some  places  the  feldspar  and 
quartz  are  nearly  or  quite  wanting,  leaving  a  granulated  hornblende  rock.  Another 
variety  of  this  rock  was  freciuently  seen  which  was  composed  of  the  same  ingredients 
but  very  tine  grained  and  compact  and  having  frequently  a  laminated  or  slaty 
structure,  the  cleavages  of  which  generally  dip  from  the  granitic  rocks  at  a  very  high 
angle 

Some  of  these  hornblende  slates  have  in  their  seams  and  cleavages  a  silky  luster, 
from  the  presence  of  mica  or  talc  in  very  fine  grains 

All  of  these  rocks  are  traversed  by  many  quartz  veins,  from  a  line  to  4  feet  or 
more  in  width,  and  with  still  larger  veins  and  dikes  of  more  recent  trap  rock.  This 
range  is  supposed  to  have  become  blended  with  the  trap  range  of  Keweenaw  point  as 
it  passes  under  the  red  sandstone  lying  between  them,  and  probably  farther  west  the 
two  are  united  in  one  range.  [These  are  the  altered  and  more  or  less  schistose  basalts 
and  accompanying-  fragmeiitals  which  are  comprised  in  the  Hemlock  formation.  [ 

Mica-siates. — Thcsc  slatcs  strctch  along  the  southerly  side  of  the  argillaceous  slates 
on  the  south  part  of  the  survey.  They  extend  from  the  Brule  River  on  a  course  east- 
uortheast  for  about  22  miles,  in  townshij)s  41  and  42  N.,  ranges  29, 31,  and  32  VV..  aud 
have  an  average  breadth  of  about  4  miles 

The  mica-slates  are  supposed  to  dip  northerly  under  the  argillaceous  slates  at  a 
high  angle,  varying  at  the  surface  from  45°  to  80° 


PREVIOUS  WORK. 


15 


This  rock  is  conii)<)se(l  of  iniwi,  ((uartz,  and  feldspar.  Its  lainiita'  are  uudiilatiiig 
or  waved,  but  its  cleavages,  on  a  large  scale,  are  even  and  regular 

Those  luica-slates  are  best  (levelo])ed  on  the  south  bouiidaiy  of  township  4L!  N., 
ranges  31  and  32  W.,  in  the  beds  and  banks  of  the  Peshakunime  and  Mesqua-cum-a- 


XXX  III 


XXXII 


XXXI 


XXX 


XXIX 


x'Xvul 


Scale  of  miles 

5 


Fig.  1. — Reproduction  of  a  portion  of  tbe  geological  map  of  the  Upper  Peninsnla  of  Michigan  hy  Wnj.  A.  Burt,  1846. 

cum-sepe,  and  at  the  falls  near  the  junction  of  the  latter  stream  with  the  Brula  River. 
[These,  according  to  our  observations,  are  the  most  altered  phases  of  the  Upper 
Huronian.] 

That  part  of  the  geological  map  accompanying  the  above  report  which 
corresponds  to  the  Crystal  Falls  district  is  reproduced  iu  fig.  1. 


16  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 


1851. 


Foster,  J.  W.,  and  Whitney,  J.  D.  Report  on  the  geology  of  the  Lake  Supe- 
rior land  district.  Part  II.  The  iron  region,  togetlier  with  the  general  geology. 
Thirty-second  Congress,  special  session,  1851;  Senate  documents,  Vol.  Ill,  No.  4, 
pp.  406,  with  maps  and  section. 

In  1851  there  was  published  a  report  by  Foster  and  Whitney  on  the 
iron  regions  of  the  Lake  Superior  land  district,  together  with  the  general 
geology.  This  gives  the  first  connected  account  of  the  results  obtained  by 
the  various  surveyors  who  had  been  engaged  on  the  Government  survey  of 
the  Upper  Peninsula  of  Michigan.  Accompanying  this  report  there  are 
two  colored  maps  and  a  section.  The  subdivisions  of  the  rocks  as  made  by 
Burt  in  the  Crystal  Falls  district  are  not  retained  in  this  report  by  Foster 
and  Whitney.  The  map  is  generalized,  and  the  hornblende-slates,  etc.,  of 
Burt  are  included  under  the  general  term  "crystalline  schists,"  and  are 
placed  by  the  authors  in  the  Azoic  system.  There  are  represented  here  and 
there  throughout  this  Azoic  area  a  trajjpean  knob  and  bed  of  marble. 

Tlie  granite  area  shown  on  Burt's  map  is  very  much  reduced  in  size, 
and  no  longer  connected  with  the  large  granite  areas  to  the  east.  The 
granite  on  the  lower  reaches  of  the  Michigannne,  in  T,  42  N.,  R.  31  W.,  is 
here  indicated  for  the  first  time.  In  these  respects  only  does  this  portion 
of  the  map  show  a  decided  advance  in  knowledge  of  the  distribution  of  the 
rocks.  A  copy  of  the  map,  showing  the  distribution  of  the  rocks  by 
symbols  instead  of  colors,  is  reproditced  as  fig.  2. 

1873. 

Brooks,  T.  B.  The  iron-bearing  rocks  (economic).  Geol.  Survey  of  Michigan, 
Vol.  I,  Part  I,  1873,  pp.  319.  With  Atlas  Plate  IV  and  general  map,  by  Romiuger, 
Brooks,  and  Pumpelly. 

The  next  mention  of  the  district  that  I  have  been  able  to  find  was 
made  in  1873  by  Maj.  T.  B.  Brooks,  in  his  report  on  the  iron-bearing  rocks 
of  Michigan. 

However,  this  report  seems  to  show  a  decided  decrease  in  knowledge 
from  that  possessed  by  Burt  concerning  the  geology  of  this  district.  It  is 
trae  that  indications  of  iron  had  been  seen,  but  the  observations  made  were 
so  meager  that  nothing  could  be  done  toward  determining  the  relations  of 
the  rocks  or  unraveling  the  structure  of  the  area. 

Upon  the  map  accompanying  the  report  (Geol.  Survey  of  Micliigan, 


PKEVIOUS  WORK. 


17 


1S73),  a  {jortion   of  wliicli  is  reproduced   iu   ti<i-.   3,    Brooks    lias  failed  to 
outline  the  granitic  areas  kuowu  to  the  previous  explorers.     Except  iu  a 


Scale  ormiles 


IS 

=1 


Fig.  3. — Enlarged  reproduction  of  a  portion  of  a  map  of  the  Lake  Superior  laud  district,  by  Foster  and  Whitney. 
G;=graDite;  T^trappean  rocks;  cT^ii'on;  ;^|P|  =  beds  of  marble. 


Metamorphic  formation. 
Azoic  system. 


Igneo  18  formation. 

(Associated  with  the  Azoic.) 
Trappean  rocks.     Granite. 


Quartz.     Crystalline  schists. 

few  places,  which  have  been  left  tincolored,  the  district  is  covered  with  the 
■color  representing  the  Huroniau. 


MON   XXXVI- 


18  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

Brooks  refers  the  ore-beariiig  rocks  to  the  Huroiiian  in  the  following 


.  wor 


■ds: 


Too  little  is  known  about  the  reoiote  Paint  River  district,  in  townships  42  and 
43,  ranges  32  and  33,  to  enable  me  to  give  anything  of  interest  regarding  its  geological 


I — 


Scale  of  miles 


Fig.  3. — Enlarged  reproductiou  of  portion  of  a  geological  map  of  the  Upper  Peninsula  of  Michigan,  by  C.  Kominger, 

T.  B.  Brooks,  and  R  Pumpelly,  1873. 


structure.  The  Iluronian  rocks  are  extensively  developed  there,  and  contain  deposits 
of  hard  hematite  ore.  I  had  the  opportunity  to  examine  only  two  localities  at  the 
Paint  River  Falls,  sec.  20,  T.  43,  R.  32,  and  sec.  13,  T.  42,  R.  33,  (p.  182). 


N2-t'l 


rO 


NZ-t'X 


o 
t 
"5  " 

I; 
I 


a. 

^  k  ^ 


Q 

*  !i  «  t 

5  o  ~  0: 

O  i  Q  I, 

Uj  Q  w  % 

10  Q  ■»  o 

k  °  fe  o 

o  t  '^  o 

m  U]  w  i 

S  S  5  5 

(t  t  ft  a 

K.  K  K  uj 

V)  "O  </l  10 


rr'tc 
\  o 


o: 
■^5"?  I- 


in 

H 
<  I/) 

Si 
2  O 

in 


\  Z  ^ 


o 


-  >  O  <- 


t  "> 

E  i 

qo 

-i  in 


PREVIOUS  WORK.  19 

II*'  also  <;-ives  liis  analysis  (No.  68,  p.  302  ot"  Brooks's  report)  of  au  ore 
sample  t'roin  the  district,  and  calls  attention  to  its  abnormally  high  water 
content,  freedom  from  silica,  and  richness  in  iron  as  compared  to  those  of 
the  more  eastern  mines  in  tlie  Menominee  region. 

1880. 

Brooks,  T.  B.  Geology  of  the  Menominee  region.  Geol.  Survey  of  Wisconsin. 
Vol.  Ill,  Part  V,  1880,  pp.  430-0.35.  Atlas  folio.  Pis.  XXVIII  ami  XXIX,  and  PI. 
XXX,  by  C.  E.  Wright. 

In  an  article  on  the  geology  of  the  Menominee  region,  which  wa.s 
written  for  the  geological  survey  of  Wisconsin,  the  same  author  brief!}' 
touched  on  that  part  of  the  adjoining  Michigan  territory  which  is  included 
in  the  district  under  consideration.  His  observations  were  thus  confined  to 
a  few  exposures  in  a  limited  portion  of  the  area.  He  attempts  to  correlate 
certain  beds  by  means  of  their  lithological  character  with  those  with  which 
he  was  familiar  in  the  Marquette  district  and  refers  them  uniformly  to  the 
higher  members  of  the  Huronian. 

They  are  also  so  referred  on  the  map  which  accompanies  the  report, 
thougli  this  is  dated  a  year  earlier  than  the  date  of  publication  of  the  report. 
That  portion  of  the  map  covering  a  small  part  of  the  Crystal  Falls  region 
is  reproduced  on  PL  IV.  The  present  survey  enables  us  to  add  very  little 
Jo  this,  aud  these  additions  are  chiefly  of  a  petrological  character. 

.      1881. 

RoMiNGER,  Carl.  Geology  of  the  Menominee  iron  region.  Geol.  Survey  of 
Michigan,  Vol.  IV,  1881. 

In  1880  Dr.  Carl  Rominger,  at  that  time  State  geologist  of  Michigan, 

spent  a  season  in  the  Menominee  district,  and  in  his  report  gives  detailed 

descriptions  of  a  few  occurrences  in  the  Crystal  Falls  district,  to  which  I 

shall  refer  later  on.      He  considers  the  rocks  in  general  to  belong  to  the 

Huronian,  and  distributes  the  beds  among  his  diorite  group,  iron-ore  group, 

and  arenaceous-slate  group,  as  given  and  defined  in  the  previous  report  on 

the  Marquette  district      No  attempt  at  more  definite  coiTelation  was  made. 

isso. 

Van  Hise,  G.  R.  An  attempt  to  harmonize  some  apparently  coutlicUng  views 
of  Lake  Superior  stratigraphy.     Am.  Jour,  of  Sci.,  3d  series,  vol.  41, 1891,  p.  133. 

On  December  30,  1890,  Prof  C.  R.  Van  Hise  read  a  paper  on  Lake 
Superior  stratigraphy  before  the  Wisconsin  Academy  of  Sciences,  Arts,  and 


20  THE  CRYSTAL  FALLS  IROX-BEARING  DISTRICT. 

Letters,  the  same  article  being  published  the  following  year  in  the  American 
Journal.  The  iron-bearing  series  of  this  district  was  in  this  article  referred 
to  the  Upper  Marquette  (Upiser  Huronian). 

1893. 

Wright,  C.  E.  Report  of  State  geologist  from  May  1,  1885,  to  June  1,  1888,  hi 
Rapt,  of  the  State  Board  of  Geol.  Survey  of  Michigau,  1893,  pp.  33-44. 

State  geologist,  Charles  E.  Wright,  in  a  report  for  the  seasons  from  1885 

to  1888,  inclusive,  merely  mentions  the  general  strike  of  tlie  rocks  of  the 

district,  and  makes  no  attempt  to  determine  their  age  nor  to  unravel  the 

structure. 

Wadsworth,  M.  E.  Sketch  of  the  geology  of  the  iron,  gold,  and  copper  dis- 
tricts of  Michigau.  lu  Rept.  of  State  Board  of  Geol.  Survey  for  years  1891-92,  1893, 
pp.  75-186. 

Dr.  M.  E.  Wadsworth,  who  on  Mr.  Wright's  dea,th  succeeded  him  as  State 
sreoloffist  of  Michig-an,  mentions  the  occurrence  of  carbonaceous  slates,  of 
granite  and  melaphyre,  and  of  conglomerate  near  Crystal  Falls,  but  does  not 
enter  into  a  discussion  of  the  relations  of  anv  of  these  rocks.  Dr.  Wads- 
worth agrees  with  the  correlation  of  Professor  Van  Hise,  and  places  the 
Crystal  Falls  ore  deposits  in  the  Upper  Marquette  series  (Wadsworth's 
Holyoke  formation)  (pp.  117,  132),  but  the  evidence  for  so  doing  is  not 
given  in  the  report.  He  also  is  the  first  to  recognize  the  volcanic  nature  of 
the  rocks  in  the  vicinity  of  Crystal  Falls  (p.  134). 

lS9o. 

RoMiNGER,  C.  Geol.  rept.  on  the  Upper  Peniusula  of  Michigan.  Geol.  Survey 
of  .Michigan,  Vol.  V,  Part.  1, 1895,  pp.  1-164. 

In  liis  report  of  work  done  on  the  Upper  Peninsula  of  Michigan  from 

1881  to  1884,  published  in  1895,  he  follows  the  same  plan,  referring  the 

various  rocks  exposed  h\  mining  operations  to  his  different  groups.^ 

Clements,  J.  Morgan.  The  volcanics  of  the  Michigamme  district  of  Michigan. 
Jour,  of  Geol.,  Vol.  Ill,  1895,  pp.  801-822. ^ 

In  a  preliminary  article  on  this  district,  by  the  writer,  published  in 
1895,  the  volcanic  character  of  the  rocks  which  cover  a  large  area  of  the 
Crystal  Falls  district  was  emphasized,  and  in  a  sketch  maj)  in  the  same 

'  Geol.  Survey  of  Micbigan,  Vol.  IV,  1881,  p.  8. 

'After  the  publication  of  this  article,  the  name  Michigamme  haviug  been  applied  to  a  formation, 
it  was  deemed  advisable,  in  order  to  avoid  confiisiou,  to  change  the  name  of  the  district  to  the  Crystal 
Falls  district. 


PREVIOUS  WORK.  21 

article  was  oiveii  an  outline  of  the  distribution  of  the  various  rocks  for  a 
portion  of  the  ilistrict,  with  their  stratig-ra[)hical  succession  (p.  803),  the  dis- 
cussion of  the  structure  and  correlation  l)eino-  left  for  the  present  report. 

The  above-mentioned  sketch  map,  with  the  maps  by  Burt,  Foster  and 
Whitney,  Brooks,  Brooks  and  Wright,  the  section  by  Foster  and  Whitney, 
and  the  section  by  Brooks,  along  the  Paint  and  Michigannne  rivers,  are  the 
oidy  maps  or  sections  which,  so  far  as  can  be  learned,  have  been  published 
of  that  part  of  the  Crystal  Falls  district  under  discussion. 

MISCELLANEOUS  REFERENCES. 

JuLiEN,  Alexis  A.  Apiit-ndi.x  A.  Lithology.  Geol.  of  Michigan,  Vol.  II,  187.3, 
PI).  1-185. 

WiCHMANN,  Arthur.  Microscopical  observations  on  the  iron-bearing  rocks 
from  the  region  south  of  Lake  Superior.  Brooks's  Geol.  of  the  Menominee  Iron 
Region,  1880,  Chap.  V,  pp.  600-65."). 

Wright,  Charles  B.  Geology  of  Menominee  Iron  Region.  Geol.  of  Wis- 
consin, Vol.  Ill,  Part  8,  1880,  pp.  665-741. 

Lane,  A.  O.  In  sketch  of  the  geology  of  tlie  iron,  gold,  and  copper  dejwsits  of 
Michigan.     Rept.  of  State  Board  of  Geol.  Survey  for  1891-92,  1893,  p.  182. 

Patton,  H.  S.  Microscopic  study  of  some  Michigan  rocks.  Uept.  of  State 
Board  Geol.  Survey  for  1891-92,  189.3,  p.  186. 

During  the  progress  of  the  Michigan  and  Wisconsin  State  surveys 
specimens  from  outcrops  were  collected,  and  descriptions  of  these  discon- 
nected specimens  are  found  in  the  State  reports. 

References  to  the  pages  on  which  the  individual  descriptions  may  be 
found  will  be  given  under  the  petrographical  discussion  of  similar  rocks 
here  described. 

UNPUBLISHED    WORK. 

In  1891  a  survey  was  organized  by  a  private  corporation,  and  put  in 
charge  of  Prof  C.  R.  Van  Hise.  He  consented  to  take  charge  of  this  work 
on  the  conditions  that  all  maps  and  notes  should  be  available  for  this  report 
and  that  no  other  compensation  was  to  be  made  by  the  company.  The 
object  of  this  survey,  known  as  the  Lake  Superior  survey,  was  to  study  that 
part  of  Michigan  of  which  Crystal  Falls  is  the  center,  in  order  to  determine 
the  feasibility  and  advisability  of  opening  up  the  mines  of  that  district. 
This  survey  was  vigorously  prosecuted,  and  an  excellent  topographic  map 
made  of  an  area  32  miles  north  and  south  and  42  miles  east  and  west,  cover- 
ing a  large  part  of  four  1.5-minute  atlas  sheets  of  the  United  States  Geological 


22  THE  CRYSTAL  FALLS  IKON-BEAKING  DISTRICT. 

Survey.  At  the  same  time,  in  connection  witli  the  topographic  work,  a 
reconnaissance  geological  survey  was  made. 

The  following  is  a  list  of  those  who  took  geological  notes  for  this 
survey:  Andrews  Allen,  A.  H.  Brooks,  W.  S.  Bayley,  J.  P.  Channing,  E.  T. 
Eriksen,  J.  R.  Finlay,  F.  J.  Harriman,  F.  T.  Kelly,  E.  B.  Matthews,  E.  R. 
Maurer,  J.  A.  McKim,  F.  W.  McNair,  W.  N.  Merriam,  and  H.  F.  Phillips.  ' 

The  following  season  was  devoted  to  a  detail  study  of  the  iron-bearing 
belts  which  had  been  outlined  by  the  reconnaissance.  This  detail  work  in 
the  western  part  of  the  district  was  prosecuted  by  parties  in  charge  of  W.  N. 
Merriam,  and  in  the  eastern  part  of  the  district  by  parties  in  charge  of 
H.  L.  Smyth.  When  they  ceased  work,  the  two  areas  mapped  were  sepa- 
rated in  the  north  b}'  about  12  miles,  and  a  narrow  belt  separated  the 
mapped  areas  to  the  south.  During  the  season  of  1894,  under  the  direc- 
tion of  Profes.sor  Van  Hise  and  assisted  by  Gr.  E.  Culver,  and  during  part 
of  the  season  by  S.  Weidman,  I  was  engaged  iu  completing  this  unfinished 
work  for  the  United  States  Geological  Survey,  preparatory  to  connecting 
this  district  with  the  Menominee  iron-bearing  district  to  the  southeast.  This 
work  was  carried  on  in  1895  by  Dr.  W.  S.  Bayley,  S.  Weidman,  and  myself, 
and  the  mapping  of  the  district  extended  as  far  as  the  Menominee  district. 

Mr.  H.  L.  Smyth  has  written  Part  II  of  the  present  report,  covering 
the  portion  of  the  district  which  was  worked  by  his  party.  My  description 
of  the  part  of  the  district  worked  by  me  is  based  largely  on  my  own  obser- 
vations. Many  of  the  facts  of  field  occurrence,  however,  mentioned  in  the 
following  paper  were  observed  and  recorded  by  the  several  men  mentioned 
above,  and  were  subsequent!)'  verified  by  my  own  observations  in  portions 
of  the  area  surveyed  by  myself,  and  by  visits  to  localities  in  other  portions. 

The  topography  of  the  greater  portion  of  the  district  was  taken  by  the 
members  of  the  Lake  Superior  Survey.  The  remainder  we  owe  to  the 
topographical  division  of  the  United  States  Geological  Survey.  The  areas 
covered  l^v  tlie  respective  organizations  are  shown  on  the  sketch  map  below 
the  topographical  map  (PI.  11). 

MODE  OF  WORK. 

As  explanatory  of  the  locations  given  in  the  paper,  it  is  perhaps  not 
out  of  place  to  give  a  brief  description  of  the  plan  of  work  followed  by  the 
Lake  Superior  Division  of  the  United  States  Geological  Survey  in  this  as 


MODE  OF  VVOKK.  23 

■well  as  in  tlie  otliiT  Lako  Superiiir  iniu-buariuy  (listricts  wliicli  lia\u  been 
previously  surveyed. 

The  Upper  Peninsula  of  Michigan  affords  an  excellent  example  of  the 
■excellence  wliicli  can  be  obtained  in  the  rectangular  land  survey,  when 
properly  carried  oiit  l)^■  the  Government.  The  section  corner  posts  originally 
established  are  in  many  cases  still  to  be  seen,  and  of  course  the  bearing 
trees  are  even  more  common.  Since  the  original  survey  the  timber  value 
has  increased  so  much  that  in  certain  forested  areas  the  section  lines  have 
been  resurveyed.  Not  uncommonly  trails  follow  the  section  lines  for  long 
distances.  Moreover,  the  roads  are  frequently  laid  out  along  the  section 
lines,  thus  giving  permanent  land  boundaries.  The  section  corners  con- 
sequently offer  the  most  reliable  points  from  which  to  make  locations. 

Ti'averses  are  made  across  each  section,  either  frcm  east  to  west  or  from 
north  to  south,  and  at  varying  intervals,  according  to  the  discretion  of  the 
geologist  and  the  exigencies  of  the  case.  Each  geologist  is  accompanied  by 
a  compassman,  whose  duty  it  is  to  determine  the  course  of  the  ti-averses  by 
means  of  a  dial  compass,  and  the  distance  traveled  by  pacing  at  the  rate  of 
2,000  steps  to  the  mile.  Corrections  are  made  at  the  corner  and  quarter 
posts.  The  compassmen  employed  are  Michigan  woodsmen,  land  lookers 
or  cruisers  as  they  are  frequently  called,  and  it  is  reniarkable  with  what 
accuracy  they  will  pace  mile  after  mile  through  swamp  and  over  rough 
hills,  windfalls,  etc. 

The  geologist  explores  the  territory  on  both  sides  of  the  line  followed 
by  the  compassman.  Ledges  are  located  by  the  geologist  pacing  to  the 
compassman  as  he  comes  opposite  him  in  a  due  east-west  or  north-south 
direction.  With  two  coordinates  thus  determined,  the  ledges  are  located 
with  reference  to  the  starting  point.  For  uniformity  and  to  facilitate  ref- 
erence and  cataloguing,  it  is  customary  to  give  the  location  with  reference 
to  the  southeast  corner  of  the  section.  Thus,  1,000  N.,  1,000  W.,  SE.  cor. 
sec.  5,  T.  42  N.,  R.  33  W.,  gives  the  location  of  the  outcrop  at  the  center 
of  the  section,  and  affords  a  means  of  finding  that  ledge  which  could  not 
be  so  accurately  and  concisely  stated  by  the  use  of  any  ordinary  land- 
marks. Moreover,  easily  recognized  landmarks,  such  as  houses,  quarries, 
etc.,  are  few,  and  exceedingly  great  changes  may  occur  very  rapidly,  such, 
for  instance,  as  those  caused  by  widespread  forest  fires,  so  that  such  a 
method  of  location  is  practically  valueless. 


24  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

MAGNETIC    OBSERVATIONS. 

It  has  long  been  known  that  many  rocks  are  possessed  of  decidedly 
magnetic  properties,  due  to  the  presence  in  them  of  varying  quantities  of 
magnetic  iron  ore.  By  the  mining  engineers  and  prospectors  this  property 
has  been  turned  to  a  practical  use  in  aiding  in  the  location  of  iron  mines 
where  the  ore  is  of  a  magnetic  kind.  It  is  only  in  the  past  three  decades 
that  this  property  has  been  used  to  any  extent  by  geologists  as  an  aid  in 
the  interpretation  of  the  structure  of  a  region.  So  far  as  I  can  learn,  the 
best  published  account  of  its  use  thus  is  in  Brooks's  report  on  the  iron- 
bearing  regions  of  Michigan.'  Conclusive  proof  of  its  geological  value 
was  given  in  the  mapping  of  the  Penokee  area,  in  1876,  by  R.  D.  Irving 
of  the  Wisconsin  survey.^  That  area  extends  for  about  60  miles  northeast- 
southwest,  and  is  on  the  average  about  4  miles  wide.  For  the  eastern  part 
of  the  Wisconsin  area  the  outcrops  are  few,  and  Irving  located  the  iron 
formation  by  magnetic  work.  Along  that  belt  have  been  sunk  shafts 
belonging  to  various  mines  which  have  raised  quantities  of  ore,  and  in  no 
case  has  a  shaft  sunk  outside  of  the  limit  indicated  by  Irving  come  upon 
paying  ore. 

By  means  of  the  dip  needle  and  solar  compass,  observations  were  taken 
which  enabled  us  to  trace  a  curving  magnetic  formation  and  connect  the 
outcrops,  which  were  separated  by  about  16  miles.  The  same  bed  was 
further  delimited,  and  the  direction  partly  checked,  by  the  occurrence,  at 
varying  distances  along  this  course,  of  outcrops  of  rocks  of  the  underlying 
formation. 

Since  the  second  part  of  this  report  contains  an  exhaustive  article  on 
the  methods  and  use  of  the  magnetic  needle,'  the  subject  is  not  further 
treated  here.  The  lines  of  maximum  magnetic  disturbance — or  briefly,  the 
magnetic  lines — are  represented  on  the  accompanying  general  map,  PI.  Ill, 
by  blue  lines  marked  with  letters  I)  and  E. 

•  Magnetism  of  rocks  .and  the  use  of  the  magnetic  needle  in  exploring  for  ore,  by  T.  B.  Brooks. 
Geol.  Survey  of  Michigiin,  Vol.  I,  Part  I,  1873,  pp.  205-243. 

■^Geol.  of  the  eastern  Lake  .Superior  district,  by  K.  D.  Irving.  Geol,  of  Wisconsin,  \o\.  Ill, 
1880,  pp.  53-238.     .\tlas  sheets,  XI-XXVI. 

3  See  Part  1 1,  Chapter  II,  by  H.  L.  Smyth,  pp.  33(j-373. 


CHAPTEE,    II 


GEOGRAPHICAL  LIMITS,  STRUCTURE  AND  STRATIGRAPHY,, 

AND  PHYSIOGRAPHY. 


GEOGRAPHICAL    lilMITS. 

The  portion  of  the  district  here  described  extends  from  the  north  line 
of  T.  47  N.  to  the  south  line  of  T.  42  N.,  and  from  the  c.enter  of  R.  31  W. 
to  the  west  line  of  R.  33  W.,  and  contains  approximately  540  square  miles. 

Upon  the  small  sketch  majj  at  bottom  of  PI.  Ill  is  outlined  the  por- 
tions of  the  district  which  have  been  studied  and  described  by  the  different 
authors. 

The  detail  character  of  the  formations  is  imknown  for  parts  of  the 
area  under  discussion.  This  is  especially  true  of  the  north,  west,  and 
southwest  parts,  where,  owing  to  the  readily  decomposable  nature  of  the 
rocks,  as  determined  by  the  few  ledges  observed,  and  to  the  drift  mantle, 
very  few  outcrops  are  to  be  found. 

STRUCTURE    AKD    STRATIGRAPHY. 

The  Crystal  Falls  district  is  not  sharply  defined  petrographically,  but 
is  continuous  with  the  Marquette  district  on  the  northeast  and  the  Menomi- 
nee district  on  the  southeast  (PI.  I).  It  is,  however,  remarkable  for  the 
vast  accumulation  of  volcanic  rocks,  which,  while  by  no  means  absent  from 
the  adjoining  districts,  do  not  there  play  so  conspicuous  a  role. 

StriTCturally  this  district  can  hardly  be  better  separated  from  the 
Menominee  and  IMarquette  districts  than  it  can  be  petrographically.  The 
important  sedimentary  troughs  of  the  two  adjacent  disti-icts  are  separated 
by  an  average  width  of  40  miles.  The  area  between  the  districts  on  a. 
direct  course  is  occupied  chiefly  by  Archean  rocks,  with  narrow  infolded 
troughs  of  Huronian  rocks  playing  a  very  subordinate  role.     At  the  east 

25 


26  THE  CRYSTAL  FALLS  IRON  BEARING  DISTRICT. 

the  Archean  is  overlain  by  the  sedimentaries  of  the  Paleozoic,  the  Cam- 
brian, and  the  Silurian.  The  connecting  Crystal  Falls  rocks  are  west  of 
this  Archean  dome. 

In  the  Marquette  district  the  essential  structural  features  have  been 
shown  ^  to  be  a  g-reat  east-west  synclinorium,  upon  which  more  open  north- 
south  folds  are  superimposed.  At  the  western  end^  of  the  district  the  ■ 
superimposed  north-south  folds  become  close,  and  the  Republic  trough  is  a 
close  fold  with  an  axis  in  an  intermediate  position.  In  the  adjoining  Crystal 
Falls  district  there  are  also  two  sets  of  folds  with  their  axes  approxi- 
mately at  right  angles  to  each  other.  The  closer  folds  are  represented  by 
the  great  anticline  in  the  central  part  of  the  district.  This  anticline  has  its 
axial  plane  trending  west  of  north  and  south  of  east,  and  the  axis  plunges 
down  both  at  the  north  and  south  ends. 

The  more  open  set  of  folds  at  right  angles  to  the  above  set,  is  repre- 
sented by  the  Crystal  Falls  syncline,  with  its  axis  striking  to  the  south  of 
west,  and  plunging  west.  Farther  south  the  axes  of  the  folds  become  much 
closer  and  more  nearl)^  east  and  west,  thus  nearly  according  in  direction 
with  the  close  folds  of  the  Menominee  district.  Thus  the  structural  features 
of  the  Crystal  Falls  district  merge  into  those  of  the  Menominee  district, 
which  joins  the  Crystal  Falls  district  on  the  southeast,  where  the  great 
structural  feature  is  a  synclinorium  similar  to  that  of  the  Marquette,  but 
with  its  axis  trending  north  of  west  and  south  of  east. 

A  glance  at  PI.  Ill  will  show  the  presence  in  the  eastern  part  of  the 
northern  half  of  the  district  of  an  oval-shaped  mass  of  Archean,  and,  nearly 
surrounding  this,  a  number  of  rock  belts. 

The  Archean  ellipse  is  11  miles  long  and  3  miles  wide  on  the  average. 
The  rocks  are  mainly  granite  and  gneiss.  They  are  cut  by  rather  infre- 
quent acid  and  basic  dikes. 

Immediatel}^  surrounding  the  Archean  is  a  quartzose  magnesian  lime- 
stone formation,  to  which  the  name  Randville  dolomite  has  been  given.^  In 
the  eastern  half  of  the  district  described  by  Sm3^th,  where  more  numerous 
exposures  are  found  than  occur  in  the  western  half,  the  formation  has  an 
estimated  thickness  of  about    lj500  feet.*     Not   only   are  the   exposures 

'  Mon.  XXVIII,  cit.,  p.  566  et  seq. 

2Loc.  cit.,  J).  570. 

2  See  Part  II,  Chapter  IV,  by  H.  L.  Smyth,  p.  431. 

<See  Part  II,  Chapter  IV,  Sec.  Ill,  p.  433. 


STUIIGTURE  AND  STUATIGKAPHY.  27 

more  numerous,  but.  owing  to  the  tact  that  the  strata  stand  on  edge,  due  to 
the  chjser  t'ohling  of  the  rock  series  here,  a  more  accurate  estimate  of  their 
thickness  can  be  made. 

According-  to  Smyth,  this  limestone  formation,  in  the  southeastern  end 
of  the  elHj)se,  at  its  upper  liorizon  becomes  mixed  with  slates,  and  these 
increase  in  quantity  until  the  formation  passes  above  into  a  slate  formation, 
called  the  Mansfield  slate.'  This  slate  formation  is  found  overlying  the 
limestone  to  the  west  of  the  central  ellipse  likewise,  but  as  few  outcrops 
have  been  found,  it  is  not  positively  known  to  exist  as  a  continuous  zone 
encircling  the  northwestern  end.  In  a  direct  line  with  its  probable  continua- 
tion to  the  north,  a  graywacke  was  found  at  one  place,  sec.  19,  T.  46,  R.  32. 
This  single  outcrop  is  insufficient  evidence  to  warrant  the  introduction  of  a 
graywacke  formation  as  the  northern  equivalent  of  a  part  of  the  Mansfield 
slates,  and  it  is  probably  but  a  phase  of  that  formation.  The  only  mine  of 
this  district  producing  Bessemer  ore  is  in  a  deposit  in  the  Mansfield  slate. 

The  close  of  the  Mansfield  Slate  time  was  marked  by  the  extrusion  of 
a  great  series  of  volcanics,  which  constitute  the  next  formation  in  the 
succession.  This  volcanic  formation  has  its  best  and  most  typical  develop- 
ment west  of  the  western  Archean  ellipse.  Because  the  Hemlock  River 
and  its  tributaries  have  exposed  good  sections  in  the  volcanics,  and  becavise 
this  river  drains  a  great  portion  of  the  volcanic  area,  the  name  "  Hemlock 
formation  "  is  applied  to  the  volcanics.  The  dip  of  the  flows  and  of  the  tuff 
beds  wherever  observed  is  about  75°  west.  The  maximum  breadth  is  about 
5  miles.  Deducting  15°  for  initial  dip,  this  would  give  the  enormous  maxi- 
mum thickness  of  23,000  feet  to  the  volcanics,  upon  the  supposition  that  no 
minor  folds  occur. 

These  volcanic  rocks  have  associated  with  them  rocks  of  unquestionably 
sedimentary  origin,  as  is  shown  by  their  well-bedded  condition  and  the 
rounding  of  the  fragments.  The  subaqueous  rocks  are,  however,  composed 
of  little-altered  volcanic  materials,  and  evidently  point  to  oscillations  of  the 
crust  during  the  time  of  volcanic  activity — such  oscillations  as  have  long 
been  known  to  be  common  in  volcanic  regions. 

Following  the  volcanics,  and  overlying  them,  probably  unconformably, 
comes  a  series  of  sedimentary  rocks,  believed  to  belong  to  the  Upper 
Huronian.     These  comprise  chloritic,  ferruginous,  and  carbonaceous  slates, 

'See  Part  ir,  Chapter  IV,  Sec.  IV. 


28  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

associated  with  quartzites,  graywackes,  and  small  amounts  of  carbonate- 
beds.  The  general  character  of  the  series  is  wliat  one  would  expect  in 
rocks  the  detritus  of  which  was  from  the  Hemlock  volcanics.  It  is  in  this 
slate  series  that,  with  the  exception  of  the  Manstield  mine,  the  ore  deposits 
of  the  Crystal  I'alls  district  are  found.  The  sedimentaries  extend  west 
from  the  Hemlock  volcanics  to  the  limits  of  the  district,  underlying  thus  a 
very  broad  expanse  of  country.  Where  exposed,  the}'  show  frequent 
changes  of  character.  This  prevents  the  identification  of  individual  beds 
for  any  considerable  distance.  Owing  to  the  imperfect  exposures  of  the 
beds  and  their  close  folding,  it  has  been  found  impossible  to  subdivide  this 
series  of  rocks  into  distinct  formations. 

The  series  has  in  places  been  highly  metamorphosed,  resulting  in  the 
production  of  gneisses  and  mica-schists,  in  places  garnetiferous  and  stauro- 
litic.  The  series  corresponds  in  a  broad  way  stratigraphically  and  litho- 
logically  to  the  Michigamme  formation  of  the  Marquette  district.^  Since, 
however,  it  has  been  found  impossible  to  subdivide  this  series,  and  because 
it  may  possibly  include  more  than  the  Michigamme  formation  of  the  Mar- 
quette district,  it  is  considered  advisable  to  speak  of  it  simply  as  the  Upper 
Huronian  series.  The  generalized  sections  through  the  western  half  of  the 
Cr}^stal  Falls  district,  which  are  given  on  Pis.  V  and  VI,  will  aid  in  the 
com^jrehension  of  the  structural  and  stratigraphical  features  thus  briefly 
outlined. 

Here  and  there  in  the  Crystal  Falls  district  isolated  patches  of  Upper 
Cambrian  Lake  Superior  (Potsdam)  sandstone  are  found.  This  occurs  in 
beds  which  are  either  horizontal  or  only  a  few  degrees  inclined  from  the 
horizontal.  They  overlie  unconformably  the  steeply  inclined  Huronian 
strata.  The  great  lapse  of  time  represent  -d  by  this  unconformity  is  indi- 
cated by  the  deposits  of  the  Keweenawan  and  Lower  and  Middle  Cambrian 
time,  found  elsewhere.  The  Lake  Superior  sandstone  grades  fi'om  the  very 
coarse  basal  conglomerate  below  into  a  moderately  coarse  sandstone  above. 
The  sandstone  is  of  a  reddish  brown  to  gray  color,  and  is  not  well  indurated 
as  a  rule,  but  is  loosely  cemented  with  ferruginous  and  in  places  calcareous 
material.  As  a  result  of  this  imperfect  induration,  the  sandstone  is  not  very 
resistent  to  the  agents  of  disintegration.  Hence  it  is  that  only  remnants 
have  been  found,  but  enough  is  present  to  indicate  that  the  greatei-  part, 


'  Fifteenth  Annual,  cit.,  p.  598,  anil  Moii.  XXVIIT,  cit..  p.  444. 


us  GEOLOGICAL  SURVEY 


MONOGRAPH    XXXVI  PL  V 


HcmZ,y.:k  P 


Alk 


Alk         /R^r 


i<-  --r;  '- 


Sketch   sliowin^    locaiioii  oT  sections 
"  on 
General  Map 


EfeS 


\/;^^0yy/^-^/y'^^i^:y^^^ 


i   IN',       \     ^^',  Vv    ,     •'V,    IV   '   V 

r-'  -,'.  s-/  /- 1  ^^  V  -  '  /-  I  ^ 


Alh 


l\\i. 


-^'J.:^-^^^—                          -^gr  Ala 

fiUiM'      -.      ~  v^v"/       V    ''■'V/   'V'V — '-r^ ;- ^ — V  /  •*  im 

MM/^  ^  /^  "_'-,nv  ^V"-.nv /-')'->  \;  n'"'-,M« 

«W   '-'/v    -'n-  '-/v,^''-'-'A    n'i''-'/n    v'  ''-  '-  ,\ 

ty-/  '  -     V-    -   ^-    I   '-    V-   ,•    '-     >1   \  -  ,'  /-'\^y    S-  ^    //-  I    '1  \ 


AlaAIn  Au  AIn  Ala 


/Rgr  Au       AInAla  -^^r- 

-i%.\-,/.'-~s-,/-/'l>-'/-''^-/,./-,/,i--\-'    -   i^'-s-'/-/-   .^'a>^-:v:v,;jaaKK-/  /--s-O/ri'-  v-/^-i'L\-.  /-'-'-  \-/-'-\-,-'-i'^-\-//-i'l\-.^/- 


^gr 


Ala   AIn 


V  /  V  ,  '  /  \  , 


1  -  ~/"_,N  v',    -' 


x-N-,-,)-/^-     s-,/-l/_N-/    /-/-■-   I-,'/-/     -,N-<~ 


N.E- 


Au 


AIn      Als    Ala 


US  BIENaCO  LITH  TJ  v 


GENERM.1ZED  SECTIONS  THROUGH  NORTH  WESTERN  PART  OE  CRYSTAL  FALLS  DISTRICT 

HORIZONTAL  SCALE,  1  LMCH  =  1  MILE.  VERTICAL  SCALE,  I  lNCH-1320  ?'EET. 

ELEVATION  OF  BASE  LINES    1000  FEET. 

NOTE:  Formations  are  brought  to  the   surface  only  whel-e  exposures  have  been  obsenved 

ALGONKIAN 


ARCH E AN 


a<ii<  &Airj; 


L0WERJ1L'R0NLAJ>J 


UPPER  HURONIAN 


Granile  Sturricon  am!  Ajibik    Koao  amlRaiidvillt 

quartzile  dolomite 


Hemlock  formaUoii   Growlaitd  ^Xe^iiiinee  UndixTded 

lonnation 


6 


us  GEOLOGICAL  SURVEY 


MONOGRAPH    XXXVI  PL  VI 


sw 


^'/i!/^</!<^<6M 


yaX^:y.,;v'./^\,!^AA^ 


PaintR  -     ^1^^ 


-S^^^^^ 


Au  Au       ''^'^ 


"  ^/v  v'vWvi  J 


Sketch  showing   locaUon  of  sections 

on 

General  Map 


^Ih^"'^^'     Ado^Au- 


N  F 


Aim  Aim 
Mir        AlsAIr  Alg      Air    AU 


^jr 


,  \  /^  <-;- 


Air  P  Air 


/Rgr 


ULius  B<e^aco  lith  wv 


ARCH E AN 


GENERALIZED  SECTIONS  TMROUCiH  SOTTHERN  PA  FIT  OF  CRYSTVl.  FALLS  DISTRICT 

HORIZONTAL  SCALE,  1  1N'CH=  1  MILF:.  A'ERTICAL  SCALE,  1  INCH-  1320  FEET. 

ELEVATION  OF  BASE  LINES   1000  FEET. 

NOTE:  Formations  are  broir^hl  to  the   surface  only  where  exposures  have  been  observed 

ALGONKIAN 


LOWER  HURON  L\N 


LrpPER  HURON  IAN 


Grajiite 


Slurgpuii  aii<l  Ajibik    KonoaudRaiidviUi: 
quailzile  dolomite 


AirnSALs   j 

Mansfield  and 
Sianio  slate 


Hemlock  formation   I'roveland  SNegaunee 
formation 


-JA-Ur---.-^ 


INTRUSIVE 


■ 


PLEISTOCENE 

UNDIVLDED 


Undivided 


Dolerite 


Gabb  ru 


STRUCTURE  AND  STRATIGRAPHY.  29 

iiiid  |)r<»l)al>ly  thu  entire  C'lystal  Falls  district,  was  covered  by  Caiiil)riaii 
deposits.     The  thickness  of  tlie  Cambrian  deposits  can  not  be  determined. 

The  next  hi<.>lier  })ortion  of  the  geological  time  scale  represented  in  the 
district  is  that  part  of  the  Pleistocene  penod  which  in  this  part  of  the 
United  States  is  characterized  l)y  the  past  existence  of  great  ice-sheets. 
The  evidences  of  the  existence  of  the  ice  ai*e  everywhere  present,  either  in 
the  rounding  and  polishing  and  scoring  on  the  surfaces  of  the  rocks 
exposed  or  in  the  character  of  the  drift  deposits.  The  direction  of  the  ice 
movement  was  clearly  from  the  northeast  to  the  southwest,  as  is  shown  l^v 
the  trend  of  the  stria',  which  were  observed  upon  the  rounded  rock  out- 
crops in  N'arious  places.  The  thickness  of  the  drift  deposit  varies  very 
materially.  In  places  it  has  been  almost  entirely  removed  by  denudation, 
if  in  such  places  it  ever  formed  anything  more  than  a  thin  veneer  upon  the 
surface.  In  other  places  it  reaches  a  very  considerable  thickness,  as  is 
shown  by  the  glacial  topography  characteristically  developed  in  T.  45  N., 
R.  32  W. 

As  the  present  report  is  confined  to  the  pre-Paleozoic  rocks,  no  detail 
description  will  be  given  of  these  Cambrian  and  Glacial  de2:)0sits,  nor  are 
they  represented  on  the  map,  except  in  those  places  where  it  has  been  found 
impossible  to  map  the  underlying  rocks.  The  generalized  columnar  section 
on  PI.  VII  gives  in  condensed  form  our  knowledge  concerning  the  formations 
mentioned. 

PHYSIOGRAPHY. 

TOPOGRAPHY. 

The  topography  in  its  large  features  is  pre-Glacial,  and  in  some  cases 
this  older  topography  is  rather  distinct.  For  instance,  in  the  case  of  the 
Deer  River  Valley,  drift  covers  the  gentle  slopes  and  bottom,  but  is  not 
sufficiently  deep  to  completely  hide  the  pre-Glacial  Deer  River  Valley. 

In  the  southwestern  part  of  the  district  west  of  Crystal  Falls,  or,  more 
generally,  west  of  the  Paint  River,  pre-Glacial  topography  is  seen  in  places. 
Here  we  find  the  drift  as  a  veneer  and  only  partly  hiding  the  bed-rock 
topography,  which  depends  mainly  on  the  strikes,  dips,  and  varying  charac- 
ters of  the  rocks. 

It  is  so  well  known  that  this  part  of  the  country  was  at  one  time 
-covered  by  ice,  that  it  is  useless  to  cite  such  proof  as  the  rounding  and 


30  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

scoring  of  the  rocks  and  the  character  of  the  di'ift  material,  a  good  portion 
of  which  can  be  readily  seen  to  have  been  brought  from  some  other  region, 
no  such  rocks  as  those  forming  it  existing  where  the  bowlders  now  lie.  The 
ice-sheet  left  a  deposit  of  di-ift,  and  we  find  the  pre-Glacial  topography  essen- 
tially modified  by  it.  As  a  result  of  this,  the  prevailing  and  most  noticea- 
ble topography  of  the  western  half  of  the  Crystal  Falls  district  is  that  of  the 
drift,  and  is  characterized  by  short  ridges  and  broken  chains  of  hills,  usually 
oval,  though  at  times  of  very  irregular  outline,  between  which  are  lakes 
and  swamps.  The  swamps  are  even  occasionally  found  on  rather  steep 
slopes,  where  a  thick  spongy  carpet  of  moss  (sphagnum)  retains  sufficient 
moisture  for  cedars  and  other  trees  and  shrubs  characteristic  of  the  Michigan 
swamps  to  grow.  The  swamps  follow  the  carpet  of  moss  up  the  hills  to  the 
spring  line. 

The  Glacial  drift  topography  is  especially  marked  where  the  drift  was 
of  considerable  depth.  These  conditions  are  well  exhibited  in  parts  of 
T.  45  N.,  Rs.  31,  32  W.,  shown  on  the  large-scale  map,  PI.  VIII.  Here, 
even  though  the  ground  is  very  heavily  timbered,  one  may  easily  trace  out 
the  sinuous  course  of  the  eskers.  When  traversing  the  country,  one  is 
constantly  descending  into  pot-holes  or  is  climbing  ridges,  some  of  them 
75  to  100  feet  high,  often  with  a  crest  only  a  few  feet,  in  some  places  not 
more  than  4  feet,  wide. 

Where  the  drift  mantle  has  been  removed,  the  rounded  character  of 
the  rock  exposures  is  usually  shown.  This  holds  good  especially  for  the 
more  resistant  rocks,  such  as  the  granites  and  massive  greenstones.  Slates 
and  tuifs,  weathering  more  readily,  have  in  numerous  cases  had  time  since 
the  ice  retreated  to  be  weathered  into  rough  broken  ledges,  some  of  which 
show  perpendicular  cliffs. 

The  elevations  range  usually  from  1,400  to  1,600  feet  above  sea-level. 
The  hills  rarely  rise  more  than  200  feet  above  the  low  ground  at  their  bases. 
The  extremes  of  height  noted  in  the  district  are  from  1,250  to  1,900  feet 
above  sea-level,  corresponding,  respectively,  to  the  valley  of  the  Michigamme 
on  the  south  and  the  watershed  between  Lake  Superior  and  Lake  Michigan 
on  the  north.  Between  these  two  extremes  there  is  a  strip  of  territory,  25 
miles  across  from  north  to  south,  in  which  the  variations  in  height  are 
within  the  limits  of  200  feet. 

A  consideration  of  the  slight  difference  of  level  which  prevails  over 


U.  S.  OEOLOGICAL  SURVEY 


MONOGRAPH  XXXVt      PL.   VII 


I 


Period.  Formation  name. 


_IO 
£LO 


Potsdam  sandstone. 


2-i 
l-O 
<  CD 

o 
U. 


Colum- 
nar SEC- 
TION. 


■Cp. 


Mansfield  siate. 


LI_Li 


^.^.^^^^^^ 


t^S 


I'll  .  r 


XJ, 


Randvilla  dolomite. 


I  ^gr. 


II"  I 


I? 


Character  cf  rocks. 


Usual  characters. 


Thickness  unknnwn  Yellowish  to  reddish  brown  sandstone,  not 
thornughly  cemented,  therefore  disintegrates  readily,  Found  in  patches 
in  many  places,  and  always  lying  either  in  beds  wh'ch  are  horizontal  or 
else  possess  slight  dip  to  the  south  This  may  represent  the  initial  dip 
with  Vkhiqh  the  beds  weie  deposited. 


A  series  of  very  great  bul  unknown  thickness.  It  consists  of  a'ternat- 
ing  beds  of  slates,  graywackes.  siderite,  and  chert  With  Ihese,  espe- 
cially associated  wilh  \he  last  two,  are  found  hematite  and  limonite  ore 
bodies  of  variable  size  and  of  great  economic  importance.  From  this 
series  is  derived  nearly  all  the  ore  supplied  by  the  Crystal  ^a'ls  distnct. 
In  the  southern  part  of  the  district,  espec-ally  well  exposed  in  the 
vicinity  of  the  Paint  and  Michigamme  rivers,  the  slates  and  graywackes 
have  bet^n  metamorphosed  into  schists  and  gneisses.  This  series  is  cut 
by  dikes  of  rock  ranging  from  acid  to  ultrabasic,  which  have,  in  places, 
metamorphosed  the  sediments. 


The  thickness  of  this  vast  pile  o'  volcanic  ejectamenta  can  not  be 
estimated  with  any  degree  of  accuracy.  It  consists  chiefly  of  in'.erbed- 
ded  acid  and  basic  lavas  and  associated  tuff  deposits,  and  the  water- 
deposited  materr  Is  derived  from  them.  Near  the  top  of  the  volcanics 
a  lent'Cula:  aiea  of  norma'  sediments,  slates  witn  lenses  of  limestone,  is 
found.     This  formation  is  cut  by  acid  and  basic  dikes 


1500 


Estimated  to  be  about  1 ,500 feet  thick.  It  consists  of  interbedded  f  rag- 
mentals,  slates,  and  graywackes  and.  associated  with  these, fen  uginous  chert 
and  carbonate  From  these  last  has  been  derived  the  ore  found  associated 
wiih  them  The  Mansfield  mine,  by  which  is  exploited  the  only  ore  body 
in  the  Mansfield  formation,  supplies  the  only  Bessemerore  of  the  Crystal 
Falls  district      These  slates  are  cut  and  metamorphosed  by  basic  dikes. 


The  thickness  's  that  estimated  for  this  formation  in  the  eastern  part 
of  the  district  by  Smyth,  The  prevailing  rock  is  quartzose  dolomite,  of 
a  veiy  friable  character. 


It  shows  the  usual  characters  of  granite.  It  is  schistose  on  flanks  of 
m.assif,  and  is  cut  by  acid  and  basic  dikes,  which  are  mafsve  ai-.d 
schistose. 


GENERALIZED   COLUMNAR   SECTION. 


PHYSIOGltAPHY.  31 

the  "Teatcr  i)iirt  of  tlie  Crystal  Falls  district  has  U-d  Smyth  to  the  conclusion 
that  this  portion  of  Michigan  liad  before  Glacial  times  been  reduced  to  the 
condition  of  an  approximate  peneplain.  (See  Part  II,  Chapter  1.)  This 
peneplain  is  a  continuation  of  tlie  peneplain  of  northern  Wisconsin,  and 
lies  between  the  northern  Michigan  base-level  on  the  nortli  and  the  central 
Wisconsin  baselevel  on  the  south,  to  both  of  which  attention  has  recently 
been  called  by  Van  Hise.^ 

DRAINAGE. 

The  greater  heights  in  the  Michigamme  district  are  in  the  northern 
part,  where  some  few  of  the  hills  rise  to  a  height  of  1,800  feet,  and 
one  to  a  maximum  of  1,900  feet  above  sea-level;  but  tlie  majority  do 
not  rise  above  1,600  feet.  The  belt  including  these  higher  elevations 
extends  about  NE-SW.  This  belt  represents  the  crest  of  the  watershed, 
from  which  all  streams  on  the  northern  side  flow  to  Lake  Superior,  and  on 
the  southeastern  side  all  flow  to  Green  Bay  of  Lake  Michigan.  A  part  of 
this  watershed  is  undivided,  and  it  is  not  uncommon  to  find  extensive  swamps 
in  which  streams  flowing  to  opposite  sides  of  the  watershed  take  their 
origin.  The  portion  of  the  Crystal  Falls  district  which  is  tributarj-  to  Lake 
Superior  is  so  small  that  it  will  be  totally  neglected  in  the  further  discussion 
of  the  drainage.  The  topographical  map,  PI.  II  shows  the  general  slope 
and  drainage  of  the  district  to  be  SSE.  The  eastern  part  of  the  district  is 
drained  by  the  Michigamme^  River  with  its  tributaries,  the  Fence  (Mitchi- 
gan),  and  the  Deer,  while  the  Paint  (Mequacumecum)  River,  with  its  main 
tributaries,  the  Hemlock  and  the  Net,  di-ains  the  west  and  northwestern  por- 
tions. The  Brule  (Wesacota)  flows  along  the  southern  part  of  the  district, 
being  for  the  most  part  just  below  the  southern  limits  of  the  present  map. 
It  forms  throughout  its  course  the  boundary  line  between  Michigan  and 
Wisconsin.  The  Paint  flows  into  the  Brule  in  sec.  12,  T.  41  N.,  R.  32  W., 
and  the  Brule  and  the  Michigamme  unite  in  sec.  16,  T.  41  N.,  R.  31  W.,  to 

'  A  central  Wisconsin  base-level,  by  C.  K.  Van  Hise :  Science,  new  ser.,  Vol.  IV,  1896,  pp.  57-59,  219. 
A  northern  Michigan  base-level :  ibid.,  pp.  217-220. 

-The  Indian  names  which  the  streams  and  lakes  of  this  district  formerly  bore  have  either  been 
dropped  or  else,  in  a  few  cases,  have  been  replaced  by  translations,  though  most  commonly  they  have 
been  replaced  by  English  names,  which  are  altogether  new.  Those  names  which  have  been  retained 
receive  various  spellings  at  the  hands  of  dift'ereut  authors,  and  even  at  the  bauds  of  the  same  writer. 
The  Michigamme  Kiver,  for  example,  is  fre<iuently  spelled  by  Burt  iu  the  same  article  Feshakmnme  and 
Pesh-a-kem-e.  The  name  Michigamme  is  also  spelled  on  various  maps  ilachigamig  and  Michirjamig. 
Whereas  the  Paint  we  find  spelled  Mequacumecum,  as  above  most  commonly,  though  Burt  spells  it 
AIesi{uacumecum  and  also  Mesijuacnm-a-cuui. 


22  THE  CEYSTAL  FALLS  IRON-BEAKING  DISTRICT. 

form  the  Meuomiuee  River.  This  last  flows  southeast  through  the  adjoin- 
ing Menominee  district,  and  is  the  boundary  hue  Ijetween  Michigan  and 
Wisconsin  from  its  source  to  its  mouth. 

A  glance  at  the  map,  PI.  II,  will  show  the  presence,  especially  in  the 
northern  half  of  the  district,  of  a  great  number  of  lakes  of  varying  sizes. 
These  lakes  of  clear  water,  with  bottoms  of  gravel,  or  most  commonly  of  a 
thick  deposit  of  decayed  vegetable  matter,  are  a  very  characteristic  feature 
of  the  landscape.  Many  are  in  the  midst  of  swamps,  sun-ounded  on  all 
sides  by  a  quaking  bog,  which  prevents  one  from  approaching  very  closely ; 
others  are  surrounded  by  steep  but  low  di-ift  hills.  The  lakes  may  or  may 
not  have  a  visible  inlet  and  outlet.  In  all  cases  the  present  water  levels 
are  considerably  below  the  original  water  levels.  In  many  cases  the  lakes 
are  but  remnants  of  much  larger  bodies  of  water.  They  are  gradually 
filling  ujD  with  silt  and  vegetable  growth.  These  lakes,  covered  with  float- 
ing lily  pads  and  surrounded  by  more  or  less  extensive  hay  marshes,  are 
favorite  places  for  the  deer,  which  in  many  parts  of  the  district  are  still 
fairly  numerous.  The  numerous  lakes  indicate  the  youthful  character  of 
the  di-ainage.  Many  of  the  streams  head  in  the  lakes.  In  other  cases  they 
flow  tlii'ough  them,  connecting  them  in  chains.  This  indicates  the  mode  of 
origin  of  the  most  of  the  streams  of  the  area.  The  youthful  character  of 
the  drainage,  is  still  further  shown  by  the  fact  that  with  but  few  excep- 
tions the  rivers  have  not  reached  rock.     They  are  still  cutting  in  drift. 

In  the  case  of  the  Deer  River  this  gradual  development  from  the 
original  condition  of  a  chain  of  lakes  to  the  present  condition  of  a  river  in 
which  the  lakes  play  very  subordinate  parts  is  well  shown.  Moreover,  its 
development  illustrates  very  well  several  of  the  stages  passed  through  by 
rivers  in  general,  and  for  these  reasons  it  may  be  well  to  describe  it  in  detail. 

The  life  history  of  the  Deer  River,'  as  it  is  to-day,  began  with  the  deposit 
of  the  di'ift,  which  destroyed  the  former  streams  of  the  district  and  concealed 
their  records.  It  appears  proljable  from  the  topography  that  the  river 
occupies  the  same,  or  approximately  the  same,  bed  in  which  its  pre-Glacial 
forerunner  moved.  The  noticeable  valley  occupied  by  the  stream  is  at  a 
maximum  about  3  miles  broad,  though  its  drainage  area  is  a  strip  averaging 

'  The  substance  of  the  following  was  presented  to  the  Wisconsin  Academy  of  Sciences,  Arts, 
and  Letters  at  the  annual  meeting,  September  27,  1895,  in  a  paper  entitled  "Some  stages  in  tlie 
(levelnpiiieut  of  rivers,  as  illustrated  by  the  Deer  River  of  Michigan."  An  abstract  of  the  paper  was 
published  iu  Amer.  Geol.,  Vol.  XVII,  lS9t>,  p.  li'6. 


8 


X  ^  - 


=^M 

X 

<H^Ps=  1 

S  =:  ^  -  ■*o 

2:; 

■i<-^  y- 

::: 

ii  -  H 

"^ 

— 

^-  '   ^ 

<  <i   r. 

PHYSIOGRAPHY.  33 

f)  or  <!  miles  in  l)rcailtli.  'I'lic  lulls  between  wliicli  the  stream  flows  are  not 
very  lii^^li  aintxc  flic  river  l)e(l,  the  niaxinuuu  elevation  bein<>-  175  feet, 
'{'he  tew  rock  outcro])s  are  in  all  cases  foiiiifl  on  the  tops  ami  flanks  of  these 
hills,  w  here  they  have  been  exposed  by  denudation.  At  one  jjoiut  only 
has  rock  been  found  in  situ  near  the  river  bed,  and  that  is  toward  the  mouth 
of  the  river.  The  conclusion  is  natural,  since  the  river  is  175  feet  below 
these  exposed  rocks  and  has  not  reached  rock,  that  it  must  be  flowing' 
thr-Hiiih  a  i)reexisting  depression  or  valle}-  parti)-  flUed  by  the  drift  of  the 
Glacial  epoch. 

The  partial  filHng  of  this  valley  at  the  time  of  the  retreat  of  the  ice 
to  the  northeast  was  accompanied  by  the  filling  of  the  depressions  in  the 
drift  by  the  water  flowing  from  the  front  of  the  melting  glacier.  After 
the  depressions  were  filled,  the  overflowing  water  naturally  followed  the 
general  southeastern  slope,  which  exists  throughout  the  area  and  is  shown 
by  the  topographical  maps  and  by  the  flow  of  the  rivers.  The  immediate 
course  of  the  water  was  determined  by  the  former  valley,  which  was  not 
completely  obliterated  by  the  drift  deposit.  Drift  barriers  across  the  valley 
separating  the  ponded  water,  or  lakes,  from  one  another  were  cut  through, 
the  material  eroded  being  spread  over  the  bottoms  of  the  lakes  below.  Thus 
was  formed  a  chain  of  lakes,  connected  usually  by  narrow  streams;  the 
processes  by  which  the  channels  were  cut  out  and  the  lakes  drained  and 
filled  up  with  the  debris  were  going  on  at  the  same  time.  The  result  has 
been  to  obliterate  the  lakes  to  a  great  extent  and  to  accentuate  the  char- 
acter of  the  stream. 

The  final  eflect  of  the  processes,  briefly  outlined,  would  be  to  destroy 
the  lakes  entirely  and  produce  a  stream. 

By  following  on  PI.  VIII  the  Deer  River  from  its  mouth  to  its  source, 

we  may  see  the  several  stages  in  its  development,  which  are  also  typical  for 

other  streams  of  the  glaciated  portions  of  the  world.     The  river  is  about  20 

miles  long  and  has  a  width  near  where  it  enters  into  the  Micliigannne  of 

20  to  30  yards.     Near  its  mouth  it  is  a  slow-flowing,  sluggish  stream,  which 

has  nearly  reached  its  base-level  of  erosion,  and  like  many  of  the  older 

streams  of  the  Coastal  Plain  region  of  the  United  States  is  gradually  filling 

portions  of  its  channel  with  the  silt  and  vegetable  matter  brought  down 

from  above. 

A  short  distance  from  its  mouth  it  resembles  such  streams  also  in  the 
HON  xxxvi 3 


34  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

meandering  character  of  its  channel.  Tliis  resemblance  is  still  further 
enhanced  by  the  presence  of  a  remnant  of  a  crescent-shaped  cut-off,  so 
characteristic  of  the  old  age  of  rivers.  Just  opposite  this  cut-off  is  a  lake, 
which  is  of  interest  on  account  of  its  possessing  two  outlets,  both  leading 
into  the  river.  Unfortunately  this  fact  was  observed  on  the  topographical 
sheet  too  late  to  permit  of  a  return  to  the  field  for  the  purpose  of  determining 
the  cause  of  the  presence  of  the  two  outlets. 

Passing  up  the  stream  we  soon  reach  the  lakes,  which  farther  on  become 
more  numerous.  The  life  history  of  these  lakes  is  inseparably  connected 
with  that  of  the  river.  They  reached  maturity  dui-ing  or  at  the  close  of  the 
Glacial  epoch,  and  since  that  time  their  history  is  that  of  decline.  This 
part  of  the  history  of  these  lakes  may  be  brieflj'  stated  as  follows:  As  the 
erosion  continixes,  the  areas  of  water  are  reduced  and  the  surrounding  swanijD 
areas  are  correspondingly  increased.  If  a  lake  were  lai-ge  and  considerable 
inequalities  existed  in  its  bottom,  two  or  more  small  lakes  connected  by  the 
stream  flowing  through  them  may  be  formed.  The  final  stage  is  a  swamp, 
traversed  by  the  slow-flowing  river. 

The  various  stages  in  the  history  of  the  lakes  are  well  illustrated  on 
the  accompanying  map,  P|.  VIII,  by  the  following  series  of  lakes.  In  Nos. 
1  and  2  the  general  character  of  such  bodies  of  water,  which  may  be  con- 
sidered essentially  as  mere  expansions  of  streams,  is  seen.  No.  3,  and  Deer 
Lake,  have  long  since  reached  maturity  and  are  advancing  rapidl}'  to  the 
point  where  they  will  each  be  separated  into  two  bodies  of  water.  No.  4 
has  already  reached  this  stage,  and  in  the  swamp  marked  A  we  have  the 
last  stage,  the  swamp,  with  the  stream  flowing  between  peaty  banks. 

On  Light  and  Liver  lakes,  in  the  lower  part  of  the  Deer  Ri-ver,  we  may 
see  all  but  the  last  of  these  stages  illustrated.  The  lakes  are  attached  to 
the  main  river  by  very  short  streams.  The  main  river  after  leaving  the 
rapids  above,  where  it  accunmlates  considerable  detritus,  enters  a  flat  por- 
tion of  its  course  partly  occu})ied  by  the  two  lakes  in  question.  Here,  its 
rapidity  being  diminished,  the  stream  deposits  the  detritus.  Thus  it  has 
gradually  built  a  delta,  now  for  the  most  part  covered  by  swamp  growth. 
This  tends  to  advance  the  shore  line,  and  thus  diminish  the  water  area. 
The  rapid  cutting  down  of  the  barrier  immediately  below  the  lakes  by  the 
swiftly-flowing  stream  tends  to  lower  the  lakes  and  thus  diminish  their 
surface  area  still  more. 


PHYSIOGRAPHY.  35 

The  couil)iiR'd  effect  of  the  draiiiiuy  and  tilHuf^  lias  been  to  separate 
what  was  tormcrK-  a  long-  narrow  lake  trending-  NE-SW.  into  three  rounded 
bodies  of  water,  two  of  which  are  connected  with  each  other,  the  larger  of 
these  two  and  the  third  bdcc  being-  connected  with  the  main  stream  by  very 
short  necks.  An  artificial  dam  has  been  built  across  the  narrow  cliannel 
below  the  lakes,  and  the  effect  has  been  to  flood  the  delta  and  unite  the 
lakes  into  one  large  bod)'  of  water,  occupying,  approximately,  the  area 
covered  h\  the  glacial  hike,  thus  restoring  the  conditions  which  existed 
before  the  natural  barrier  had  been  trenched. 

In  the  remainder  of  the  course  of  the  Deer  River  the  tendency  of  other 
artificial  dams  to  restore  the  river  to  its  original  condition,  that  of  a  series 
of  connecting  lakes,  is  well  shown.  These  dams  were  built  by  lumbermen 
at  the  foot  of  the  lakes  or  swamps  when  it  was  desired  to  retain  a  large 
body  of  water  at  these  places.  When,  on  the  other  hand,  the  desire  was  to 
enable  the  logs  to  pass  rapids,  a  dam  (marked  B  on  the  map)  was  built  near 
the  head  of  the  rapids.  The  l)ack  water  would  bring  the  logs  to  the  dam, 
and  on  opening-  the  gates  the  flood  would  carry  them  over  the  rapids  into 
the  deeper  water  beyond.  The  Deer  River  thus,  after  having  reached  a 
somewhat  advanced  stage,  has  been  rejuvenated  by  the  Michigan  lumbermen. 

A  study  of  the  small  tributaries  shows  the  same  condition  of  things, 
although  not  on  so  large  a  scale  nor  so  perfectly  as  in  the  main  stream. 

The  source  of  the  Deer  River  is  in  the  copious  sjjrings  which  rise  out 
of  a  spongy,  marshy  piece  of  ground  less  than  125  yards  distant  from 
Bone  Lake,  and  about  20  feet  below  the  usual  water  level  of  Bone  Lake, 
and  are  really  fed  by  the  lake  water  percolating  through  the  drift  and 
appearing  at  this  point.  From  the  springs  there  is  a  depression  which  leads 
up  to  the  lake.  The  highest  point  of  this  depression  was  about  3  feet  above 
the  normal  water  level  of  the  lake. 

The  outlet  of  Bone  Lake  is  Ihe  Fence  River.  The  river  leaves  the 
lake  at  a  point  three-quarters  of  a  mile  distant  from  the  head  of  the  Deer 
River  Valley.  In  order  to  obtain  a  supply  of  water  for  driving-  the  Fence 
River,  Bone  Lake  has  been  converted  into  a  reservoir.  A  dam  was  built 
at  tbe  outlet  which  raised  the  water  about  4  feet,  and  the  result  was  to  turn 
some  of  the  water  of  the  lake  into  the  Deer  River,  necessitating  also  a  dam 
across  this  small  valley  near  the  lake  shore.  At  present  only  a  few  strokes 
of  the  shovel  would  be  necessary  in  order  to  turn  the  water  of  the  flooded 


36  THE  CRYSTAL  FALLS  IROIST-BEARING  DISTRICT. 

lake  from  tlie  Fence  into  the  Deer  River,  thus  gaining  for  it  a  drainage  area 
extending  7  miles  farther  north  and  including  three  large  lakes,  the  main 
sources  of  tlie  water  suj)ply  of  the  western  branch  of  the  Fence. 

I  ha^-e  no  data  which  would  enable  me  to  show  that  the  valley  at  the 
head  of  Deer  River  was  ever  a  channel  for  the  waters  of  Bone  Lake.  I  am 
incUned  to  believe  that  such  was  not  the  case.  For  had  it  existed  with  the 
present  slope,  20  feet  in  375  feet,  or  even  a  much  lower  one,  the  water 
would  have  had  a  marked  erosive  power,  and  it  would  have  cut  back  its 
channel  mucli  more  rapidly  than  the  Fence,  which  for  a  mile  below  the  lake 
is  a  comparatively  sluggish  stream,  and  would  have  eventually  captured 
Bone  Lake  and  its  feeders. 

The  Deer  River  is  still  continuing  the  process  of  lengthening  its  chan- 
nel, and  the  springs  which  give  it  birth  are  gradually  undermining  the 
barrier  at  its  head,  so  that  it  is  possible  that  it  will,  unless  artificially 
restrained,  obtain  much  more  water  from  Bone  Lake  than  it  does  at  present. 
A  change  in  atmospheric  and  other  conditions,  which  would  insure  a  state 
of  equilibrium  Ijetween  the  incoming  and  outgoing  waters,  thus  preserving 
the  waters  of  Bone  Lake  at  their  present  level,  would  be  favorable  for  the 
final  successful  robbery  of  the  upper  Fence  River  system  by  the  Deer 
River.  This  favorable  condition,  as  may  be  readily  seen,  would  be  greatly 
increased  in  proportion  as  the  increase  of  inflowing  over  outflowing  water 
raised  the  level  of  the  lake. 

TIMBER  AND  SOIL. 

The  district  was  at  one  time  very  heavily  timbered,  with  hard  wood 
and  ])ine,  the  former  predominating  on  the  whole.  Along  the  flood  j^lains 
of  the  large  streams  one  finds  sandy  j)ine  barrens  where  once  there  were 
heavy  pine  forests.  On  the  headwaters  the  pine  are  found  scattered 
through  the  hard  wood.  Individually  these  trees  are  very  much  larger  and 
better  tlian  the  thick  and  therefore  smaller  growth  of  the  plains.  Lumber- 
ing, which  had  been  confined  for  years  to  the  main  drainage  channels  of 
the  district,  has  of  late  been  rapidly  extended,  following  all  the  ramifica- 
tions of  the  tributary  streams,  until  at  present  there  remains  in  this  district 
only  a  few  years'  cut  of  pine  at  the  very  headwaters  of  the  rivers. 
Following  the  lumbermen  comes  the  forest  fire,  which  finds  its  most  nourish- 
ing food  in  the  dry  resinous  pine  tops  left  by  thein.     The  fires,  once  started. 


riJYSIOGliAPUY.  37 

are  not  confined,  however,  to  the  cut  pine,  but  .sjjread  to  tlie  adjacent 
standing-  ])ine  and  even  into  the  hard-wood  forests,  carrying  destruction 
with  tliein,  and  leaving  but  the  gaunt,  l)are,  and  blackened  truidcs  to  mark 
the  sites  of  what  were  forinerh'  thick  forests. 

The  i)ine-covered  areas  have  a  thin  soil  and  are  jtoorly  adapted  to 
agriculture.  The  areas  covered  with  hard  wood  have,  on  the  contrary,  soil 
well  adajtteil  to  the  crops  of  the  latitude. 

The  advance  of  the  lumberman  has  necessitated  the  damming  and 
clearing  of  streams  and  the  blasting  of  channels  to  permit  the  floating  of 
the  logs,  and  this  has  driven  the  fish>  especially  the  sjjeckled  trout,  which 
formerly  crowded  all  the  streams,  into  the  smallest  and  most  inaccessible 
ones.  Rutfed  grouse,  Bonasa  umbellus,  and  deer  are  still  rather  plentiful  in 
certain  portions  of  the  area,  although  the  pot-hunter  with  set  guns,  spring 
nooses,  and  pitfalls  is  rapidly  exterminating  them.  The  deadh-  character 
of  such  appliances  is  brought  vividly  to  mind,  when,  as  happened  in  my 
own  case,  one  is  suddenly  arrested,  while  following  a  deer  trail  through  the 
underbrush,  by  a  hay  wire  noose  around  his  neck,  and  he  may  be  thankful 
if  the  bent  sapling,  having  been  bent  so  long-  as  to  lose  its  elasticity,  fails 
to  spring  up  and  render  the  device  effective. 


CHAPTER   III. 
THE  ARCHEAN. 

DISTRIBUTIOlSr,  EXPOSURES,  AXD   TOPOGRAPHY. 

The  granite  described  in  this  chapter  belongs  to  the  oldest  system  in 
the  district,  and  forms  the  western  elliptical  core  designated  on  PL  III  as 
Archean.  It  is  surronnded  by  sedimentary  strata,  which  have  a  quaqua- 
versal  dip  away  from  the  granite  as  a  center.  The  portion  of  the  Crystal 
Falls  district,  in  which  the  granite  outcrops,  is  about  13  miles  long  by 
3  miles  wide,  its  longest  axis  extending  in  a  NW.  and  SE.  direction  and 
covering  parts  of  Ts.  44,  45,  and  46  N.,  Rs.  31  and  32  W. 

The  exjjosures  of  granite  are  especially  numerous  in  the  southeast  part 
of  the  o^•al  area,  where,  owing  to  the  proximity  of  large  streams,  the  Fence 
and  Deer  rivers,  and  the  consequent  increased  erosion,  the  drift  has  to  some 
extent  been  removed.  In  the  northwest  part  of  the  area,  with  rare  excep- 
tions, all  the  rocks  are  deeply  covered  with  drift. 

In  general  the  topography  of  the  area  is  that  of  the  drift,  but  in  the 
southern  part  it  is  seen  to  have  been  considerably  influenced  by  the  char- 
acter of  the  iinderlying  rocks.  The  granite  usually  outcrops  in  small, 
rounded,  and  isolated  knobs,  whose  relations  to  one  another  can  onh'  he 
conjectured.  Where  an  occasional  knob  is  composed  of  massive  granite 
and  more  or  less  gneissoid  granite,  the  exposed  surface  is  so  small  as 
to  prevent  the  observer  from  determining  the  relations  between  the  two. 
Cutting  the  massive  and  schistose  granite  are  certain  long  narrow  masses 
of  dark-colored  rocks  of  rather  fine  gi-ain,  and,  with  few  exceptions,  very 
schistose.  From  their  geological  occurrence  it  was  concluded,  in  »\nte  of 
their  appearance,  that  they  are  dike  rocks  cutting  the  granite.  The  follow- 
ilig  paragrajjli,  quoted  from  the  inanuscript  notes  of  G.  O.  Smith,  describes 
very  cleai'ly  their  field  occurrence: 

The  gaps  in  this  grauite  ridge  seem  to  indicate  greenstone  dikes,  as  here  the 
granite  usually  has  a  facing  of  the  greenstone  more  or  less  extensive,  and  often  in 
the  center  of  the  gap  there  are  several  small  areas  of  greenstone.    In  all  cases  the 
38 


RELATIONS  OF  THE  ARCIIEAN.  39 

greenstone  is  markedly  more  affected  by  weathering  than  is  the  granite.  A  stndy  of 
the  rehitions  at  the  tew  points  of  contact  did  not  yield  mncli  more  than  negative 
resnlts,  but  these  pointed  to  the  intrusive  character  of  the  greenstones. 

UEIiATIONS   TO   OVERJOYING   FORMATIONS. 

The  relations  of  tlie  granite  to  the  sedhnentary  rocks  might  be  explained 
in  two  ways;  the  former  may  serve  as  the  base  of  the  latter  rocks,  or  it  may 
penetrate  them.  The  occurrence  of  the  granite  in  an  elliptical  sliape,  with 
sediments  surrounding  it  showing  quacpiaversal  dips,  might  be  regarded  as 
evidence  of  its  intrusion  in  the  Huronian  sediments,  and  on  this  theory  it 
would  follow  that  the  granite  is  of  Huronian  or  post-Huronian  age.  If 
intrusive,  it  should  be  found  to  penetrate  and  metamorphose  those  sedi- 
ments. Against  the  intrusive  character  of  the  granite,  and  in  favor  of  its 
pre-Huronian  age,  are  the  following  facts:  (1)  There  is  a  total  absence  in 
the  surrounding  sedimentary^  strata  of  any  dikes  which  are  lelated  to  the 
granite.  (2)  There  is  a  total  alisence  of  any  metamorphic  action,  so  far  as 
observed,  in  the  sedimentaries.  (3)  On  the  east  flank  of  the  granite  core, 
on  the  west  bank  of  the  west  branch  of  the  Fence  River  in  the  SW.  corner 
sec.  1,  T.  45  N.,  R.  32  W.,  is  a  recomposed  granite,  which  passes  up  into  a 
tine  sericitic  quartzite,  with  false  bedding.  These  rocks  evidently  derived 
their  material  from  the  granite,  and  hence  mark  the  beginning  of  sedimenta- 
tion in  this  area. 

Thus  the  positive  evidence  confirms  the  negative,  and  since  the  granite 
underlies  the  oldest  sedimentary  rocks,  whose  age  has  been  determined  to 
be  Huronian,  the  former  is  classified  as  Archean,  that  term  being  used  here 
to  designate  those  rocks  of  undoubted  igneous  character  which  form  the 
foundation  upon  which  rest  the  oldest  determinable  sedimentary  rocks. 
It  is  not  the  province  of  this  paper  to  enter  into  a  speculative  discussion  of 
the  origin  of  the  Archean  rocks  of  the  district.  For  such  a  discussion  the 
reader  is  referred  to  Professor  Van  Hise's  exhaustive  disquisition  on  the 
Principles  of  North  American  pre-Cambrian  Geology,^  where  the  conclusion 
is  reached  that  "the  Archean  is  igneous  and  represents  a  part  of  the  original 
criist  of  the  earth,  or  its  downward  crystallization."'"  The  Archean  has 
gradually  reached  the  surface  by  the  removal  by  erosion  of  the  superjacent 
rocks. 

I  Sixteenth  Ann.  Rept.  U.  S.  «eol.  Survey,  Part  I,  1896,  pp.  571-874. 
-  Loc.  cit.,  p.  752. 


40  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT, 

PETROGBAPHICAIi  CHARACTERS. 

The  rocks  of  the  Archean  comprise  biotite-granite,  gueissoid  biotite- 
grauite,  and  acid  and  basic  dikes. 

BIOTITE-GRANITE  (GRANITITE). 

The  rock  occupying  the  main  and  central  ])art  of  the  Archean  area  is 
a  biotite-ffranite.  This  rock  is  also  found  to  some  extent  on  the  border  of 
the  area.  The  rocks  of  this  kind  vary  in  color  from  light-graj'  rocks  to 
those  having  various  tints  of  red,  depending  usually  upon  the  degree  of 
alteration.  They  vary  also  from  medium  to  coarse  grain.  Some  varieties 
show  a  decided  porphyritic  texture,  and  in  some  cases  also  an  approach  to 
a  laminated  structure.  The  porphyritic  character  is  due  to  the  presence  of 
large  crystals  of  feldspar,  which  stand  out  from  the  surrounding  granitic 
groundmass,  thus  producing  a  typical  granite-porphyry.  The  feldspar  phe- 
nocrysts  lie  with  their  longer  axes  parallel,  and  thus  help  to  produce  an 
imperfect  laminated  structure.  This  parallel  structure  in  the  granite- 
porphyry  is  apparently  analogous  to  the  flow  structure  of  the  volcanic 
rocks,  and  probably  was  produced  by  movements  in  the  magma  before  it 
had  reached  even  a  viscous  state,  as  we  find  that  the  phenocrysts  give  no 
evidence  of  having  undergone  excessive  mashing  or  torsion.  The  different 
textural  varieties  grade  into  one  another  in  such  a  way  as  to  indicate  that 
the}^  are  merely  modifications  of  the  same  magma.  In  addition  to  these 
textural  varieties,  which  are  original,  we  find  in  certain  places  a  passage 
from  massive  to  schistose  rocks,  in  which  the  schistosity  is  of  dynamic 
origin,  i.  e.,  of  secondary  nature. 

In  the  thin  sections  these  rocks  show  the  normal  granitic  texture  and 
the  usual  mineral  constituents  which  characterize  biotite-granites.  The 
chief  minerals  are  orthoclase,  microcline,  plagioclase,  quartz,  and  biotite. 
Zircon  and  apatite  are  the  accessory  minerals  present,  and  the  secondary 
minerals  include  epidote-zoisite,  chlorite,  muscovite,  rutile,  and  iron  pyrites. 

Quartz  occurs  in  grains  forming  the  cement  and  molding  around  the 
other  minerals.  In  one  of  the  granites  it  has  a  peculiar  saccharoidal  char- 
acter macroscopically,  and  under  the  microscope  such  portions  are  resolved 
into  very  fine  aggregates  of  quartz  grains. 

The  quartz  is  also  frequently  found  in  round  blebs  of  varying  size 
included  in  the  best  crystallized  feldspar  crystals.     Thus  the  crystallization 


PETKOGKAPHIOAL  CnAKACTEKS  OF  AKCHEAN..  41 

of  the  (|uartz,  unless  such  qunrtz  rej)reseuts  the  "([uartz  tie  corrosion"  of 
the  French  autliors,  conthiued  through  the  entire  time  occupied  by  the  crys- 
tallization of  the  feldspars,  since  it  is  included  in  the  ohlest  feldspar  of  the 
rocks,  and  also  forms  the  matrix  in  which  lie  the  youngest  feldspars.  Undu- 
lator\-  extinction,  so  general  in  the  quartzes,  shows  that  the  rocks  have  been 
subjected  to  pressure,  and  in  some  cases  it  has  been  sufficient  to  produce 
the  extreme  cataclastic  structure  of  very  greatly  mashed  rocks. 

The  quartz  includes  numerous  gas  and  fluid  inclusions,  the  latter 
frequently  with  dancing  bubbles  and  forming  negative  crystals,  by  means 
of  which  it  is  easy  to  orient  the  irregular  grains.  The  quartz  of  one  of  the 
specimens  was  found  to  contain  liquid  inclusions,  each  of  which,  besides  the 
usual  bubble,  held  a  small  rectangular  crystal.  These  crystals  are  trans- 
parent, with  a  light  greenish  tinge.  A  crystal  similar  in  ajDpearance  found 
in  the  same  quartz  individual  is  partly  inclosed  by  a  large  (j -shaped  bub- 
ble, and  gave  inclined  extinction,  though  no  further  optical  tests  could  be 
made  upon  it. 

Three  kinds  of  feldspar  are  present :  (1)  A  finely  striated  plagioclase ; 
(2)  a  feldspar,  unstriated,  or  at  most  showing  Carlsbad  twins,  and  presumed 
to  be  orthoclase ;  and  (3)  microcline,  these  last  two  being  frequently  inter- 
grown  after  the  manner  of  pertliite.  The  plagioclase  was  the  first  feldspar 
to  crystallize.  It  is  invariably  so  altered  that  the  twinning  laminae  are 
nearly  obliterated,  thus  preventing  accurate  measurements.  It  is  probably 
oligoclase;  and  if  so,  it  is  highly  probable  that  much  of  the  white  mica 
produced  by  its  alteration  is  pai'agonite  instead  of  muscovite,  a  fact  not 
determinable  microscopically.  The  phenocrysts  are  orthoclase,  usualh' 
in  Carlsbad  twins,  and  thus  at  first  sight  appear  to  have  been  the  first  feld- 
spar to  crystallize;  but  I  find  that  these  ])henocrysts  not  uncommonly 
inclose  small  rectangular,  more  or  less  automorphic,^  crystals  of  plagioclase, 
which  is  in  reality  the  oldest  feldspar.  Hence  these  orthoclases,  notwith- 
standing their  porphyritic  character,  are  later  than  a  part  of  the  plagioclases. 
One  phenocryst  with  Carlsbad  twinning  was  observed  in  which  one  part  of 

'  Automorph,  Xenomorph;  Uber  die  Eruptivgesteine  iiu  Gebiete  tier  Schlesisch-Maehrischen 
Kreideformatiou,  by  Carl  E.  11.  Kohrbach :  Tsch.  Jliu.  Pet.  Mit.,  Vol.  VII,  188(3,  p.  18. 

Idiomorph,  AUotriomorph ;  Rosenbusch  :   Mik.  Phys.,  Vol.  II,  1887,  2tl  ed.,  p.  11. 

L.  V.  Pirssou  has  recently  proposed  in  a  paper,  read  before  the  Geological  Society  of  America, 
on  A  Needed  Terra  in  Petrology,  the  term  anbedra  for  minerals  which  do  not  possess  crystallographic 
outlines  and  are  xeuomorpbic,  in  contradistinction  to  those  which  we  properly  call  crystals  and  which 
are  automorphio :  Geol.  Soc.  Am.  ,Vol.  VII,  1896,  p.  492,  and  Am.  Jour.  Sci.,  4th  series,  Vol.  II,  189G,  p.  150. 


42  THE  CRYSTAL  FALLS  lEOX-BEARING  DISTRICT. 

the  individual  shows  microclinic  striations.  The  other  part  was  untwinned, 
and  near  the  center  of  the  phenocryst,  bisected  by  the  Carlsbad  twinning- 
plane,  was  found  a  rectano-ular  plagioclase  crystal. 

The  mierocline  is  usually  the  best  crystaUized  feldspar  in  the  gTound- 
niass,  and  also  by  far  the  freshest.  In  the  few  cases  in  which  it  was 
observed  in  contact  with  plagioclase,  the  latter  molded  it,  and  is  therefore 
older  than  the  mierocline,  which  in  its  turn  is  older  than  the  orthoclase. 
In  one  ease  a  mierocline  individual  showing  the  lattice  structure  over  a 
portion  of  its  surface  possesses  no  twinning  lamellaj  in  another  portion,  the 
twinning  lamellpe  fading  until  they  totally  disappear.  Thus  no  sharp 
delimitation  is  apparent  between  the  twinned  and  untwinned  portions  of  the 
individual. 

In  most  slides  all  the  feldspars  are  much  altered,  but  even  in  those  in 
which  the  mierocline  is  fresh  the  plagioclase  and  orthoclase  alwaj's  show 
alterations,  the  plagioclase  altering  most  easily  and  usually  being  so  changed 
that  it  is  with  difficulty  that  one  can  recognize  the  twinning  lamellse.  Hence 
some  of  them  may  have  been  taken  for  the  nonstriated  orthoclase.  In  an 
early  stage  of  the  alteration  of  the  feldspars  minute  dark  ferrite  particles 
which  impregnate  them  are  hydrated,  and  this  gives  the  feldspars  a  more  or 
less  distinctly  red  tinge.  In  a  more  advanced  stage  of  alteration,  muscovite 
and  a  httle  epidote-zoisite  are  produced.  Another  alteration  of  the  feldspar 
is  alwavs  associated  with  marked  pressure  phenomena,  and  hence  is  pre- 
sumed to  be  the  result,  partially  at  least,  of  dynamic  action.  This  is  the 
partial  or  complete  granulation  of  the  feldspar  and  the  production  from 
that  mineral,  with  the  addition  from  other  sources  of  the  iron  and  magnesia 
necessarv,  of  secondary  white  mica  and  quartz,  and  some  biotite.  It  is 
highly  possible  that  some  of  the  small  limpid  grains  considered  to  be 
secondary  quartz  are  really  an  acid  feldspar.  Orogenic  movements  are 
also  indicated  by  the  bending  of  twinning  lamellae,  and  were  probably  the 
partial  cause  of  the  twinning. 

Biotite  occurs  in  plates,  and  as  a  rule  shows  better-developed  crystals 
than  does  the  feldspar,  though  it  frequently  occurs  in  decidedly  ragged 
flakes.  It  is  strongly  pleochroic,  showing  absorption  in  the  following  colors: 
Pale  straw  yellow  to  yellowish  brown,  for  rays  vibrating  perpendicular  to 
cleavage,  to  very  dark  chocolate  brown  and  greenish  brown  for  those  par- 
allel to  cleavage.     In  the  case  of  the  biotite  showing  a  greenish  color  this 


P1«:TK()GKA1'111CAL  CHAUACTKRS  of  AKCIIEAN.  43 

seems  to  be  the  result  of  ine'ipient  alteration",  since  the  edges  of  the  flakes 
are  ragu'ed,  and  in  many  cases  almost  the  entire  biotite  of  the  section  is 
altered  to  a  chlorite,  which  shows  ordinary  white  to  li<>-ht  o-reenish  pleoch- 
roism,  with  the  sinuiltaneous  production  of  epidote  and  l)undles  of  needles 
with  high  single  and  double  refraction,  having  yellowish  or  l)rownish  color. 
These  needles  are  taken  for  rutile.  The  biotite  is  found  usually  lying 
between  the  feldspar  and  quartz  grains  almost  as  though  it  had  l)een  tlie  last 
product  of  crystallization.  It  has  suffered  crashing  with  the  other  minerals. 
Apatite  and  zircon  were  observed  in  a  few  crystals.  No  onginal  iron 
ore  was  seen.  As  intimated  above,  by  the  use  of  the  term  "epidote-zoisite" 
the  exact  character  of  this  secondary  material  is  not  always  determinable. 
In  some  instances  parts  of  an  epidote  crystal  show^  the  dee})  blue  inter- 
ference color  of  zoisite,  apparently  indicating  a  mixture  of  the  zoisite  and 
epidote  molecules,  the  latter  predominating  in  the  crystals.^  The  remaining 
secondary  minerals  mentioned  as  occurring  in  the  granite  show  their  usual 
characters. 

GNEISSOID  BIOTITE-GRANITE,  BORDER  FACIES  OF  GRANITE. 

About  the  central  area  of  biotite-granite  just  described,  and  in  part 
formina-  the  border  of  the  Archean  area,  are  rocks  having  a  gneissic 
structure.  With  these  are  associated  the  biotite-granites.  The  gneissoid 
rocks  in  general  are  markedly  darker  in  color  than  the  granites,  showing 
normally  a  rather  dark  gray.  They  vary  little  from  one  another  in  texture 
and  are  much  fmer  grained  than  the  granites.  The  fine-grained  condition 
of  these  schistose  and  banded  rocks  has  perhaps  a  great  deal  to  do  with 
their  dark  color,  though  this  is  primarily  owing  to  the  amount  of  biotite 

present. 

In  some  of  the  specimens  the  bands  can  be  readily  distinguished  under 
the  microscope,  and  are  seen  to  contain  a  white  mica  and  a  nxuch  smaller 
amount  of  biotite.  These  two  minerals  are  present  in  fine  films  between 
the  crushed  quartz  and  feldspar  grains,  gi\'ing  to  the  rocks  a  very  decided 
schistose  character.  These  mica  folia  are  much  more  numerous  in  certain 
areas  than  in  others,  thus  producing  a  more  or  less  perfect  banding.  The 
mica  plates  are  not  all  regularly  parallel,   although  ordinarily  having  a 

'On  some  granites  from  British  Columbia  and  the  adjacent  parts  of  Alaslca  and  the  Ynkon 
district,  by  F.  D.  Adams :  Canadian  Record  Sci.,  Sept.,  1891,  p.  3+6. 


44  THE  GEYSTAL  FALLS  lEON-BBAKING  DISTKICT. 

tendency  to  this  arrangement,  and  are  usually  parallel  to  the  banding. 
The  most  perfect  schistosity  is  thus  developed  parallel  to  the  micaceous 
bands.  The  banding  and  the  schistose  structure  are  plainly  of  secondary 
oris'in,  the  result  of  dynamic  action. 

Others  of  the  gnessoid  granites,  however,  when  examined  under  the 
microscope,  are  decidedly  massive,  and  it  is  only  on  a  large  scale  that  the 
banding  shows  distinctly.  In  such  cases  the  cause  of  the  banding  could  not 
be  determined,  and  might  by  some  be  ascribed  to  differentiation,  though, 
from  the  association  of  these  gneissoid  granites  with  those  just  described,  it 
is  assumed  that  the  banded  str  icture  is  due  to  dynamic  action.  If  this  be 
the  case,  however,  a  complete  recrystallizatiou  has  taken  i)lace,  and  slight 
dynamic  effects  are  now  shown.  The  strike  of  the  banding,  wherever  it 
was  taken,  was  uniform,  varying  from  N.-S.  to  nearly  N.  45'^  W.,  agreeing, 
on  the  whole,  with  the  trend  of  the  Archean  oval  area. 

The  microscope  shows  that  the  constituent  minerals  of  the  gneissoid 
granites  are  the  same  as  those  which  compose  the  granites  just  described. 
These  show  also  the  same  relations  to  one  another  and  the  same  general  char- 
acters as  in  the  granites,  except  where  mashing  has  completely  obliterated 
the  original  texture,  and  hence  no  further  description  of  them  is  necessary. 
The  crushing  to  which  the  gneissoid  granites  have  been  subjected  is 
very  clearly  shown  in  the  present  cataclastic  condition  of  the  quartz  and 
feldspars. 

As  stated  above,  both  the  gneissoid  granite  and  the  granite  proper  are 
found  in  the  border  area  of  the  Archean.  In  those  rocks  in  which  the  con- 
tact shows  a  gradual  transition  from  the  banded  rock  to  the  unbanded,  the 
micaceous  bands  are  clearly  secondary,  and  are  the  result  of  the  crushing 
of  the  original  granite,  these  lines  representing  macroscopic  and  microscopic 
shearing  planes  along  which  the  feldspar  and  quartz  have  been  thoroughly 
granulated,  and  sericite  and  some  biotite  produced,  as  was  found  to  be  the 
case  also  in  some  of  the  granites.  These  rocks  thus  agree  in  their  dynamic 
origin  with  a  similar  but  apparently  more  extensive  and  better  developed 
gneissoid  border  facies  in  the  Morbihan  (Brittany)  granites,  which  have 
been  described,  and  whose  origin  has  been  so  cleai-ly  demonstrated  by 
BaiTois.^  Numerous  other  similar  cases  have  been  described  recently  from 
the  Canadian  granite  massifs  and  from  Sweden  and  other  districts. 


'  Anu.  Soc.  G6ol.  du  Nord.,  1887,  p.  40. 


ACID  DIKES  IN  ARCIIEAN.  45 

ACID  DIKES  IN   ARCHEAN. 

Observations  upon  diki's  of  acid  rocks  {•uttini^-  the  Arcliean  <»-r;uiite  are 
very  i'ew,  and  we  may  suppose  this  to  be  partly  due  to  tlieir  occurrence  in 
isolated  knobs,  which  prevented  the  determination  of  the  relations  of  adjacent 
exposures  of  rocks  of  sliji'litly  different  character.  Some  few  dikes  were, 
nevertiieless,  ol)serve<l,  and  are  j^Tauites  varying  from  medium  to  coarse 
gi-ained,  grauolitic'  to  })orphyritic  rocks.  The  porphyritic  facies  is  the 
most  conuuon.  The  dikes  do  not  show  differences  from  the  main  mass  of 
the  Archean  granite  suflKcieiit  to  warrant  detailed  petrographical  description. 
The  following  description  of  one  mass  of  granite-porphyry  is  given,  as  it  offers 
good  proof  of  its  relation  to  the  schistose  border  facies  of  the  granite.  In 
this  case  the  gneissoid  rock  is  found  as  inclusions  in  the  granite-porphyry,  as 
is  illustrated  in  the  accompanying  diagrammatical  sketch,  fig.  4,  taken  from 
a  ledge  in  the  field.  In  this  sketch  the 
sharply  outlined  angular  and  lenticular 
areas  represent  the  gneiss  included  in  the 
gi'anite-porph)Ty.  The  jjlienocrysts  of 
this  granite-porphyry  have  a  parallel 
arrangement,  the    long    direction    of  the 

phenOCryStS    agreeing    also  with  the     trend       Fio  4— Granite  imrpl.yry  with  mclusions  of  gneis- 
soid granite. 

of  the  longer  axes  of  the  inclusions.     The 

banding  and  foliation  in  the  inclusions  strike  at  a  right  angle  to  the  flow- 
age  structure  of  the  granite.  The  lines  of  separation  between  the  areas  of 
gneiss  and  the  granite,  as  shown  in  this  outcrop,  are  sharp,  and  |)oint  to 
their  nature  as  inclusions,  and  such  is  accepted  as  the  true  explanation 
of  their  angular  character  and  sharp  outlines.  As  this  por^jhyritic  granite 
was  intruded  throu"h  the  border  facies  of  the  Archean  oranite,  these  fras"- 
ments  were  taken  up,  and  were  so  arranged  as  to  agree  with  the  direction 
of  movement  in  the  intruding  mass.  This  occurrence  shows  this  granite- 
porphyry  to  he  younger  than  the  great  mass  of  Archean  granite,  whether 
we  consider  the  inclusions  to  be  a  border  facies  of  the  Archean  granite, 
derived  from  it  by  dynamic  action,  and  thus  of  secondary  origin,  or  to  have 
resulted  from  differentiation  of  the  molten  magma. 

'  This  terra  has  been  proposed  by  a  committee  on  nomenclature  for  the  geologic  folios  of  the 
United  States  Geological  Survey,  for  use  in  place  of  "granitic." 


46 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 


BASIC  DIKES   IN  THE  ARCHEAN. 

The  influence  of  the  dikes  on  the  character  of  the  topography  has 
already  been  mentioned.  They  occur  in  long  narrow  bands  of  varying 
widths,  and  with  one  exception  are  markedly  schistose.  Considering  the 
granite  on  a  large  scale  as  an  approximately  homogeneous  mass,  we  would 
expect  to  find  lines  of  weakness,  which  might  be  indicated  by  the  arrange- 
ment of  the  dikes.  No  such  definite  arrangement  can  be  seen,  however,  as 
the  dikes  are  found  to  extend  in  all  directions.  A  good  example  of  their 
mode  of  occurrence  may  he  seen  in  tig.  5,  which  also  illustrates  very  clearly 
their  hifluence  on  the  topography.  '  A  small  valley,  in  sec.  1,  T.  44,  R.  32, 
througli  which  a  brook  flows,  is  occupied  by  the  main  dike,  from  which 

diverge  the  smaller  ones, 


SC! 


iGnANlTE(CflTACLASTlC) 


? 


Scale  ormiles 


Fio.  5.— Illustration  of  the  effect  ou  tile  ttipograpiiy  of  the  differential  erosion 
of  basic  dikes  and  granite. 


penetrating  the  granite  on 
both  sides.  These,  not 
having  been  much  more 
deeply  eroded  than  the 
granite,  do  not  form  chan- 
nels deep  enough  to  be 
shown  on  a  map  with  a 
1 0-foot  contour  interval. 
It  is  without  doubt  owiny 
to  the  fact  that  the  dikes 
weatlier  so  much  more 
readily  than  does  the  granite  that  we  may  partly  explain  the  comparative 
scarcity  of  exposures.  The  depressions  separating  the  granite  knobs  are 
believed  to  indicate  in  many  cases  the  position  of  dikes,  liut  being  now 
filled  with  g-lacial  deposits,  the  underlying  dike  rocks,  if  such  are  present, 
are  covered.  Thus  we  find  them  exposed  onl}'  where  erosion  has  cleared 
this  debris  away,  or  where  portions  of  the  dike  still  border  the  steep  sides 
of  the  granite  on  the  sides  of  dej^ressions. 

The  dikes  may  be  classified  as  (1)  earlier  dikes,  showing  a  schistose 
structure,  and  with  no  trace  of  igneous  textures,  and  (2)  later  massive  dikes, 
showing  original  igneous  textures. 

(1)   SCHISTOSE   DUCES. 

The  general  character  of  these  rocks  occurring  as  dikes  may  be  briefly 
mentioned.     The)'  are  schistose,  tor  the  most  part  fine  grained,  and  black 


BASIC  DIKES  IN  ARCnKAN.  47 

ill  color.  The  ci»iis>titueiits  of  tliu  schistose  eruptives,  arranged  according 
to  tlieir  rehitive  importance,  arc  liiotite,  liornlilende,  clilorite,  (juartz,  feld- 
spar ('?),  cah'ite,  epidote,  iron  oxide,  sphene,  and  ninscovite. 

Tile  clear  limpid  grains  whicli  form  the  gronndmass  are  nndoubtedly 
for  the  most  jiart  (jnartz.  No  satisfactory  results  were  obtained  in  the  tests 
for  feldspar,  but  it  is  highh'  probable  that  some  is  associated  with  the 
quartz.  Dark  chocolate-brown  to  light-brown  biotite  is  almost  an  invari- 
able constituent.  In  some  cases  it  is  aQCompanied  by  a  little  chlorite,  which 
apj)ears  not  to  have  been  derived  from  the  biotite.  In  a  few  rare  instances 
biotite  is  absent  altogether,  chlorite  taking  its  place.  The  biotite  and  chlo- 
rite are  usually  found  between  the  quartz  grains.  They  have  a  parallel 
arrangement,  and  this  gives  the  rock  its  schistosity.  Biotite  and  epidote 
are  found  included  in  the  grains  of  quartz  of  the  groundmass.  Muscovite 
is  rarely  present,  but  when  found  is  in  medium-sized  automorphic  plates. 
Ragged  pieces  of  ore,  either  ilmenite  or  titaniferous  magnetite,  and  sphene, 
secondarv  to  these,  are  found  in  almost  all  specimens,  and  in  a  few  instances 
iron  pyrites  was  observed.  Calcite  is  invariably  present  in  irregular,  fairly 
large  grains,  almost  equaling  the  quartz  in  quantity.  Epidote  is  found  in 
large  qutmtity,  both  in  crystals  and  in  irregular  grains,  the  crystals  occurring 
among  the  bunches  of  biotite  and  included  in  the  grains  of  quartz.  The 
large  amount  of  epidote  in  association  with  the  calcite  seems  to  point  to  the 
very  basic  character  of  the  feldspar  of  the  original  rock. 

A  bluish-green  hornblende  is  rather  frequently  associated  with  the 
mica.  In  rocks  in  which  the  hornblende  predominates  mica  is  always  pres- 
ent, but  the  reverse  is  not  true,  the  most  micaceous  rocks  being  entirely  free 
from  the  hornblendic  component. 

The  hornblende  is  found  in  lai'ge  prismatic  individuals  without  terminal 
faces.  This  mineral  contains  some  of  the  other  constituents  of  the  rock  in 
which  it  is  found,  such  as  quartz,  epidote,  and  more  rarely  ircin  oxides. 
The  interspaces  between  the  hornblende  crystals  are  filled  with  irregular 
biotite  flakes  and  with  grains  of  quartz,  epidote,  and  iron  oxide.  This 
hornblende  is  apparently  one  of  the  last,  if  not  the  last,  mineral  to  develop. 
The  hornblendic  rocks  are  not  nearly  so  schistose  as  the  micaceous  ones. 

The  secondarv  origin  of  the  hornblende  is  clearlv  shown  in  one  of  the 
sections  which  is  traversed  by  a  fissure;  the  hornblende  can  be  seen  extend- 
ing into,   and    in  places    crossing,   this    fissure.      The  other  minerals  are 


48  THE  CRYSTAL  PALLS  mON-BEARING  DISTRICT. 

presumed  to  be  secondary,  but  this  can  not  be  proved  tor  them.  The 
schistose  character  of  the  rocks  is  evidence  of  dynamic  action.  The  pres- 
ence of  undulatory  extinction  was  noticed  in  the  quai'tz  of  some  specimens, 
but  its  absence  is  the  rule.  However,  from  the  absence  of  great  pressure 
phenomena,  and  the  remarkably  fresh  condition  of  the  minerals  composing 
the  basic  rocks,  which  contrasts  strongly  with  the  generallv  altered  condition 
of  the  minerals  of  the  more  refractory  acid  rocks  including  tliem,  it  would 
appear  that  complete  recrystallizatiqn  has  occurred.^ 

The  schistose  structure  can  undoubtedly  be  referred  to  the  dynamic 
action  which  resulted  in  the  upturning  of  the  sedimentaries  and  caused  the 
de^■elo[)ment  of  schistosity  in  certain  portions  of  the  border  of  the  granite. 
This  dynamic  action  was  in  all  probability  also  the  chief  force  in  the  pro- 
duction of  the  secondary  minerals. 

The  schistosity  of  the  dikes  does  not  agree  in  direction  with  the  gen- 
eral strike  of  the  schistosity  throughout  the  entire  district,  but  is  always 
nearly  pai-allel  to  the  long  extension  of  the  dikes.  These  dikes  represent 
belts  of  weakness,  and  it  is  therefore  natural  that  the  movements  should 
occur  along  these  belts  rather  than  across  them. 

This  schistosity  of  the  dikes  also  furnishes  a  slight  clue  as  to  their  age. 
Younger  than  the  granites  they  cut,  they  must  have  occupied  their  present 
position  at  the  time  the  dynamic  revolution  took  place  which  resulted  in 
the  development  of  schistosity  in  the  granite,  as  well  as  in  the  sedimentaries. 
It  is  impossible  to  bring  the  date  of  their  intrusion  within  narrow  limits. 
It  seems  very  probable,  however,  that  they  were  formed  at  the  time  of  the 
extrusion  of  the  basic  Hemlock  volcanics,  though  it  is  impossible  to  proye 
their  connection  with  them. 

(2)   MASSIVE   DIKES. 

The  only  dike  rock  which  retains  to  some  extent  its  original  texture  is 
a  much-altered  medium-grained  dolerite  (diabase).  The  alterations  it  has 
undergone  are  those  usual  for  such  basic  types  of  rock,  and  this  one  exhibits 
nothing  peculiar  or  of  special  interest.  An  ophitic  texture,  while  still  recog- 
nizable, is  more  or  less  obscured  by  the  uralite  which  has  developed  out  of 
the  pyroxene.  The  remnants  of  the  original  plagioclase  feldspar  ])resent 
show  exceedingly  slight  pressure  effects.     The  alteration  processes  would 

'Principles  of  North  American  pre-Cambriau  Geology,  cit.,pp.  706-707. 


RfiSUMfi  OF  AIJCHEAN.  49 

tlierefore  seem  to  have  been  due  to  the  action  of  percolating  water,  without 
special  mechanical  influence.  Hence  we  may  date  the  intrusion  of  tliia 
particular  dike  after  the  orogenic  movements  which  affected  the  granite 
core,  rendering  portions  of  it  schistose,  and  crushing  all  of  it  to  a  greater 
or  less  extent.  These  movements  are  presumed  to  have  taken  place  just 
prior  to  or  during  Keweenawau  time;  and  therefore  the  age  of  this  dike  is 
Keweeuawan  or  post-Keweenawan.' 

In  the  above- described  granite  massif  we  have  a  rock  of  pre-Huronian 
ao-e,  as  shown  by  its  relations  to  the  overlying  sedimentaries.  It  possesses 
in  general  a  coarse  granular,  and  in  places  porphyritic  texture.  Along  its 
border  it  contains  portions  which  are  much  finer  grained,  darker  than  the 
rest  of  the  mass,  and  very  well  banded.  The  boundaries  between  the 
banded  rock  and  the  granite  at  times  are  sharp,  but  frequently  are  very 
indefinite.  This  banded  schistose  portion  is  found  to  be  due  to  pressure, 
causing  the  gradual  passage  from  the  granular  granite  to  the  gneissoid, 
schistose  granite. 

One  instance  of  undoubted  inclusion  of  gneissoid  granite  by  a  true 
granite  was  observed.  If  the  gneissoid  granite  was  derived  by  pressure 
from  the  Archean  granite,  then  the  particular  granite  dike  which  includes 
the  fragments  must  be  of  later  age  than  the  great  mass  of  granite  of  the 
Archean  area. 

The  Ai'chean  is  cut  by  basic  dikes  of  two  ages.  The  earlier  ones  were 
rendered  schistose,  and  the  production  of  this  secondary  structm-e  was 
accompanied  by  a  total  obliteration  of  the  primary  igneous  texture  and 
the  production  of  a  large  amount  of  mica  and  hornblende.  All  the  dikes 
were  probably  injected  at  the  time  of  the  volcanic  activity  when  the  vol- 
canics  of  the  higher  series  were  ejected,  but  no  proof  of  their  connection 
can  be  produced.  They  were,  however,  injected  before  the  folding  of  the 
.area  took  place,  as  shown  by  their  having  been  rendered  schistose  by  it. 

A  single  dike  belonging  to  the  later  series  was  studied.  It  is  massive, 
and  therefore  was  irrupted  after  the  folding  which  produced  the  schistosity 
in  the  earlier  series  of  dikes.  It  belongs  probably  to  a  Keweeuawan  or 
post-Keweenawan  period  of  eruption. 

'  For  a  discuasiou  of  the  orogenic  movements  which  aft'ected  the  Crystal  Falls  district,  the  reader 
is  referred  to  p.  158  et  seq. 

3I0N   XXXVI 4 


CHAPTEE   IV. 

THE  LOWER  HURONIAN  SERIES. 

This  series  is  represented  in  the  Crystal  Falls  district  by  the  following 
formations,  given  in  order  from  the  Dase  upward:  The  Randville  dolomite, 
the  Mansfield  slate,  and  the  Hemlock  formation.  At  the  beginning  of  the 
deposition  of  the  Lower  Huronian  series  the  entire  district  was  covered  by 
the  pre-Cambrian  sea,  with  the  possible  exception  of  a  small  island  in  the 
Archean  area. 

SECTION    I.— THE    RANDVILLE    DOLOMITE. 

The  best  exposures  of  this  dolomite  are  found  near  the  center  of  the 
district  east  of  the  western  ellipse  and  in  the  extreme  southeastei-n  part  of 
the  district  in  the  Felch  j\lountain  range.  Both  areas  are  described  by 
Smyth,  to  whom  we  owe  the  name,  and  the  reader  is  referred  to  his  descrip- 
tion on  p.  406  and  p.  431  for  the  detail  characterization  of  the  formation. 
It  will  suffice  for  our  purpose  to  state  that  it  is  a  medium-grained  crystallme 
dolomite. 

The  few  outcrops  which  I  shall  mention  are  important  as  showing  the 
relations  of  the  formation  to  the  underlying  rock,  but  are,  petrographically 
considered,  rather  exceptional  phases  of  the  formation.  Hence  my  descrip- 
tion will  be  brief. 

DISTRIBUTION,  EXPOSURES,  AND   TOPOGRAPHY. 

The  area  in  which  the  Randville  dolomite  immediately  underlies  the 
drift  is  a  continuous  zone  adjacent  to  and  surrounding  the  Archean  core. 
The  belt  varies  slightly  in  width  along  the  sides  of  the  ellipse.  At  the 
ends  it  is  two  or  three  times  the  width  at  the  sides.  This  is  due  to  the 
lower  dip  of  the  beds  at  the  ends.  Exposures  are  found  m  the  area  studied 
by  me  only  on  the  northeast  and  southwest  flanks  of  the  granite  core. 

The  west  branch  of  the  Fence  River  follows  the  limestone  area  for  a 
short  distance  in  the  northeastern  part  of  the  district,  skirting  the  Archean 

50 


PETKOGHAPIIICAL  CUAUACTERS  OF  KANDVILLE  DOLOMITE.      51 

•  -•raiiito.     On    the    whole,    however,   the    Kiiudville    (loh»inite    has   hud    uo 
uuirked  effect  on  the  topoyrapliy  or  the  drainage. 

PETROGRAPIIICAI.   CHARACTERS. 

Ill  general  the  Randville  dolomite  consists  petrographicall}'  of  a  fine- 
grained dolomite,  with  some  quartz.  This  grades  down  through  a  calca- 
reous quartzite  by  increase  of  quartz  into  a  true  quartzite.  The  nearer  the 
granite,  the  more  quartzitic  is  the  formation.  At  the  southeast  corner  of 
sec.  2,  T.  45  N.,  R.  32  W.,  on  the  west  bank  of  the  west  branch  of  the 
Fence  River,  is  a  ver}-  good  exposure  of  the  quartzite.  Its  derivation 
from  the  underlying  granite  is  here  shown.  The  rock  is  a  very  fiue-grained, 
almost  novaculitic,  quartzite.  It  shows  current  bedding  in  some  places, 
though  no  true  bedding  was  observed.  Immediately  below  this  quartzite 
is  a  very  schistose  rock,  in  which  one  can  readily  distinguish  macroscopic- 
ally  rounded  to  lenticular  quartz  areas,  with  masses  of  sericite  flakes 
between  them.  The  contact  between  tlie  quartzite  and  the  schistose  rock 
seems  very  sharp  when  viewed  from  a  short  distance,  but  is  found  to  l)e 
indefinite  when  closely  examined.  A  close  search  was  made  along  a  con- 
tact for  pebbles  from  the  granite,  but  such  were  not  found.  However, 
small  rounded  pieces  of  vein  quartz,  most  probably  derived  from  the  granite, 
were  observed.  The  schistose  rock  in  its  turn  grades  down  into  a  graj-ish 
granite,  which  is  also  more  or  less  schistose.  We  have  here  evidently  a 
transition  from  the  granite,  through  the  intermediate  schistose  recomposed 
granite,  to  the  true  sedimentary  rock  above.  The  meaning  of  this  transi- 
tion is  considered  below. 

Under  the  microscope  the  cause  of  the  schistosity  of  the  rock  inter- 
mediate between  the  granite  and  the  quartzite  is  plain.  Quartz  and  sericite, 
with  some  feldspar,  are  alone  present  in  it.  The  quartz  is  grayish  and 
granulated,  and  mashed  out  into  oval  areas  representing  original  quartz 
grains.  Various  fragments  constituting  the  areas  are,  however,  angular 
and  more  or  less  equidimensional,  and  when  not  so  never  have  a  definite 
orientation  of  their  longer  axes.  Between  these  large  areas,  but  not  between 
the  individual  small  fragments  constituting  the  areas,  sericite  is  abundant. 
When  the  sericite  is  not  predominant,  the  flakes  lie  in  a  fine  mass  of  quartz 
grains,  each  of  which  agrees  in  long  direction  with  the  mica  plates  and 
large  oval  quartz  areas.     The  sericite  flakes  are  both  included  in  this  quartz. 


52  THE  CRYSTAL  FALLS  IRON-BEAfiING  DISTRICT. 

and  also  lie  between  the  grains.  In  one  instance  fragments  of  the  original 
feldspar  were  found  in  the  midst  of  such  an  area.  These  quartz-sericite 
areas  are  unquestionably  of  secondary  origin,  and  the  minerals  have  devel- 
oped in  connection  with  pressure.  They  were  probably  produced  from 
feldspar  which  existed  in  the  original  gi-anite. 

Whether  this  schistose  rock  was  formed  from  a  weathered  but  not 
transported  granite,  from  an  arkose  or  feldspathic  sandstone,  or  from  the  solid 
granite,  it  is  impossible  to  say.  A  similar  sericite-schist  which  developed 
from  recomposed  granites  has  been  described  by  Van  Hise  as  occurring  at 
several  localities  in  the  Marquette  district.^  In  these  cases  at  places  the 
fragmental  characters  are  still  sufficiently  clear  to  admit  of  the  statement 
that  the  rocks  are  sedimentary.  In  the  Crystal  Falls  rock  mashing  has 
destroyed  all  original  characters.  The  rock  occupies  an  intermediate  posi- 
tion between  a  metamorphosed  sedimentary  and  a  metamoiphosed  eruptive, 
and  grades  on  the  one  liand  into  the  sedimentary  and  on  the  other  into 
the  eruptive.  This  makes  it  impossible  to  say  whether  it  belongs  exclusively 
to  the  one  or  to  the  other,  or  in  part  to  both.  Similar  relations  .  in  other 
parts  of  Michigan  were  explained  by  Rominger"  as  cases  of  progressive 
metamorphism  of  sediments,  the  granite  being  supposed  to  be  the  extreme 
stage  of  alteration  of  the  sedimentary  rock.  Later  the  finding  of  basal 
conglomerates  at  or  near  these  localities  has  shown  conclusively  that  this 
explanation  is  incorrect,  and  it  has  been  abandoned  by  Rominger. 

The  quartzite,  which  immediately  overlies  the  rock  of  doubtful  char- 
acter, is  composed  of  angular  grains  of  quartz,  between  which  are  plates  of 
sericite  which  have  an  imperfect  parallelism,  thus  giving  a  certain  degree 
of  schistosity  to  the  quartzite,  possibly  enough  in  places  to  warrant  its 
being  called  a  quartz-schist.  The  rock  shows  no  conclusive  microscojjical 
e\'idence  of  a  sedimentary  origin,  but  differs  from  the  cherts,  with  which  it 
might  be  confused,  in  the  size  of  the  grains  and  in  the  presence  of  sericite. 
This  rock  was  originally  probably  chiefly  composed  of  quartz  sand,  with 
some  feldspathic  material  from  the  disintegrated  granite.  Coincident  with 
the  pressure  which  produced  the  striking  schistosity  in  the  underlying  rock, 
this  sand  was  also  mashed,  resulting  in  the  production  of  sericite  and  quartz 

'  Mou.  U.  S.  Geol.  Survey  Vol.  XXVIII,  1897,  p.  226. 

-The  Marquette  irou  district,  by  Carl  Rominger:  Geol.  of  Michisau,  Vol.  IV,  Parti,  1878-1880, 
pp.  15-52. 


lM:TROGKAPniCAL  CHARACTERS  OF  RANDVILLE  DOLOMITE.      53 

trom  the  feldspiir  and  in  the  rrusliing-  of  the  quartz  grains,  thus  completely 
destro}'ing-  the  rounded  chistic  grains  and  oljliteratiug  all  the  sedimentary 
character  of  the  rock,  except  the  macroscopic  structure  of  cun-ent  bedding. 

On  the  west  side  of  the  granite  ellipse,  at  N.  1750,  W.  1550,  sec.  12, 
T.  44  N.,  R.  32  W.,  about  100  yards  from  the  granite,  to  the  nortli,  and 
lower  down  on  the  slope  of  the  same  hill  on  which  the  granite  is  found,  is 
found  a  carbonaceous  quartzite  or  quartzose  dolomite.  The  strike  is  N. 
25°-35°  W.  The  surfiice  only  is  seen,  so  that  the  dip  could  not  be  taken. 
Microscopical  examination  shows  the  rock  at  the  eastern  side  of  the  exposure 
to  be  made  up  of  quartz  grains  held  together  by  a  fine-grained  carbonate 
cement.  This  grades  up  to  the  west  by  increase  of  calcite  and  correspond- 
ing diminution  of  quartz  to  a  quartzose  dolomite. 

At  N.  500,  W.  1550,  sec.  1,  T.  44  N.,  R.  32  W.,  one-fourth  mile  distant 
from  the  granite,  is  seen  another  outcrop  of  a  very  dense  quartzose  dolo- 
mite, appearing  macroscopically  almost  like  a  vitreous  quartzite,  but  really 
with  just  enough  quartz  grains  in  it  to  enable  the  qualifying  term  "quartzose" 
to  be  appropriate!}-  used.  The  brown  ferruginous  crust  on  the  weathered 
surfaces  point  to  a  percentage  of  iron  in  the  magnesium-calcium  carbonate. 
The  pure  limestones  are  to  be  sought  slightly  farther  away  from  the 
Archeau  shore,  where  the  conditions  were  more  favorable  for  the  production 
of  a  pure  nonclastic  sediment. 

REIiATIOlSrS  TO  UNDERLYING  AND  OVERLYING  FORIMATIONS. 

At  only  the  one  place  cited  above  has  a  contact  between  the  granite 
and  the  Randville  dolomite  been  found.  It  is  probable  that  unconformable 
relations  exist,  even  though  no  basal  conglomerate  has  been  discovered  as 
evidence  of  wave  action  on  the  Archean  coast. 

Relations  between  the  Randville  dolomite  and  the  overlying-  forma- 
tions  have  not  been  observed  in  the  part  of  the  district  studied  b}-  me. 

THICKNESS. 

Reliable  data  for  estimating  the  thickness  of  the  Randville  dolomite 
have  only  been  obtained  in  that  area  surveyed  by  Smyth.  (See  p.  433.) 
According  to  his  estimate,  the  formation  possesses  a  maxinunn  thickness  of 
1,500  feet. 


54  THE  CRYSTAL  FALLS  lEON-BEARmG  DISTRICT. 

SECTION    II.— THE    MANSFIELD    SLATE. 

The  formation  of  the  Lower  Huronian,  which  is  next  higher  than  the 
Randville  dolomite,  is  composed  of  sedimentary  beds,  in  which  a'  slate  pre- 
dominates. 

This  formation  is  fonnd  in  its  most  t^q^ical  development  in  a  narrow 
valley  through  which  the  Michigamme  River  flows,  and  in  which  the  village 
of  j\Iansfield  and  a  mine  of  the  same  name  are  situated.  The  valley  and  the 
slates  are  well  known  in  the  Crystal  Falls  district  on  account  of  their  eco- 
nomic importance.  For  this  reason  the  name  "Mansfield  slate"  is  here 
applied  to  this  formation. 

DISTRIBUTIONS,  EXPOSURES,  AXD  TOPOGRAPHY. 

The  part  of  the  valley  occupied  by  the  Mansfield  slates  begins  at  the 
northern  section  line  of  sees.  17  and  18,  T.  43  N.,  R.  31  W.,  and  extends  due 
soiith  for  3  miles  to  the  southern  section  line  of  sec.  29  of  the  same  township. 
The  slate  belt  is  widest  at  the  north,  being  over  one-fourth  mile  wide  on 
the  westren  side  of  section  17.  To  the  south  it  gradually  dimini.shes  in 
width,  until  it  finally  disappears  in  sec.  29.  The  strike  of  the  sedimentary 
rocks  is  almost  due  north-south,  except  in  a  few  places  where  the  rocks 
have  been  gently  flexed  and  the  strike  varies  a  few  degrees.  The  dip  is 
high  to  the  west,  ranging  from  65°  to  80°. 

The  influence  of  the  Mansfield  slate  belt  upon  the  topography  is 
strikingly  shown  by  the  depression  in  which  the  slates  are  found,  and 
which  contains  the  Michigamme  River.  The  slates  are  surrounded  on 
all  sides  by  igneovis  rocks  which  form  fairly  high  hills,  those  to  the  west 
being  composed  of  rocks  of  volcanic  origin,  those  to  the  north,  east,  and 
south  being  intrusive,  and  later  than  either  the  sedimentaries  or  the  vol- 
canics.  The  Michigamme  River  flows  south  through  sec.  1,  T.  43  N.,  R. 
32  W.,  and  meets  the  east  and  west  ridge  of  intrusives  in  the  northeastern 
part  of  sec.  12  of  the  same  township  and  range.  It  cuts  through  this  at  an 
oblique  angle,  changing  its  course  to  the  southeast  In  sec.  7,  T.  43  N., 
R.  31  W.,  it  leaves  the  intrusives  and  penetrates  a  short  distance  into  the 
volcanic  rocks,  their  contact  not  being  able  to  cause  a  change  in  the  course 
of  the  river,  owing  to  the  slight  diff'erence  in  resisting  power  between  the 
intrusives   and  the  volcamcs.      Still  flowing  to  the   southeast,  it  finds  at 


THE  MANSFIELD  SLATE.  55 

the  Michigamnu'  (lain,  on  the  section  line  between  sees.  7  and  18,  near  the 
southeastern  and  northeastern  corners,  respectively,  the  contact  between 
the  three  kinds  of  rock,  the  sedimentaries,  the  volcanics,  and  the  intrusives. 
Where  the  water  leaves  the  eruptive  and  enters  the  sedimentary  area  the 
more  easily  erodible  nature  of  the  rocks  of  the  latter  is  well  shown  by  the 
■falls  which  have  been  formed,  the  volcanics  constituting  the  barrier  over 
which  the  water  plunges  into  a  deep  basin  worn  from  the  slates.  Crossing 
the  slates  in  the  same  direction,  i.  e.,  southeast,  the  river  strikes  squarely 
ao-ainst  the  intrusive  dolerites  and  is  deflected  to  the  south,  following  the 
contact  between  the  two  rocks  for  a  short  distance,  then  gradually  working 
to  the  west  into  the  center  of  the  sedimentary  area,  the  river  takes  an 
almost  directly  southerly  course,  with  only  minor  bends.  In  the  slates  the 
river  has  fairly  low  flat  banks  on  both  sides.  In  the  southern  portion  of 
the  area  the  valley  is  narrower,  owing  to  the  progressive  narrowing  of  the 
sedimentary  belt.  As  soon  as  the  river  leaves  the  Mansfield  slate  belt, 
it  resumes  the  sinuous  course  it  had  before  the  Mansfield  belt  is  entered, 
and  flows  between  high  banks  through  the  intrusives,  out  through  the  sand 
plains  near  Lake  Mary. 

POSSIBLE  CONTINUATION  OF  THE    MANSFIELD  SLATE. 

In  sec.  10,  T.  44  N.,  R.  32  W.,  about  7  miles  northwest  of  the  extreme 
northern  end  of  the  Mansfield  area  of  slate,  there  are  one  or  two  exposures 
of  much  crumpled  interbedded  brown  and  black  slates.  Their  strike  is  about 
N.  16°-20°  W.,  but  owing  to  their  plicated  condition  the  dip  varies  from  55° 
southwest  over  to  85°  northeast.  The  average  dip,  however,  is  presumed 
to  be  to  the  southwest,  which  is  in  accord  with  the  general  structure  of  the 

area. 

The  slate  exposures  are  surrounded  by  coarsergrained  basic  intrusives, 
dolerites,  which  outcrop  within  short  distances  on  all  sides.  The  nearest 
sedimentary  beds  are  quartzose  dolomite  ledges  which  outcrop  1 J  miles  to 
the  east,  in  sees.  1  and  12,  T.  44  N.,  R.  32  W.,  rather  close  to  the  Archean 
granite.  A  section  across  the  Lower  Huronian  rocks  at  this  point  shows 
the  Archean  granite  overlain  by  quartzose  dolomite,  which  is  in  its  turn 
overlain  b3'  the  slates.  The  relations  which  these  rocks  bear  to  one  another 
are  those  which  similar  ones  bear  to  one  another  near  Michigamme  Moun- 


56  THE  CRYSTAL  FALLS  IKON-BE ARING  DISTRICT. 

tain/  and  the  slates  of  the  two  areas  are  consid  red  to  he  of  the  same  age. 
Since  the  slates  correspond  stratigrai^hically  to  the  slates  of  the  Michigamnie 
Mountain  and  to  those  of  the  Mansfield  area,  they  have  been  connected  ou 
the  map  with  the  slates  of  Michigannne  Mountain  by  a  narrow  belt  included 
between  dotted  lines;  but  this  belt  is  not  based  on  any  connecting  exposures. 
These  two  ledges  of  slate  are  taken  as  the  northernmost  outcrops  of  the 
Mansiield  slate  formation,  although  a  number  of  miles  north  and  in  direct 
continuation  of  them  along  the  strike  there  was  found  a  single  doubtful  out- 
crop of  a  graj'wacke,  showing  neither  strike  nor  dip.  Whether  it  represents 
a  shallower  water  deposit  contemporaneous  with  the  slates  it  is  impossible 
to  say.  However,  on  such  slight  evidence  it  was  not  deemed  advisable  to 
continue  the  slate  belt  to  this  point. 

PETROGRAPIIICAL   CHARACTERS. 

A  petrographical  description  of  the  Mansfield  slate  belt  must  neces- 
sarily be  very  brief,  owing  to  the  small  area  and  to  tlie  scarcity  of  the 
exposures. 

The  rocks  of  the  Mansfield  slate  belt  are  graywackes,  clay  slates, 
phyllites,  siderite-slates,  cherts,  ferruginous  cherts,  and  iron  ores,  with  the 
various  rocks  which  have  been  derived  from  them  l^y  metamorphism. 
They  vary  from  coarse-grained  rocks  to  very  fine  grained  slaty  ones.  The 
latter  predominate,  and  for  that  reason  this  belt  is  called  a  "slate "  belt.  The 
color  of  the  rocks  varies  from  an  olive  green  and  purplish  lilack  to  bright 
red  for  those  which  are  very  ferruginous  and  more  or  less  altered. 

The  ordinary  detrital  rocks  may  be  divided  into  the  coarser  and  the 
finer  kinds.  The  first  are  the  graywackes,  and  the  second  are  the  ordinary 
clay  slates  and  ^shyllite.  There  is,  however,  a  gradation  from  the  one  to 
the  other. 

GRAYWACKE. 

The  graywackes  consist  largely  of  grains  of  quartz  and  feldspar  of 
unquestionably  detrital  origin.  Associated  with  these  is  a  large  amount 
of  mica,  chlorite,  and  actinolite,  with  invariably  more  or  less  rutile.  This 
last  is  in  minute  grains  as  well  as  in  crystals.  Many  of  the  crystals  show 
fine  knee  twins,  triplets,  and  more  rarely,  heart-shaped  twins.     Tourmaline 

'  See  Part  II,  Chapter  IV,  Sec.  IV,  by  H.  L.  Smyth. 


I'ETROGRArillCAL  COAHACTEKS  OF  MANSFIELD  SLATE.  57 

is  sometimes  pirst'Ut.  The  feiTO-niagnesiau  minerals  dc-velop  cliiefly  from 
the  alteration  of  the  feldspar,  and  from  the  finer  detritns  which  is  jjresmiied  to 
have  existed  between  the  grains.  As  a  consequence,  the  secondary  minerals 
lie  between  the  original  grains.  Many  of  the  quartz  grains  are  enlarged, 
and  here  the  secondary  minerals  are  included  in  the  new  areas  of  the  enlarged 
grains.  In  numerous  cases  the  new  quartz  occupies  about  as  much  space 
as  the  original  grains  themselves.  This  shows  very  clearly  the  porous 
character  of  the  original  sandstone.  All  original  grains  of  the  rocks  show 
signs  of  extensive  mashing.  Some  specimens  contain  a  large  amount  of 
tourmaline  in  long  slender  crystals,  which  penetrate  both  the  feldspar  and 
the  quartz  grains.  The  presence  of  tourmaline  is  especially  interesting  as 
indicating  that  these  sedimentaries  may  have  been  subjected  to  a  certain 
amount  of  fumarole  action.  According  to  the  proportion  in  which  the 
vai-ious  minerals  have  developed,  we  obtain  sericite-,  actinolite-,  or  chlorite- 
schists  produced  from  the  graywackes. 

CLAY  SLATE  AND   PHYLLITE. 

The  clay  slates  are  dull  and  lusterless  and  are  black,  olive  green,  or 
red  in  color.  They  are  usually  impregnated  with  more  or  less  iron  pyrites 
in  large  macroscopical  crystals.  One  can  distinguish  in  them  quartz,  Avhite 
mica,  a  few  needles  of  actinolite,  rutile,  hematite,  with  a  small  proportion 
of  a  dark  ferruginous  and  carbonaceous  interstitial  material. 

The  amount  of  iron  which  these  clay  slates  contain  varies  considerably. 
In  some,  hematite  is  present  in  such  quantity  as  to  cause  the  slates  to  be 
appropriately  called  hematitic  slates.  Such,  for  instance,  is  the  one  forming 
the  foot  wall  of  the  Mansfield  ore  body.  The  iron  oxide  gives  to  the  slates 
a  very  bright  red  color  where  they  are  weathered.  Tliese  weathered 
hematitic  slates  are  very  commonly  known  in  the  district  as  red  slates,  or 
as  "paint  rock"  or  "soapstone,"  though  rocks  of  very  different  character  are 
also  at  times  designated  by  these  names. 

The  phyllites  have  a  silky  luster  and  a  bluish-black  color.  They  are 
composed  essentially  of  white  mica  quartz,  some  feldspar,  innumerable 
minute  crystals  of  rutile  and  dark  ferruginous  specks.  These  seem  to  differ 
from  the  rocks  called  here  clay  slates  only  in  that  they  are  more  completely 
crystalline,  the  interstitial  material  of  the  slates  having  disappeared. 


58  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

ORIGIN    OF   CLAY    SLATE   AND   PHYLLITE. 

The  origin  of  the  clay  slates  of  the  Mansfield  formation  is  probably  to 
be  looked  for  in  the  disintegration  and  decay  of  the  Archean  granite,  and 
the  subsequent  metamorphisra  of  the  resulting  clay.  For  between  the 
srranites  and  the  slates  no  other  rock  masses  are  known  to  have  existed  from 
which  the  clay  could  have  been  derived.  The  phylUtes  are  presumed  to 
have  resulted  from  the  metamorphism  of  the  clay  slates. 

PRESENT    COMPOSITION     NECESSARILY    DIFFERENT    FROM     THAT     OF     ROCK    FROM 

WHICH   DERIVED. 

It  is  a  well-recognized  principle  of  rock  weathering  that  in  the  altera- 
tion of  rocks  near  the  surface  of  the  earth  there  is  a  relatively  rapid 
diminution  in  the  quantity  of  the  more  soluble  constituents.  Hence  a  clay 
shows  a  lower  percentage  of  alkalies  and  alkaline  earths  than  is  found  in 
the  parent  rock,  with  an  increase  in  the  percentage  especially  of  alumina 
and  water.  This  relation  is  made  clear  by  Adams  in  a  statement  of  the 
comparison  of  the  composition  of  certain  slates  and  granites:^  "On  com- 
paring the  analyses  of  a  series  of  granites  with  those  of  a  series  of  slates, 
as,  for  instance,  those  given  in  Roth's  '  Gesteins  Analyzen,'  the  latter  are  seen 
to  be  on  an  average  considerably  higher  in  alumina  and  much  lower  in 
alkalies,  while  at  the  same  time  they  are  lower  in  silica,  which  has  been 
separated  both  as  sand  and  in  combination  with  the  alkalies  which  have 
gone  into  solution,  and  in  most  cases  contain  more  magnesia  than  lime 
instead  of  more  lime  than  magnesia,  as  is  usual-  in  granites."  Adams  con- 
cludes further,  after  a  comparison  of  the  alkalies  in  the  slates  and  granites, 
that  "The  slates  thus  contain  on  an  average  about  two-thirds  of  the  amount 
of  alkali  present  in  the  average  granite."^  An  examination  of  series  of 
analyses  of  granites  shows  that  while  the  percentages  of  soda  and  potassa 
vary  considerably,  now  the  one  being  predominant,  now  the  other,  on  the 
whole  in  the  typical  granites  the  potassa  is  higher  than  the  soda.'  This  is 
the  relation  which  we  would  expect  in  the  case  of  an  ideally  pure  granite. 


1  A  further  contribution  to  our  knowledge  of  the  Laurentiau,  by  F.  D.  Adams:  Am.  Jour.  Soi.,  3d 
aer.,  Vol.  L,  1895,  p.  65. 

"Loc.  cit.jp.  65. 

■^'Zirkel  states  that  iu  the  weathering  of  granites  the  soda  is  much  more  readily  removed  than 
is  the  potassa;  Lehrbuch  der  Petrographie,  Vol.  II,  1894,  p.  32. 


PETROGRAPDICAL  CnARACTERS  OF  MANSFIELD  SLATE. 


50 


ill  wliicli  no  aiiorthoclase  replaces  the  orthoclase.  As  a  consequence  of 
tlie  easier  solubility  of  the  soda,  this  relation  between  the  two  alkalies, 
soda  and  potassa,  is  maintained,  and  is  often  made  more  strildng  in  the 
clay  slates.  An  average  of  31  analyses  of  clay  slates  taken  from  various 
sources  shows  two  and  one-half  times  as  much  potassa  as  soda.  In  the  case 
of  the  Mansiield  slate  this  difiPerence  has  been  increased,  so  that  there  is 
ten  times  as  much  potassa  as  soda  present. 

ANALYSIS   OF   MANSFIELD   SLATE. 

Mr.  George  Steiger,  of  the  United  States  Geological  Survey,  has 
prepared  a  complete  analysis  (No.  1  in  the  following  table)  of  a  typical 
specimen  of  the  Mansiield  clay  slate.  Analyses  Nos.  2  and  3  were  pre- 
pared by  W.  Maynard  Hutchings,'  and  numbered  by  him  Nos.  2  and  5, 

respectively. 

Analysis  of  the  Mansfield  clay  slate. 


Constituent. 

1. 

2. 

3. 

SiO- 

60.28 

.69 

22.61 

2.53 

.45 

Trace. 

.13 

.04 

1.35 

5.73 

.54 

.60 

3.62 

.03 

None. 

.97 

59.28 

53.57 

TiO. 

Al,03 

21.85 

1      5.80 

24.53 
6.51 

FeaOa 

FeO 

MnO 

CaO  

.45 

.76 

BaO            .        . 

MeO 

1.24 
4.13 
1.18 

I      6.25 

1.81 
4.34 

.97 

7.65 

K,0 

NajO   

H.Q  at  100    

HiO  above  100^' 

PjO, 

CO.                       .            .  . 

c                      

Total  

99.57 

100. 18 

100. 12 

COMMENTS   ON    ANALYSIS. 

That  which  is  the  most  striking  about  the  analysis  is  the  relative  pro- 
portion of  the  alkaline  earths,  lime,  and  magnesia,  the  latter  being  present 

'Notes  on  the  composition  of  clays,  slates,  etc..  and  nn  sonip  points  in  their  contact  metamor- 
phism:  (ieol.  M.ag.,  Vol.  I,  1894,  p.  38. 


60  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

iu  the  greater  quantity.  As  a  rule,  in  all  of  the  igneous  rocks  (and  to 
the  igneous  rocks  all  clay  slates  owe  their  ultimate  origin),  except  in  the 
nonfeldspathic  ultrabasic  ones,  the  reverse  condition  exists,  namely,  the 
magnesia  subordinate  in  quantity  to  the  lime.  The  difiference  in  amount 
of  soda  and  potassa  is  very  striking  and  should  be  noticed,  in  view  of  cer- 
tain points  to  which  attention  will  be  called  in  subsequent  pages.  The 
percentage  of  alumina  is  higher  than  is  usital  in  the  clay  slates.  It  will  be 
noticed  that  considerable  water  is  present,  but  in  consideration  of  the  char- 
acter of  the  rock  this  is  to  be  expected.  If  anything,  the  value  is  rather 
lower  than  would  be  expected,  indicating  a  possible  loss  of  water  due  to  the 
rock  having  already  undergone  some  dynamic  action.  The  carbon  present 
is  considered  as  offering  trustworthy  evidence  of  the  presence  of  organic  life 
at  the  time  of  the  deposit  of  the  slates,  though  no  more  satisfactory  evidence 
of  the  existence  of  life  has  been  found. 

COMPARISON  OF  ANALYSIS  OF  MANSFIELD  CLAY  SLATE  WITH  ANALYSES  OF  CLAYS. 

During  the  last  few  j^ears  there   have  appeared   in   the    Geological 

Magazine,  from  the  pen  of  Mr.  W.  Maynard  Hutchings,  some  very  elaborate 

and  suggestive  articles  upon  the  composition  of  clays,  shales,  and  slates, 

and  from  one  of  these  ^  I  have  taken  two  analyses  of  Carboniferous  clays 

for  comparison  with  the  Mansfield  clay  slate.     These  two  analyses,  Nos.  2 

and  3,  p.  59,  are  from  the  very  fine  grained  clays,  in  which  the  quartz  was 

not  distinguishable  with  the  microscope,  and  are  the  analyses  showing  the 

highest   and   lowest   percentages    of   silica.     Mr.    Hutchings    says    of   his 

analyses  that  the  samples  were  di-ied  at  220°  F.,  and  that  the  titanic  oxide 

was  not  determined  but  is  contained  in  the  silica  and  alumina.     Concerning 

the  clays,  he  writes: 

From  these  analyses  it  will  be  seeu  that  these  clays  would  be  capable,  chemically 
considered,  of  transformatiou  iuto  very  typical  "  clay-slates."  Mineralogically  they 
are  clay-slates,  having  already  undergone  all,  or  nearly  all,  the  mineral  changes 
requisite  to  constitute  the  normal  (unaltered)  slates.  Nothin.g  more  is  needed  but 
physical  changes,  such  as  compacting,  arrangement  of  mica  iu  a  plane,  increase  of 
size  of  mica,  etc.^ 

The  great  similarity  of  these  clays  with  the  Mansfield  clap  slate  is  very 
evident.     The  only  material  difference  which  exists  between  them  is  in  the 

'Notes  on  the  compositiou  of  clays,  ."ilates,  etc.,  autl  on  some  points  iu  tlieir  contact  metamor- 
phism,  by  W.  Maynard  Hutchings :  Geol,  Mag.,  Vol.  1, 1894,  p.  38. 
•Loc.  cit.,  p.  38. 


rETllOGUAPUlOAL  CHAKACTERS  OF  MANSFIELD  SLATE. 


61 


liiyhor  pLTcentiii^-e  of  water  (•(tntaiued  in  tliu  clay.s.  This  difference  is 
natural,  clays  iisuall}'  containing-  about  twice  as  much  water  as  do  the 
slates. 

COMPARISON  OF  ANALYSIS  OF  MANSFIELD  CLAY  SLATE  WITH  ANALYSES  OF  OTHER 

CLAY  SLATES. 

In  tlie  following  talkie  there  are  given,  for  purposes  of  comparison  with 
the  Mansiield  clay  slate,  analyses  of  typical  clay  slates,  roofing  slates  from 
the  Cambrian  of  Vermont  and  New  York. 

Analyses  of  tyjncal  clay  slates. 


Constituent. 


SiO^  

TiO, 

AUG, 

Fe.20, 

FeO 

MnO 

CaO  

BaO  

MgO 

KjO 

NasO 

H,0  at  100^  ... 
H2O  above  100^ 
P2O5 

CO; 

FeS, 

C 


Total 


60.28 

.69 

22.61 

2.53 

.45 

Trace. 

.13 

.04 

1.35 

5.73 

.54 

.60 

3.62 

.03 

None. 


.97 


99.57 


62.37 

.74 

15.43 

1.31 

5.34 

.22 

.77 

.07 

3.14 

4.20 

1.14 

.34 

3.71 

.06 

.87 

.06 

Trace. 


3. 


59.70 

.79 
16.98 

.52 
4.88 

.16 
1.27 

.08 
3.23 
3.77 
1.35 

.30 
3.82 

.16 
1.40 
L18 

.46 


99.80 


100. 05 


67.61 

.56 

13.20 

5.36 

1.20 

.10 

.11 

.04 

3.20 

4.45 

.67 

((.45 

ft  2.  97 

.05 

None. 

.03 


100.00 


67.89 

.49 

11.03 

L47 

3.81 

.16 
1.43 

.04 
4.57 
2.82 

.77 

a.  36 

6  3.21 

.10 
1.89 

.04 


100. 08 


aHjO  at  110-. 


b  U2O  above  110^. 


No.  1.  Black  slate,  Bp.  32497,  N.  450,  W.  1620,  sec.  17,  T.  43  N.,  R.  31  W.,  Michigan.  Analyzed  by 
George  Steiger. 

No.  2.  Sea-green  slate,  Griffith  &  Nathaniel  Quarry,  South  Poultney,  Vermont.     W.  F.  Hillebraud. 

No.  3.  Black  slate,  American  Black  Slate  Company,  Benson,  Vermont.     W.  F.  Hillebraud. 

No.  4.  Red  slate,  three-fourths  mile  south  of  Hampton  Village,  New  York.     W.  F.  HiUebrand. 

No.  5.  Cireen  slate,  three-fourths  mile  northwest  of  Janesville,  Washington  County,  New  York. 
W.  F.  Hillebraud. 

Nos.  2,  3,  4,  and  5  taken  from  Analyses  of  rocks  and  analytical  methods,  1880-1896,  Clark  and  HiUe- 
brand: Bull.  U.  S.  Geol.  Survey,  No.  148.  Nos.  2  and  3  are,  respectively,  C  and  F,  p.  277,  aud  Nos.  4  and  5 
are  A  aud  D,  p.  280. 


62  THE  CEYSTAL  FALLS  lEOK-BEAElNG  DISTEICT. 

The  strong  similarity  between  the  composition  of  these  clay  slates  is 
at  once  apparent,  and  needs  no  further  comment.  The  only  marked  differ- 
ence between  the  Huronian  clay  slate  and  the  Cambrian  ones  is  the  higher 
percentage  of  alumina  present  in  the  former. . 

SIDERITE-SLATE,  CHERT,   FERRUGINOUS    CHERT,  AND    IRON    ORES. 

The  two  most  interesting  kinds  of  rock  from  the  Mansfield  slate  belt 
are  those  known  as  the  siderite-  or  sideritic  slates  and  the  cherts  or  ferrugin- 
ous cherts,  according  to  the  quantity  of  iron  carbonate  and  iron  oxide 
present.  These  alternate  with  each  other,  and  are  found  also  interstratified 
with  the  fragmental  slates,  and  thus  there  can  be  no  question  as  to  their 
sedimei^tary  character.  The  siderite-slates  are  of  a  light  to  dark  gray  color. 
They  are  well  laminated,  and  in  some  places  cleave  rather  readily  along 
the  laminae,  though  at  other  places  they  break  with  an  almost  conchoidal 
fracture.  The  weathered  siderite  slates  are  covered  by  a  crust  of  reddish- 
brown  hydrated  iron  sesquioxide. 

Microscopically  the  siderite  slates  are  composed  of  siderite,  or  of  sider- 
ite and  exceedingly  fine  grained  cherty  silica.  Roundish  rhombohedra  of 
siderite  compose  the  purer  sideritic  portions.  If  one  passes  from  the  pure 
to  the  less  pure  slates,  the  siderite  gradually  diminishes  in  quantity,  the 
silica  grains  increase  correspondingly,  and  the  rock  grades  into  the  chert 
which,  in  bands,  is  commonly  associated  with  iron  carbonate  in  the  Lake 
Superior  region.  As  the  carbonate  alters  to  the  oxide  or  hydrated  oxide 
ferruginous  cherts  are  produced.  The  cherts  are  white  to  red,  depending 
on  the  amount  of  iron  oxide  present.  The  manner  in  which  the  siderite 
alters  to  limonite  and  hematite,  and  the  various  steps  of  the  process  have 
been  so  well  described  and  beautifully  illustrated  in  Monograph  XXVIII, 
that  the  reader  is  referred  to  that  volume  for  further  information.  None  of 
the  brilliant  red  jasper  or  jaspihte,  such  as  that  found  in  the  Marquette 
district,  is  associated  with  the  Mansfield  slates.  Iron  ores  of  economic 
importance,  however,  are  found  associated  with  these  slates,  and  are 
described  in  detail  farther  on.  None  of  the  sideritic  slates,  ferruginous 
cherts,  or  ores,  although  interbedded  with  the  fragmental  slates,  show  any 
evidence  of  fragmental  origin  so  far  as  the  indi\'idual  grains  of  the  minerals 
composing  them  are  concerned. 


PETKOGUAPIIICAL  CUAHAGTEliS  OF  xMANSFIELD  SLATE.  63 

RELATIONS   OF    SIDERITE-SLATE,   FERRUGINOUS  CHERT,   AND  ORE   BODIES  TO 

CLAY   SLATES. 

Owing-  to  the  scarcity  of  tlie  outcrops  of  the  sedinientaries  in  thu  i\Iaus- 
tic'ld  Valley,  it  is  practically  impossible  to  decipher  the  relations  of  the 
individual  lieds.  Neither  the  study  of  the  surface  exposures  nor  the  expo- 
sures in  the  mine  workings  liave  given  definite  results.  Tliat  the  heds  repre- 
sent interbedded  strata  is  well  understood,  but  the  sequence  of  the  strata  is 
indeterminable.  It  is  of  especial  interest  to  determine,  so  far  as  possible, 
the  relations  of  the  ferruginous  rocks,  in  order  that  the  possible  iron-ore 
deposits  associated  with  theni  may  be  found.  A  cross  section  through  the 
Mansfield  mine  from  east  to  west  shows  the  following  relations:  The  foot- 
wall  of  black  heinatitic  slate  is  overlain  by  25  to  30  feet  of  fernaginous 
chert  and  iron  ore.  This  stratum  is  succeeded  by  "red  slate,"  so  called  by 
the  miners,  which  is  probably  weathered  greenstone  impregnated  with  iron. 
This  is  followed  b}'  a  conglomerate,  and  this  by  amygdaloidal  greenstone, 
of  the  overlying  volcanic  formation.  The  ore  body  extends  north  and 
sovith,  agreeing  thus  with  the  strike  of  the  slates.  All  drifts  end  on  the 
north  in  mixed  ore,  and  on  the  south  in  mixed  ore,  with  "quartz-rock"  and 
"lime-rock"  of  the  miners  in  some  places.  From  these  facts  we  may  justly 
conclude  that  the  ore-bearing  ferruginous  cherts  exist  in  beds  in  the  slates 
or  as  lenticular  masses  which  agree  in  dip  and  strike  with  the  surrounding 
slates.  This  conclusion  is  confirmed  b}-  test  pits  along  the  strike  of  the 
exposed  beds,  which  have  disclosed  similar  ferruginous  cherts  at  various 
places  for  a  distance  of  half  a  mile  to  the  north. 

RKLATIONS  OF  MANSFIEIiD   SLATE  TO  ADJACENT  FORMATIONS. 

RELATIONS  TO   INTRUSIVES. 

The  Mansfield  slates  are  surrounded  on  three  sides— east,  north,  and 
south — by  coarse-grained  basic  eruptive  rocks.  The  fact  that  the}'  are  so 
surrounded  by  these  rocks,  which  cut  them  off  in  the  direction  of  their 
strike,  points  to  the  later  origin  of  these  eruptives.  Moreover,  the  quartzitic 
character  of  some  of  the  sedinientaries  shows  that  they  could  not  have  been 
derived  from  the  eruptives  which  stratigraphically  underlie  them,  for  iu 
these  no  quartz  is  found.  The  quartzitic  character  would  thus  seem  also 
to   indicate   that  the  slates  are  older  than  the  intrusives.     Wherever  the 


64  THE  CEYSTAL  FALLS  lEON-BEAEING  DISTEICT. 

igneous  rocks  and  slates  are  in  contact  or  in  close  association,  the  latter 
have  been  metainorpliosed,  and  adinoles,  spilosites,  and  desmosites  have  been 
formed  which  are  similar  to  those  described  as  occurring  in  other  areas 
along  the  contact  zone  of  basic  intrusives.  Although  no  single  instance  of 
a  dike  penetrating  the  slates  has  been  found,  it  can  hardly  be  doubted  from 
the  relations  which  have  been  outlined  that  the  slates  are  older  than  the 
intrusive  dolerites. 

RELATIONS   TO   VOLCANICS. 

The  sedimentaries  are  overlain  by  volcanics,  both  lava  flows  and  tufa- 
ceous  deposits.  In  these  tuff's,  at  the  northeast  corner  of  sec.  7,  T.  43  N., 
R.  31  W.,  angular  black-slate  fragments  have  been  found  similar  in  every 
respect  to  the  slates  of  the  Mansfield  belt.  From  this  it  is  clear  that  at 
least  some  of  the  volcanics  are  younger  than  part  of  the  slate  formation. 
In  section  29  similar  relations  obtain,  the  <»nly  difference  being  that  the 
masses  of  slate  and  graywacke  are  inclosed  in  rather  larger  fragments  in  a 
volcanic  conglomerate,  and  still  retain  very  closely  their  normal  strike.  In 
the  conglomerates  near  the  Mansfield  mine  are  found  chert  fragments  and 
in  some  places  fragments  of  iron  oxide.  These  latter  were  evidently  not 
included  as  oxide,  but  as  fragments  of  cherty  carbonate.  Like  the  great 
mass  forming  the  ore  body,  the  fragments  have  since  their  deposition  been 
altered,  forming  iron-oxide  bodies  of  small  size.  Further  discussion  of  the 
relations  between  the  volcanics  and  slates  will  be  found  under  the  heading 
"  Hemlock  formation." 

STRUCTURE   OF  THE  ]MA]SrSFIELD  AREA. 

It  has  already  been  seen  that  the  Mansfield  rocks  strike  north  and 
south  and  have  a  high  westerly  dip.  The  two  possible  explanations  of  this 
structure  which  are  compatible  with  the  facts  in  other  portions  of  the  area 
are  (1)  that  they  form  a  westward  dipping  monocline,  and  (2)  that  they  are 
the  western  limb  of  an  anticline. 

THICKNESS. 

As  the  sedimentaries  forming  the  Mansfield  belt  now  dip  west  at  a  very 
high  angle,  and  as  there  is  no  evidence  of  duplication  of  strata  due  to  fold- 
ing, I  feel  comparatively  safe  in  giving  an  estimate  of  their  thickness.  The 
belt  is  widest  at  the  north  end,  and  there  has  a  breadth  of  about  1,950  feet. 


THICKNESS  OF  MANSFIELD  SLATF. 


65 


Till-  i'.vi'i-aj^-o  dip  of  tliu  beds  is  80^,  and  this  gives  a  maxiumm  thickuess 
(if  l,!lO()  feet.  Toward  the  south  the  belt  rapidly  narrows,  initil  it  is  cut 
out  l)y  tile  intruding  (hilerites.  A  thickness  of  1,500  feet  is  probably  not 
tar  from  the  average. 

To  the  east  of  the  ^Mansfield  slates  is  a  belt,  varying  in  width  up  to 
about  1,200  feet,  in  which  are  found  large  masses  of  metamorphosed  slates, 
surrounded  by  intrusive  dolerite.  In  this  belt  the  slate  masses  still  show 
a  general  north-south  strike,  with  slight  variations  to  the  east  or  west,  and  a 
westward  dip.  One  might,  perhaps,  consider  this  a  slate  area  which  has 
been  ciHn])letely  saturated  with  intrusives.  If  it  should  be  so  considered, 
this  thickness  should  lie  added  to  the  estimated  thickness  of  the  slates  as 
above  given,  but  as  intrusives  predominate  in  it,  the  slate  being,  as  it  were, 
merely  incidental,  I  have  preferred  not  to  include  it  in  the  belt  with  the  slate. 

t)RK    DEPOSITS. 

Although  a  great  deal  of  exploring  for  iron  ore  has  been  done  in  the 
Mansfield  slates,  only  one  lai-ge  body  of  ore  has  thus  far  been  discovered,  in 
which  is  the  Mansfield  mine.  This  mine 
is  situated  on  the  west  liank  of  the  Michi- 
gamme  River,  in  sees.  17  and  20,  T.  43 
N.,  E.  31  W.  The  mine  was  apparently 
prospering  wlien,  on  the  nigdit  of  Septeni- 
ber  28,  1893,  a  cave-in  occurred,  letting 
in  the  waters  of  the  jMichigamme  River 
and  drowning'  28  miners. 


N 


MAI,N 

SHAFT 


For  two  Lours  after  the  caving  occurred, 
the  bed  of  the  river  below  the  mine  was  bare, 
the  water  tlowing  into  the  mine  worlviugs. 
The  accompanying  tigure,  fig.  C,  prepared  by  J. 
Parke  Channing,  October  8,  1893,  shows  the 
relative  position  of  the  shaft  and  the  river,  and 
the  couceutric-  cracks  caused  by  the  caving  of 
the  mine.  (Plan  copied  from  address  of  presi- 
dent:   Proc.   Lake  Superior  Inst.  Min.   Eng., 

Vol.  Ill,  1895,  plate  opposite  p.  42. )      The  timber  shaft  is  near  the  center  of  tliese 
cracks. 

After  the  caving  the  mine  remained  idle  until  recently.  At  tlie  present 
writing  the  DeSoto  Mining  Company  has  obtained  control  of  tlie  mine  and, 
I  understand,  have  freed  it  from  water. 

MON   XXXVI 5 


Fig.  6.  -  Concentric  cracks  formed  by  the  caving  in 

of  tlio  5I.iiiatield  mine. 


6(3  THE  CRYSTAL  FALLS  IKOXBEARING  DISTRICT. 

Ill  May  tbey  began  the  task  of  diverting  the  cbauuel  of  the  river  to  a  point  several 
hundred  feet  sonth  of  the  old  course.  They  have  dredged  out  a  cut  2,050  feet  in 
length  by  100  feet  wide  and  18  feet  deep.  At  the  upper  end  of  the  new  channel  a 
cofferdam  containing  14,000  cubic  yards  of  earth  has  been  constructed,  and  where  the 
waters  join  the  old  outlet  several  hundred  feet  below  the  mine  another  embankment 
has  been  constructed  across  the  course  of  the  old  bed  that  has  8,000  cubic  yards  of 
earth.  This  task  was  a  very  expensive  one,  and  it  has  been  well  completed,  the  old 
channel  being  perfectly  dry. 

The  turning  of  the  river's  course  brings  out  with  startling  distinctness  the  criminal 
negligence  or  carelessness  of  those  who  were  working  the  mine  at  the  time  of  the 
accident.  The  upper  tier  of  timbers  in  the  mine  are  plainly  seen,  as  also  the  ground 
that  had  been  cut  out  to  receive  the  set  that  was  being  gotten  into  place  when  the 
waters  broke  through.  This  shows  the  miners  had  worked  up  to  within  12  feet  of 
the  water  of  the  river.  A  great  crack  in  the  formation  shows  where  the  water  first 
gained  entrance.  The  ore  made  up  the  bed  of  the  stream — was  a  portion  of  the  bed 
in  fact — and  the  walls  of  the  mine  were  nearly  vertical.  The  ore  deposit  had  a  width 
of  about  20  feet.  The  water  pressure  must  have  been  considerable,  and  the  blasting 
of  the  ore  (as  it  is  hard,  and  explosives  are  needed  to  loosen  it)  shattered  the  thin 
piotection  over  the  miners,  permitting  the  water  to  find  ready  and  unimpeded 
entrance  Into  the  mine.  An  engineer  could  not  have  been  employed  and  the  wildest 
sort  of  guessing  must  have  been  done  by  those  who  had  the  work  in  charge. 
No  sane  man  would  have  permitted  the  opening  of  the  deposit  so  near  the  river's 
bottom." 

Owing  to  the  long  abandonment  of  the  mine,  the  dii'ect  sources  of  infor- 
mation have  been  closed.  For  a  description  of  the  ore  body  I  am  com- 
pelled to  rely  on  such  data  as  are  available  from  existing  notes  and  plats. 
I  am  especially  indebted  to  a  manuscript  description  of  the  mine  by 
J.  Parke  Channing,  and  to  Mr.  C.  T.  Roberts,  of  Crystal  Falls,  for  plats  of 
the  mine.  The  sketch  of  the  mine  here  introduced,  PI.  IX,  is  compiled 
from  an  original  drawing  of  J.  Parke  Channing,  reproduced  on  the  plate 
cited,  and  from  data  obtained  from  other  sources. 

GENERAL   DESCRIPTION   OF  MANSFIELD   MINE  DEPOSIT. 

The  Mansfield  mine  has  au  ore  body  varying  from  16  to  32  feet  in 
width.  It  is  in  ahnost  vertical  position ;  it  has  well-defined  foot  and  hang- 
ing walls  composed  of  impervious  rock;  it  has  a  somewhat  indefinite  longi- 
tudinal extent.  The  ore  is  Bessemer  and  occurs  in  an  iron-bearing  formation, 
which  corresponds  in  every  particular  to  those  of  the  other  iron-bearing 

'  Report  of  Commissioner  of  Mineral  Statistics  of  Michigan,  George  A.  Newett,  for  1896,  p.  84. 


::^^^g«ftas»«g?g»- ■=»;«<:  r- 


feK^^>^.^, 


5    il 


ORE  DEPOSITS  IX  MANSFIELD  SLATE.  67 

districts  ot"  the  Lake  Superioi-  region.  'I'lie  ore  was  iirst  toiuid  in  a  test  pit 
wliicli  passed  through  0  feet  of  drift.  The  main  working'  shaft  was  then 
located  about  100  feet  west  of  this  j)oiut.  It  was  put  down  to  a  deptli  of 
4()0  feet  before  ore  was  struck.  From  this  shaft  crosscuts  were  driven  east 
at  average  intervals  of  70  feet,  and  the  ore  body  was  met  at  a  distance  vary- 
ingf  from  74  feet  at  the  first  level  to  10  feet  at  the  sixth  level.  The  cross- 
cuts,  in  every  case  after  leaving  the  greenstone,  pass  through  so-called  red 
slate,  at  the  maximum  about  25  feet  thick,  before  ore  is  reached,  this  rock 
constituting  the  hanging  wall.  From  these  data  the  dip  of  the  ore  body 
may  be  calculated  to  be  about  80°  W.,  agreeing  well  with  the  observed  dip 
of  the  slates,  which  outcrop  over  the  area.  The  thickness  of  the  ore,  as 
shown  by  the  cross  sections,  averages  about  25  feet.  The  extreme  variation 
in  thickness  ranges  from  two  sets,  or  16  feet,  to  four  sets,  or  32  feet.  The 
strike  of  the  slates  is  north  and  south,  and  the  trend  of  the  ore  body 
agrees  with  this.  This  brings  its  southern  end  under  the  original  course 
of  the  Michigamme  River  as  the  stream  bends  slightly  to  the  west,  south 
of  the  shaft.  An  examination  of  the  longitudinal  (north-south)  section 
through  tlie  ore  body  does  not  determine  whether  or  not  it  has  a  pitch. 
The  southern  boundary  is  nearly  vertical  from  top  to  bottom,  while  the 
northern  boundary  lengthens  about  140  feet  between  the  first  and  the  fifth 
levels. 

In  the  northern  end  of  the  mine — that  is,  in  line  with  the  strike  of  the 
sedimentaries — the  ore  body  terminates,  in  a  more  or  less  irregular  Avay,  in 
so-called  mixed  ore.  This  mixed  ore  continues  to  the  north  for  over  half  a 
mile,  as  shown  by  the  numerous  test  pits  which  have  been  bottomed  in  it. 
To  the  south  of  the  mine  shaft  the  ore  body  proper  extends  for  200  feet.  It 
then  changes  its  character,  becoming  a  lean  non-Bessemer  ore.  A  long  drift 
(335  feet)  at  the  second  level  was  run  through  this  ore,  and  after  leaving  it 
penetrated  a  mixed  ore,  the  so-called  lime  rock  (sideritel)  and  quartz  rock 
(chert?)  of  the  miners.  Three  crosscuts  along  this  drift  show  the  ore  body 
to  vary  from  20  to  30  feet  in  thickness,  with  the  same  foot  and  hanging  wall 
as  for  the  remainder  of  the  mine.  The  same  condition  exists  also  lower 
down,  as  shown  by  a  drift  from  the  fourth  level,  260  feet  south.  The 
figures  on  PL  IX,  giving  longitudinal  and  cross  sections  of  the  mine,  show 
clearly  the  dimensions  of  the  ore  body. 


68  THE  CRYSTAL  FALLS  lEON-BEARING  DISTRICT, 

RELATIONS  TO   SURROUNDING  BEDS. 

The  foot  wall  of  the  ore  is  a  black  slate,  described  as  being  rich  in 
hematite  and  liearing  large  crystals  of  iron  pyrite.  No  crosscuts  have 
been  driven  for  any  distance  into  the  foot  wall,  so  that  it  is  impossible  to 
say  what  thickness  of  the  hematitic  black  slate  there  may  be  before  the 
greenish  pvritiferous  slate  begins.  In  places  a  gray  "soapstone"  takes  the 
place  of  the  black  slate  as  the  foot  wall. 

The  dump  obtained  by  sinking  the  shaft  in  the  material  overlying  the 
ore  shows  lai-ge  masses  of  conglomerate,  the  pebbles  of  which  are  rounded 
and  predominantly  of  volcanic  rocks,  with  pebbles  of  chert  and  slate  from 
the  iron  formation  and  slates  below.  These  fragments  are  well  rounded. 
The  microscope  also  shows  quartz  grains  with  secondary  enlargements,  so 
that  there  can  be  no  doubt  that  the  rock  is  a  true  conglomerate.  Similar 
conglomerates,  except  that  the  sedimentary  fragments  are  w^anting,  have 
been  noticed  farther  north  along  the  west  side  of  the  river.  Just  west  of 
the  Ijridge  at  IMansfield,  near  the  mine,  there  is  also  a  small  exposure  of  con- 
glomerate, which  shows  an  alternation  of  coarse  and  fine  sediments,  with  a 
strike  nearly  north  and  south,  and  a  dip  of  80^  W.  To  the  west,  above  this 
conglomerate,  and  not  more  than  15  to  20  feet  distant,  are  found  the  lavas 
of  the  Hendock  volcanics.  According  to  the  mine  captain,  the  succession 
west  from  the  ore  body  in  the  hanging  wall  is  20  to  25  feet  of  paint  rock, 
or,  as  it  is  usually  called,  red  slate,  then  conglomerate,  then  greenstone. 
It  is  difficult  to  diagnose  the  paint  rock,  as  no  specimens  are  to  be  had,  but 
it  is  highly  probable  that  it  is  a  ferruginous  and  extremely  altered  lava 
sheet.  Similar  rocks  are  commonly  found  thus  altered  in  association  with 
the  ores  in  the  Penokee-Gogebic  and  Marquette  districts.  Lending  weight 
to  this  conclusion  is  the  fact  that  in  some  places  an  amygdaloidal  green- 
stone has  been  exposed  in  test  pits  immediately  above  the  iron-bearing 
formation. 

COMPOSITION  OF  ORE. 

The  Mansfield  mine  up  to  the  present  time  has  raised  only  Bessemer 
ore,  and  is  the  only  mine  in  the  Crystal  Falls  district  which  has  supplied 
any  considerable  quantity  of  ore  of  this  character.  An  average  of  a  num- 
ber of    analyses  gives  the  following  composition  for  the  Bessemer  ore: 


OllE  DEPOSITS  IN  MANSFIELD  SLATE.  69 

]\lft;illir   iron,    04.80;   pliospliorus,   0.037;    .silica,   3.70.'     According-  to   Dr. 
N.  P.  Ilulst,- those  ore  deposits  in  tlie  Menominee  range  which  have  ijoorlv 
defined  walls  carry  a  uiininiuni  of  phosphorus.     This  body,  however,  .shows 
that  the  same  conditions  do  not  exist  at  the  Mansfield  mine,  since,  while  it 
has  both  sharply  defined  foot  and  hanging  walls,  it  contains  ])ut  a  low 
l)er  cent  of  phosphorus.     P'rom  an  e.xamination  of  the  analyses  from  which 
the  aliove  average  was  obtained  I  find  that  the  percentage  of  phosphorus 
shows  a  marked  increase  in  the  lower  levels  of  the  mines  over  that  of  the 
higher,  and  there  is  also  a  slight  corresponding  decrease  in  the  content  of 
metallic  iron.      Increase  of   phosphorus  with   depth  is  also  found  in  the 
adjoining  Menominee  range,  as  noted  by   Messrs.   E.   F.   Brown, ^  of  '^he 
Pewabic  mine,  and  Per  Larsson,^  of  the  Aragon.     It  is  impossible  to  state 
whether  or  not  this  distribution  is  due  to  the  action  of  percolatino-  water,  as 
suggested  by  Hulst,"  Larsson,*'  and  other  Michigan  mining  engineers.     Onlv 
a  large  number  of  good  analyses  from  carefully  selected  ores  and  asso- 
ciated rocks  and  a  detailed  study  of  conditions  of  occurrence  could  lead  to 
any  accurate  determination  of  the  reason  for  such  distribution,  and  a  dis- 
cussion of  these  reasons  is  by  no  means  warranted  by  the  few  and  imper- 
fect analyses  of  the  Mansfield  ores,  which  I  have  been  able  to  obtain.     The 
ore  body  changes  in  composition  to  the  south  of  the  shaft,  as  shown  by  the 
drifts  in  this  direction.     The  ore  in  this  part  of  the  mine  contains  more  phos- 
phorus, alumina,  and  calcium,  and  less  iron.     This  low-grade  lean  ore  then 
passes  over  into  the  banded  chert  and  ore  nnxed  with  the  lime  and  quartz 
rock  mentioned  above. 

MICROSCOPICAL  CHARACTER  OF  THE  ORES  AND  ASSOCIATED  CHERT  BANDS. 

The  ore  varies  from  a  soft  limonitic  hematite  to  a  moderately  hard 
hematite.  It  is  for  the  most  part  opaque  under  the  microscope,  but  in 
places  shows  bright-red  to  brownish-red  color  in  transmitted  li(i-ht  In 
incident  light  the  ore  for  the  most  part  shows  a  dull-brown  or  reddish  color, 
though  in  places  it  has  a  bright  metallic  reflection.     In  places  in  the  ore 

'  An  average  of  62  per  cant  metallic  irou  and  .030  per  cent  phosphorus  is  reported  in  Report  of 
Commissioner  of  Mineral  Statistics  of  Michigan  (G.  A.  Newett)  for  1896,  p.  85. 

•The  geology  of  that  portion  of  the  Menominee  range  east  of  the  Menominee  River,  by  X.  P. 
Hulst:  Proc.  Lake  Superior  lust.  Min.  Eng.,  Vol.  I,  1893,  p.  28. 

'Distribution  of  phosphorus  and  system  of  sampling  at  the  Pewabie  mine.  Irou  .Mountain,  by 
E.  F.  Brown :  Proc.  Lake  Superior  Inst.  Min.  Eug.,  Vol.  Ill,  1895,  p.  49. 

*0p.cit.,p.52.  ^0p.cit.,p.28.  '0p.cit.,p.53. 


70  THE  CRYSTAL  FALLS  IRON-BEAEIXG  DISTRICT. 

are  spots,  in  wliicli  is  a  large  quantity  of  chert  mixed  with  iron  oxide.  As 
such  ferruo-inous-chert  areas  increase  in  quantity  the  ore  grades  into  the 
ferruf>-inous  chert  and  chert  which  is  found  associated  with  it  in  bands  and 
lenticuLir  areas. 

ORIGIN    OF    THE    ORE    DEPOSITS. 

The  mode  of  occurrence  and  general  characters  of  the  ore  body  hav- 
ino-  been  described,  we  are  now  prepared  to  determine  the  cause  of  concen- 
tration of  the  iron  at  this  particular  point  and  the  source.  From  the 
description  it  was  seen  that  the  appearance  of  the  body  of  ore  was  that  of 
a  bedded  deposit.  The  microscopical  examination  shows,  however,  that 
the  ore  presents  no  evidences  of  clastic  origin.  An  examination  of  the 
cherts  and  rocks  of  the  area  which  are  interbedded  with  the  ore,  and 
also  a  studv  of  the  southern  contact  of  the  ore  body,  shows  that 
the  ore  is  a  chemical  deposit,  or  the  result  of  a  replacement  process, 
by  which  the  original  rock  was  largely  removed,  and  its  place  taken 
by  the  present  ore.  It  has  been  shown  (p.  62)  that  the  siderite  bands 
pass  into  hematitic  and  limonitic  chert  bands.  It  has  been  seen  that  in 
the  southern  end  of  the  mine  the  lean  ore  merges  into  a  mass  of  ore  bedded 
with  chert  and  mixed  with  a  rock  called  by  the  miners  lime  and  quartz 
rock.  I  interpret  this  rock  to  be  banded  siderite  and  chert,  possibly  with 
some  quartzite  bands,  all  of  which  are  found  outcropping  at  the  surface. 
The  siderite  evidently  has  been  changed  into  iron  oxide  and  the  silica 
replaced  by  iron  oxide,  the  banding  of  the  original  rock  not  having  been 
destroyed  thereby.  Irving^  considered  siderite  to  be  the  source  of,  similar 
ore  and  associated  chert  and  jasper.  Van  Hise=  has  fully  explained  the 
process  of  the  concentration  of  the  ores  of  the  Penokee-Gogebic  and 
Marquette  districts,  and  has  applied  the  explanation  to  the  other  districts 

'  Origin  of  the  ferruginous  schists  and  iron  ores  of  the  Lake  Superior  region,  by  R.  L).  Irving : 
Am.  Jour.  Sci.,  3d  series,  Vol.  XXXII,  1886,  pp.  255-272. 

-The  iron  ore  of  the  Marquette  district  of  Michigan,  by  C.  R.  Van  Hise:  Am.  Jour.  Sci.,  3d  series, 
Vol.  XLIII,  1892,  pp.  116-132. 

Iron  ores  of  the  Penokee-Gogebio  series  of  Michigan  and  Wisconsin,  by  C.  R.  Van  Hise:  Am. 
Jour.  Sci.,  3d  series,  Vol.  XXXVII,  1889,  pp.  32-48. 

The  Penokee  iron-bearing  series  of  Michigan  and  Wisconsin,  by  R.  D.  Irving  and  C  R.  Van  Hise, 
Tenth  Ann.  Rept.  U.  S.  Geol.  Survey,  Part  I,  1890,  pp.  341-507. 

The  Penokee-Gogebic  iron-bearing  series  of  Michigan  and  Wisconsin,  by  R.  D.  Irving  and  C.  R. 
Van  Hise:  Mou.  U.  S.  Geol.  Survey,  Vol.  XIX,  1892,  pp.  245-290. 

The  Marquette  iron-bearing  district  of  Michigan,  by  C.  R.  Van  Hise  and  W.  S.  Bayley,  with  a 
chapter  ou  the  Republic  trough,  by  H.  L.  Smyth :  Mon.  U.  S.  Geol.  Survey,  Vol.  XXVIII,  1897,  pp.  400-405. 


ORK  DEPOSITS  IN  MANSFIELD  SLATE.  71 

in  tlio  Liiko  Suporior  ivg-ion.  I  shall  not  do  more,  therefore,  than  to  add 
that  the  investij^ations  in  this  area  have  sliown  the  probable  correctness  of 
this  explanation. 

It  is  very  interesting-  from  an  historical  standpoint  to  note  that  as  far 
back  as  1868  Credner  had  made  the  suggestion,  with  reference  especially  to 
the  ^Marquette  district,  tliat  the  ores  were  derived  from  an  original  iron 
carbonate.  The  following-  quotation  will  show  his  idea  of  the  processes  of 
development  of  the  ore:^ 

Spliaerosiderit  wurde  aus  kohlensjiurereichen  Gewiisseu  abgesetzt,  duioh  eine 
theilweise  Oxydatiou  desselbeu  eiitstaud  Magneteisenstein,  durch  weitere  Anfnalime 
von  Sanerstoff'  das  Gemenge  vou  Magneteisenstein  und  liotbeisensteiu  uiid  endlich 
reiner  Eotlieiseiisteiu;  aus  diesem  sporadisch  durch  Zntritt  von  Wasser  Brauneisen- 
stein. 

Credner's  suggestion  seems  to  have  been  lightly  considered  by  other 
workers  in  that  area.  In  1886  Irving-^  suggested  the  theory  of  replacement 
of  an  original  fen-uginous  carbonate  to  explain  the  Penokee-Gogebic  iron 
ores.  This  theory  has  since  then  lieen  elaborated  by  Van  Hise,  and  shown 
to  have  a  wider  application  to  the  other  Lake  Superior  ore  districts.  He 
has  also  traced  the  iron  to  its  source  in  the  rocks  removed  by  denudation, 
and  shows  why  it  occurs  in  the  positions  in  which  the  ore  bodies  are  at 
present  found  to  occur.  Moreover,  Van  Hise  has  also  explained  the  process 
of  development  in  detail,  and,  what  is  perhaps  far  more  important,  the 
reason  certain  ores  develop  and  not  others.  In  its  essentials,  however,  the 
process  is  the  same  as  that  suggested  by  Credner  in  the  lines  quoted  above, 
though  in  them  no  suggestion  of  the  replacement  to  which  is  due  the 
em-ichment  of  the  ore  bodies  is  made. 

Much  of  the  iron  of  the  Mansfield  ore  is  presumed  to  have  resulted 
directly  from  the  alteration  of  the  feiTuginous  carbonate  in  place,  but  a  large 
amount  was  brought  in  from  above  by  infiltrating  waters.  The  ferruginous 
matter,  which  was  taken  into  solution  during  the  denudation  of  the  area,  has 
been  carried  down  by  percolating  waters  and  deposited  at  places  favorable 
for  its  accumulation.  The  beds  are  now  on  edge,  off'ering  the  most  favorable 
condition  to  percolation.     The  conclusion  is  obvious  that  these  deposits 


'  Die  vorsilurischen  Gebikle  der  "  Oberen  Halbiusel  von  Michigan  "  in  Nord-Amerilca,  by  H. 
Credner:  Zeitschv.  dentsch.  geol.  Gesell.,  Vol.  XXI,  1869,  p.  547. 

-On  the  origin  of  the  ferruginous  schists  and  iron  ores  of  the  Lalse  Superior  region,  by  R.  D. 
Irving:  Ani.  Jour.  Sci.,  Vol.  XXXII,  1886,  p.  263. 


72  THE  CRYSTAL  FALLS  IKON-BEAEING  DISTRICT. 

were    formed    after   the  beds  were  tilted,  and  the    iron  derived  from  the 
upward  extension  of  the  rocks,  which  has  been  removed  by  erosion. 

CONDITIONS  FAVORABLE  FOR  ORE  CONCENTRATION. 

The  conditions  favorable  for  the  accumulation  of  ore  deposits  have 
been  ascertained  by  Van  Hise  from  studies  in  the  other  iron-bearing  dis- 
tricts of  the  Lake  Superior  region.    He  summarizes  these  results  as  follows:^ 

[1]  The  iron  ore  is  contiued  to  certain  definite  horizons,  known  as  tlie  iron-bearing 
formations.  .  .  .  frtj  All  ore  bodies  have  been  found  to  be  distributed  very  irregu- 
larly in  these  iron-bearing  formations.  Thi.s  is  due  to  the  fact  that  they  are  secondary 
concentrations  produced  by  downward  percolating  waters,  and  the  ore  bodies  therefore 
occur  at  the  places  where  water  is  concentrated,  in  accordance  with  the  laws  of  the 
underground  circulation  of  waters,  [b]  These  places  are  just  above  an  impervious 
formation,  at  the  contact  of  the  Upper  Huroniau  and  Lower.Huroniau  aud  where  the 
rocks  are  .shattered.  \c\  The  impervious  basement  formation  m.ay  be  a  surface 
volcanic,  a  subsequent  intrusive,  an  argillaceous  stratum,  or  any  other  impermeable 
formation,  [d]  These  impervious  basements  are  most  efl'ective  when  they  are  in  the 
form  of  pitching  troughs,  thus  concentrating  the  waters  from  the  sides  along  a  well- 
defined  channel.  These  pitching  troughs  may  be  formed  by  a  single  one  of  the  above 
rocks  or  by  a  combination  of  two  or  more  of  them.  The  horizon  marked  by  the  uncon- 
formity between  the  Upper  and  Lower  Huronian  is  a  great  natural  zone  of  percolating 
waters.  Here  oftentimes  the  basement  formation  of  the  Upper  Huronian  is  itself  a 
lean  ore,  having  derived  its  material  from  the  Lower  Huronian,  but  in  this  case  a 
secondary  concentration  has  occurred  in  order  to  produce  the  present  ore  bodies. 
[e]  Finally,  as  a  result  of  folding,  the  iron-bearing  formations  have  been  shattered, 
thus  producing  natural  water-courses.  More  frequently  than  not,  more  than  one  of 
these  classes  of  phenomena  are  found  together  where  the  great  ore  bodies  occur,  and 
in  many  cases  all  are  combined.  The  original  source  of  the  iron  ores  has  been  ascer- 
tained to  be  in  many  cases  a  lean  carbonate  of  iron,  often  with  a  good  deal  of 
carbonate  of  calcium  and  magnesium,  formed  as  an  ocean  deposit. 

Van  Hise  adds  to  the  above  statement  that  generally  the  ore  bodies, 
as  a  result  of  their  methods  of  concentration,  somewhere  reach  the  rock 
surface. 

The  Mansfield  ore  body  has  well-defined  foot  and  hanging  walls  of 
normally  impervious  rock.  The  iron-bearing  formation  is  much  fractured. 
We  thus  have  certain  of  the  conditions  favorable  to  the  concentration  of  an 
ore  body.  Wliether  a  trough  is  completed  by  a  slight  cross  fold  in  the 
formation,  or  possibly  by  an  intersecting  dolerite  dike,  has  not  been 
determined. 


'Foiirteeuth  Ann.  Kept.  U.  S.  Geol.  Survey,  Part  I,  1893,  pp.  107-108. 


OKIi  DEPOSITS  IN  MANSFIELD  SLATE.  73 

EXPLORATION. 

Exploration  has  developed  no  other  deposits  ulong-  the  Mansfield  slate 
belt.'  It'  other  deposits  exist,  it  is  hig'hly  jjrobable  that  they  extend  to  the 
rocK  surface — that  is,  are  covered  by  the  drift  mantle  alone. 

The  intervals  between  possible  ore  bodies  along  the  strike  of  the  slates 
ar'^  probably  occupied  by  mixed  chert  and  ore  or  ferruginous  chert.  Explora- 
tions should  extend  from  the  impervious  slate  below  the  iron-bearing  forma- 
tion to  the  impervious  rock  above  the  iron-bearing  formation.  In  order  to 
explore  the  belt  thoroughly,  rows  of  pits  cross-sectioning  the  formation  ought 
to  be  made  at  intervals  not  greater  than  100  feet,  and  even  with  such  inter- 
vals an  important  deposit  might  be  missed,  for  it  frequently  happens  that  at 
the  surface  of  the  rock  an  ore  deposit  is  smaller  than  it  is  at  a  moderate 

depth. 

SECTION   III.— THE  HEMLOCK  FORMATION. 

This  formation,  the  most  interesting  petrographically  in  the  Crystal 
Falls  district,  consists  almost  exclusively  of  typical  volcanic  rocks,  both 
basic  and  acid,  with  crystalline  schists  derived  from  them.  Sedimentary 
rocks  play  a  very  unimportant  role.  With  one  exception  they  have  been 
formed  directly  from  the  volcanics,  and  occur  interbedded  with  them. 
Cutting  through  the  volcanics  are  intrusive  rocks,  which  likewise  include 
both  basic  and  acid  kinds.  Chemically  the  intrusive  and  extrusive  rocks 
show  very  close  relationships.  The  name  Hemlock  has  been  given  to  this 
volcanic  formation  because  the  river  of  that  name  flows  through  it  for  a 
number  of  miles,  and  in  places  affords  excellent  exposures. 

DISTRIBUTIOSr,   EXPOSURES,   AXD  TOPOGRAPHY. 

Beginning  in  sec.  36,  T.  46  N.,  R.  32  W.,  the  place  where  the  Hemlock 
formation  enters  the  part  of  the  district  studied  by  ine,  the  formation  has  a 
width  of  one-half  of  a  mile.  From  this  place  the  formation  has  a  north- 
western coui'se  for  about  5  miles,  gradually  widening.  It  then  bends  to  the 
west,  and  after  a  short  distance  to  the  south,  which  course  it  follows  for 
about  9  miles.  In  township  45  N.,  Rs.  32  and  33  W.,  the  belt  has  a  maxi- 
mum width  of  5  miles.     At  the  end  of  the  southern  course  the  formation 

'  Since  tbe  above  w.as  Tvritten  I  hare  been  informed  that  Mr.  George  .T.  Maas,  of  Negaunee,  has, 
with  a  diamond  drill,  located  a  body  of  bessemer  ore  30  feet  thick  on  lot  6,  sec.  20,  T.  43  N.,  E.  31  W., 
1  mile  sonth  of  the  Mansfield  mine. 


74  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

bends  to  the  southeast,  and  continues  with  this  general  trend  for  about  16 
miles  into  T.  42  N.,  R.  31  W.,  where  my  field  study  of  it  ended.  At  the 
north  the  belt  runs  into  the  eastern  half  of  the  district  described  by  Smyth, 
and  swings  south,  which  course  is  followed  for  some  15  miles.  The 
entire  belt  thus  foiius  an  oval  suiTOunding  the  sedimentaries,  except  in  the 
southeastern  part  of  the  district.  Another  area  of  Hemlock  volcanics  is 
found  in  T.  43,  Rs.  32  and  33,  just  north  of  Crystal  Falls.  This  area  is 
about  one-half  a  mile  wide  just  north  of  the  city  of  Crystal  Falls,  but  rap- 
idly widens  as  it  is  followed  to  the  west  until  at  the  western  limits  of  the 
area  it  is  about  3J  miles  wide.  A  third  small  isolated  area  is  found  in  sees. 
17,  18,  19,  and  20,  T.  42  N  ,  R.  32  W,  and  sec.  24,  T.  42  N.,  R.  33  W., 
about  4  miles  south  of  Crystal  Falls. 

The  topography  of  the  Hemlock  formation  is  exceedingly  rough  where- 
ever  erosion  has  succeeded  in  cutting  through  the  drift  mantle.  This  occurs 
only  adjacent  to  some  of  the  streams.  The  rough  topography  at  these 
places  is  due  to  differential  erosion  working  upon  rocks  approximately  on 
edo-e,  and  of  varying  hardness.  The  valleys  usually  indicate  the  location 
of  beds  of  tuff  and  the  higher  grounds  are  almost  universally  occupied 
by  dense  rocks  forming  the  lava  flows,  or  of  the  coarse-grained  massive 
intrusive  rocks.  In  a  few  places,  however,  the  thoroughly  consolidated  and 
indurated  tuffs  form  high  hills.  In  traversing  the  Hemlock  formation  one 
makes  an  abrupt  ascent,  followed  by  a  sharp  descent  into  a  nan-ow  swamp, 
then  another  ascent,  and  so  on.  Exposures  appear  for  the  most  part  in 
small  areas  along  the  edges  of  the  swamps  and  scattered  over  the  faces  of 
the  hills.  These  are  fairly  numerous,  but  so  small  and  disconnected  as  to 
prevent  the  tracing  out  of  the  individual  flows,  although  this  might  be  pos- 
sible if  the  traverses  were  made  at  very  short  intervals  and  the  area  mapped 
in  ffreat  detail. 

°  THICKNESS. 

As  has  been  seen,  the  belt  of  eruptives  varies  in  width  from  one- 
half  of  a  mile  to  nearly  five  miles.  The  dip  of  the  rocks  is  about  75°  W. 
The  enormous  thickness  of  25,500  feet  which  these  data  would  give  is 
probably  illusory. 

In  the  case  of  the  assumption  of  the  thickness  of  a  series  of  lava  flows 
and  tuffs,  it  is  important  that  the  initial  dip,  which  these  deposits  must  have, 
be  considered.     This  dip  varies  greatly,  depending  on  the  slope  of  the 


THICKNESS  OF  HEMLOCK  FORMATION.  75 

cone,  whic'li  iu  its  tiivn,  is  dept'iideiit  on  the  viscosity  of  the  lava  and  the 
j)resenco  of  varying'  (jnantities  of  fragmental  jirodncts.  If  we  assume  these 
pre-Cambrian  volcanic  products  to  have  had  an  initial  dip  of  15'^,  I  lielieve 
we  are  within  limits  for  products  consisting,  as  these  do,  of  what  was  i)rob- 
ably  moderately  viscous  basalt  and  vast  masses  of  fragmental  material. 
This  estimate  is  based  on  the  assumption  that  the  volcanics  here  represented 
were  deposited  for  the  most  part  upon  the  westward  slope  of  a  volcano,  or 
a  series  of  volcanoes.  This  initial  dip  of  15°  is  then  to  be  deducted  from 
the  present  dip,  75°,  of  the  flows.  Taking  this  into  consideration,  we  get  a 
thickness  of  23,000  feet  for  the  volcanics. 

It  is  highly  probable  that  the  rocks  have  been  subjected  to  close 
folding,  and  for  this  reason  also  the  apparent  thickness  would  be  much 
greater  than  the  true  thickness.  The  schistose  character  of  some  of  the 
rocks  shows  clearly  that  they  have  been  severely  mashed,  and  this  mashing 
was  probably  produced  in  connection  with  folding.  It  is  probable  that  this 
possible  maximum  thickness  shoiUd  be  very  materially  reduced,  jDOssibly 
to  one-half  or  one-third  of  the  amount.  However,  even  the  maximum 
above  calculated  is  probably  paralleled  by  the  vast  masses  of  volcanic 
material  accumulated  in  certain  volcanic  areas,  such  as  those  of  Hawaii  or 
Iceland.     Geikie  writes:^ 

The  bottom  of  these  Iceland  Tertiary  basalts  is  everywhere  concealed  under  the 
sea.  Yet  their  visible  portion  shows  them  to  be  probably  more  than  .3,000  meters  in 
thickness. 

An  especial  interest  belongs  to  this  Icelandic  plateau  because  volcanic  action  is 
still  vigorous  upon  it  at  the  present  day. 

RELATIONS  TO  ADJACENT  FORMATIONS. 

In  the  northern  part  of  the  Crystal  Falls  district  the  volcanics  overlie 
the  quartzose  dolomite  formation  known  as  the  Randville  dolomite.  In 
the  central  part  of  the  district,  through  which  the  Deer  River  runs,  as  shown 
in  section  G-H,  PI.  VI,  outcrops  are  so  scarce  that  it  has  been  found 
impossible  to  trace  the  boundaries  of  the  formations  with  any  degree  of 
accuracy.     Consequently  this  part  of  the  district  is  mapped  as  Pleistocene. 

From  the  few  outcrops  of  slate,  probably  equivalent  to  the  Mansfield 
slate,  which  have  been  found  in  the  Deer  River  area,  it  has  been  thought 


'The  Tertiary  basalt-plateaux  of  northwestern  Europe,  by  Sir  A.  Geikie;  Quart.  Jour.  Geol. 
Soo.  London,  Vol.  LII,  1896,  p.  395. 


76  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

higlilv  probable  that  this  slate  iu  an  extremely  plicated  condition  may 
underlie  the  volcanics  of  this  area,  and  it  is  so  represented  in  section  G-H, 
Plate  VI.  As  evidence  of  this,  in  T.  43  N.,  R.  31  W.  the  volcanics 
overlie  the  Mansfield  slate  unconformably. 

In  places  test  pits  have  disclosed  an  amygdaloidal  lava  flow  immedi- 
ately overlying  the  Mansfield  slates.  At  one  place,  at  the  northeast  corner 
of  sec.  7,  T.  43  N.,  R.  31  W.,  angular  fragments  of  the  underlying  black 
slate  have  been  found  in  the  tufaceous  deposits  of  the  Hemlock  volcanics. 
Farther  south,  along  the  contact  just  west  of  the  Mansfield  mine,  a  con- 
glomerate is  exposed,  which  contains  fragments  of  slate,  lava,  and  rounded 
grains  of  quartz  with  secondary  enlargements.  The  rock  is  evidently 
water  deposited.  There  is  also  obtained  from  the  workings  of  the  mine  a 
conglomerate,  taken  from  just  alcove  the  ore,  which  consists  of  lava  frag- 
ments and  pieces  of  chert  and  ore,  as  mentioned  on  pp.  64,  68.  From  these 
occurrences  it  is  clear  that  some  of  the  sedimentaries  are  unquestionably 
older  than  some  of  the  volcanics,  and  yet  the  conglomerates  bearing  the 
fragments  of  ore  and  slate  contain  also  fragments  of  lava,  showing  the 
existence  of  some  of  the  volcanics  before  the  deposition  of  this  conglom- 
erate. The  only  explanation  of  all  of  the  facts  which  has  occurred  to  me 
is  as  follows:  After  the  ore-bearing  Mansfield  slate  was  deposited,  an  erosion 
interval  occurred.  Then  followed  a  volcanic  outbreak.  It  is  highly 
probable  that  this  outburst  began  far  north  of  the  Man.sfield  mine,  coincident 
with  the  upheaval  which  resulted  in  the  erosion  of  the  Mansfield  slate. 
The  volcanic  ejectamenta  were  mixed  with  the  sedimentary  fragments  and 
all  together  were  rounded  and  bedded,  forming  in  places  conglomerates. 
In  places  along  the  shore  lava  flows  descended,  some  reaching  into  the  sea 
and  covering  the  sedimentaries  along  the  shore  where  no  conglomerate  had 
been  formed.  At  other  places  deposits  of  scoriae,  etc.,  including  fragments 
of  slates  from  the  sedimentaries  through  which  the  volcano  burst,  were 
made,  and  thus  deposits  of  tuff  are  found  overlying  the  sedimentaries. 
The  various  deposits,  though  really  separated  by  a  slight  physical  break, 
are  practically  conformable  with  the  series  below,  all  having  a  north-south 
strike  and  a  high  westward  dip. 

The  formations  which  underlie  the  volcanics  in  the  northern  and 
southern  parts  of  the  district  are  of  different  character.     This  difference 


RELATIO^TS  OF  HEMLOCK  FORMATION.  77 

niav  be  oxplaiucd  by  supposing  the  voleaiiocs  broke  out  in  tlie  nortliern 
])art,  while  the  Manstielil  shite  was  still  being  deposited  in  the  south. 
Gradually,  however,  the  volcanic  activity  spread  toward  the  south,  proba- 
l)ly  following- a  fissure  along  the  pre-Cambrian  shore,  and  igneous  materials 
buried  the  Mansfield  slate.  Hence,  while  on  the  whole  these  volcanics  are 
younger  than  the  Mansfield  slates,  some  of  the  lower  of  them  ai-e  con- 
temporaneous with  some  of  the  upper  Mansfield  beds.  The  volcanics 
invariably  overlie  the  Randville  dolomite,  and  are  unque.stionably  of  later 
age  than  that  formation. 

The  Hemlock  volcanics  are  overlain  throughout  their  extent  b}-  the 
Upper  Huronian  series  of  graywackes  and  slates.  Near  the  contact  line 
with  the  volcanics  wherever  the  Huronian  outcrops,  or  has  been  exposed 
by  exploration,  it  has  been  found  to  be  characterized  bv  a  line  of  magnetic 
attraction.  By  means  of  magnetic  observations  the  line  of  contact  has  been 
traced,  where  owing  to  lack  of  exposures  it  would  have  been  otherwise 
impossible  to  comiect  the  isolated  outcrops. 

RELATIOlSrS  TO  USTTRUSIVES. 

High  ridges  composed  of  dolerite  are  found  extending  in  a  general  north- 
west and  southeast  direction  through  the  volcanics.  That  these  masses 
were  forced  up  through  the  Hemlock  formation  is  indicated  by  the  folding 
which  they  cause  in  certain  places.  Such  rocks  are  unquestionably  younger 
than  the  volcanic  series.  There  may  be  seen  also  on  the  map,  in  T.  44  N., 
R.  32  W.,  a  number  of  isolated  knobs.  These  are  also  doleritic,  and  are 
presumed  to  be,  like  the  larger  ridges,  intrusive  in  the  volcanics. 

The  dolerites  have  in  their  turn  been  cut  by  acid  dikes.  These  are 
coarse  micropegmatitic  granites.  Similar  acid  dikes  have  been  found 
cutting  the  surrounding  volcanics.  This  set  of  acid  dikes  may  be  looked 
upon  as  the  youngest  intrusive  igneous  rocks  occurring  in  the  Hemlock 
volcanic  formation. 

Cutting  the  volcanics  are  also  basic  dikes  varying  from  fine  to  moder- 
ately coarse  grain.  It  is  well  known  that  during  a  volcanic  epoch  the  out- 
poured lavas  and  clastic  volcanic  deposits  are  penetrated  by  dikes  coming 
from  the  same  magma.  Whether  or  not  these  dikes  are  of  this  origin,  and 
are  hence  contemporaneous  with  the  later  volcanics,  or  are  of  later  age,  and 


78  THE  CRYSTAL  FALLS  IRON-BEAEING  DISTRICT. 

correspond  to  the  coarse  dolerites,  it  is  impossible  to  determine  with 
certainty.  They  are  presumed,  however,  to  form  an  integral  part  of  the 
Hemlock  volcanics,  as  no  connection  between  the  dikes  and  the  unques- 
tionably intrusive  dolerites  could  be  traced  in  the  field. 

VOLCANIC   ORIGIN. 

In  spite  of  numerous  occuiTences  of  ancient  volcanics  which  have 
recently  become  known,  the  late  Professor  Dana  makes  the  following 
statement:^ 

It  is  not  yet  certain  that  a  volcano  ever  existed  on  tlie  continent  of  Ifortli 
America  before  the  Cretaceous  period;  for  the  published  facts  relating  to  supposed 
or  alleged  rolcanic  eruptions  in  the  course  of  the  Paleozoic  ages  are  as  well  explained 
on  the  suppo.sition  of  outflows  from  fissures  and  tufa  ejections  under  submarine 
conditions;  and  none  of  the  accounts  present  evidence  of  the  former  existence  of 
a  volcanic  cone,  that  is,  of  an  elevation  pericentric  in  structure  made  of  igneous 
ejections.  . 

The  presence  in  the  Hemlock  formation  of  a  quantity  of  pyroclastics, 
great  in  proportion  to  the  solid  lavas,  and  the  absence  of  any  great  sheets 
of  lava,  so  important  a  product  of  great  fissure  eruptions,  seem  to  point  to 
the  derivation  of  the  Hemlock  rocks  from  a  volcano  or  volcanoes  situated 
near  the  border  of  the  contemporaneous  Huronian  sea,  rather  than  from  a 
simple  fissure.  While  some  of  the  eruptives  may  have  been  submarine, 
the  occurrence  of  large  quantities  of  clearly  subaerial  deposits  shows  that  the 
eniptives  were  largely  on  the  land.  Thus  it  appears  that  neither  a  fissure 
flow  nor  a  submarine  volcano  will  wholly  explain  the  Hemlock  formation. 
However,  it  is  hig'hly  probable  that  this  volcanic  outburst,  which  piled 
masses  of  volcanic  material  upon  the  land,  was  accompanied,  as  have  been 
all  or  nearly  all  the  great  outbursts  of  recent  times,  by  submarine  lava 
flows  and  tuff"  ejections.  No  such  clear  evidence  of  the  presence  of  a  Pale- 
ozoic or  pre-Paleozoic  volcano  on  the  North  American  continent  has  been 
adduced  as  that  given  by  the  English  geologists  for  certain  volcanoes 
in  the  British  Isles.  But  while  the  j^i'esence  of  a  central  cone  with  peri- 
centric arrangement  in  the  Hemlock  district  is  not  conclusively  j^i'oven, 
the  presumption  in  favor  of  such  a  cone  or  cones  having  existed  is  certainly 
strong. 

'Manual  of  Geology,  by  J.  D.  Dana:  4th  ed.,  1895,  p. 938. 


VOLCANIC  OUIGIN  OF  HEMLOCK  FORMATION.  79 

An  attempt  was  made  to  locate  the  vent  or  vents  from  wliicli  the 
material  was  derived,  l)iit  no  evidence  could  be  found,  unless  we  consider 
tlie  vents  to  have  been  where  the  accunudations  were  the  greatest.  The 
coarse-grained  rocks  which  were  first  supposed  to  represent  the  plugs  of 
ancient  volcanoes,  on  careful  and  detailed  examination  appear  to  be  later 
intrusives,  or  else  are  indeterminate. 

CXiASSIFICATIOlSr. 

The  general  character  and  distribution  of  the  Hemlock  formation  hav- 
ing been  given,  we  may  now  proceed  to  a  petrographical  consideration  of 
the  rocks  comprising  it.  This  will  be  given  in  more  detail  than  for  the 
other  rocks  of  the  Michigamme  district  because  this  great  pre-Cambrian 
volcanic  formation  possesses  peculiar  interest. 

The  rocks  of  the  Hemlock  formation  are  chiefly  of  direct  igneous 
origin.  Some  interleaved  sedimentary  rocks  occur,  which,  however,  with  a 
single  exception  are  composed  of  fragments  of  the  igneous  rocks.  For  the 
sake  of  easy  reference,  the  usual  classification  into  igneous  and  sedimentary 
rocks  will  be  used.  The  massive  igneous  rocks  are  subdivided  according  to 
chemical  composition  into  acid  and  basic  rocks.  The  acid  rocks  include 
rhyolite-porphyries,'  aporhyolite-porphyries,  and  acid  pyroclastics.  The 
basic  rocks  include  altered  uonporphyritic  basalts,  porphyritic  basalts,  and 
variolite  and  basic  pyroclastics.  The  sedimentary  rocks  are  divided  into 
the  volcanic  sedimentaries  and  the  nonvolcanic  sedimentaries  or  normal 
sedimentaries.  The  first  include  tuffs  and  ash  beds — the  aeolian  deposits, 
and  volcanic  conglomerates — subacpieous  deposits.  The  normal  sedimen- 
taries are  represented  by  slates  and  limestones.  Various  schists  are  locally 
produced  from  these  numerous  kinds  of  rocks  through  metasomatic  changes 
and  dynamo-metamorphic  action.  Many  of  these  schists  resemble  one 
another  very  closely,  though,  as  will  be  seen  later,  they  are  derived  from 
both  the  massive  rocks  and  from  the  elastics.  These  have  been  described 
in  connection  with  the  rocks  from  which  they  have  been  derived. 

'According  to  a  late  riiliug  of  the  Director  of  the  United  States  Geological  Survey,  based  on  the 
recommendation  of  a  committee  on  nomenclature  for  geologic  folios,  "porphyry"  is  to  be  used  only 
with  a  textural  significance.  Hence  "quartz-porpbjry,"  according  to  this  ruling  should  no  longer  be 
used  as  a  rock  name.  The  rhyolite-porphyries  here  described  are  what  have  been  kuown  as  normal 
quartz-poriihyries. 


80 


THE  CRYSTAL  FALLS  lEONBEAKING  DISTRICT. 


The  following  table  will  show  the  arrangement  outlined  above,  which 
will  be  followed  in  the  descriptions: 

Classification  of  the  rocks  of  the  Hemloch  formation. 


Igneous . 


.    . ,       Uavas  *  Rhyolite^poiphy ry  ....  J  gchistose  acid  lavas  . 

fAcid  ..  <'^''''""'  (  Aporhyolite-porphyry  .  S 

(  Pyroclastics 

(  Nonporphyritic 

(  Lavas Metabasalt <;  Poipbyritic 

Basic  .  .'  .  ( Vaiiolitic 


Pyroclastics . .  Eruptive  breccia 
Volcauic  sediments 


Flow  breccia  . 


Sedimentary 


Normal  sediments  , 


)  Eohan  deposits J  Ash  beds 

I  Subaqueous  deposits..  .Conglomerates 

S  Slate 

(  Liiuestone , 


Crystalline 
schists. 


ACID   VOLCAlSriCS. 


The  acid  volcanics  are  comparatively  unimportant  in  quantity.     They 
may  be  conveniently  subdivided  into  the  lavas  and  pyroclastics. 


ACID    LAVAS. 


The  acid  lavas  occur  in  such  small  quantity  as  to  make  it  impossible 
without  very  great  exaggeration  to  place  them  upon  the  accompanying 
small-scale  general  maps,  though  they  have  been  introduced  upon  the 
detail  maps  wherever  the  scale  permitted.  They  usually  form  isolated 
ridges,  and  their  relations  to  the  surrounding  basic  volcanics  are  obscured 
by  lack  of  exposures.  The  trend  of  the  individual  ridges  agrees  with  the 
general  strike  of  the  banding  in  the  basic  tuffs.  Moreover,  in  nearl}^  all 
cases  the  isolated  exposures  which  are  closest  together  lie  in  such  relations 
to  one  another  that  when  connected  the  large  sheets  thus  formed  follow  the 
strike  of  the  tuff  banding,  as  do  the  individual  ridges,  and  they  are  there- 
fore confidently  assumed  to  be  the  isolated  portions  of  acid  flows  inter- 
bedded  with  the  basic  volcanic  rocks. 

The  rock  types  represented  are  the  two  closely  related  rocks — the 
rhyolite-porphyry  and  the  aporhyolite-porphyry.  Under  the  rhyolite- 
porphyries  are  included  the  porphyritic  acid  lavas,  which  have,  so  far  as 
can  be  determined,  an  original  holocrystalline  groundmass.  Under  the 
aporhyolite-porphyry,  following  Miss  Bascom's  use  of  cqm,^  I  include  those 


1  structures,  origin,  and  nomenclature  of  the  acid  volcanic  rocks  of  South  Mountain,  Penn- 
sylvania, by  Miss  Florence  Bascom :  Jour.  Geol.,  Vol.  1, 1893,  p.  816. 


ACID  V'OLGANICS  OF  HEMLOCK  FORMATION.  81 

acid  lavas  which  are  now  hkewise  hoh)cr)stalhue,  but  wliich  owe  this 
cliaracter  to  the  devitriHcatiou  of  an  orig-iual  ghissy  base,  supposing-  tliem 
in  tlieir  original  vitreous  condition  to  have  corresponded  to  tlie  modern 
liyalorhyolite-porphyries. 

RHYOLITE-PORPHYRY. 

The  rhyolite-porphyries  on  fresh  fracture  are  dark  grayish-blue  to  lilack. 
From  this  the)'  grade  with  advancing  alterations  through  chocolate  brown 
to  purplish.  The  weathered  surface  A^aries  from  white  to  reddish.  The 
weathering  has  in  one  case  brought  out  very  well  the  fluxion  banding  of 
the  rock.  Tlieir  texture  is  very  pronouncedly  porphp-itic.  The  quartz  and 
feldspar  crystals  stand  out  ^ilainl}-  from  the  groundmass,  which  is  usually 
dense  with  a  somewhat  resinous  luster.  The  porphyritic  quartzes  average 
perhaps  the  size  of  a  small  pea,  and  hence  are  macroscopically  very  plainly 
visible.  They  frequently  stand  out  on  the  weathered  surface  and  show 
their  cry.>-tal  forms,  and  in  other  cases  we  see  the  angular  cavities  out  of 
which  they  have  fallen,  like  the  kernel  from  the  nut. 

Under  the  microscope  the  rocks  are  seen  to  be  typical  rhyolite-por- 
phyries. The  phenocrysts  are  chiefly  corroded  dihexahedral  crystals  of 
quartz.  Crystals  of  plagioclase  and  orthoclase  are  less  common.  These 
lie  in  a  very  fine-grained  holocrystalline  groundmass,  composed  largely  of 
feldspar  and  quartz,  with  some  zircon  in  small  crystals,  and  here  and  there 
magnetite. 

These  are  presumed  to  be  the  original  constituents  of  the  groundmass. 
Associated  with  them  are  considerable  quantities  of  secondary  chlorite, 
epidote,  biotite,  muscovite,  calcite,  and  reddish  to  brown  alteration  products 
of  the  magnetite.  Included  in  the  groundmass  are  here  and  there  oval 
areas  of  finely  crystalline  secondary  quartz,  probably  filling  former 
amygdaloidal  cavities. 

In  thin  section  the  crystal  contours  of  the  quartz  phenocrysts  are 
more  or  less  rounded,  with  here  and  there  embayments  of  the  groundmass 
projecting  into  them.  The  crystal  form  is,  however,  always  cleai'ly 
marked.  In  some  cases  the  individuals  have  been  broken  before  the  cool- 
ing of  the  magma,  the  fragments  of  an  individual,  though  now  separated, 
being  seen  to  conform  to  one  another.     That  they  have  been  subjected  to 

23ressure  is  shown  by  the  undulatory  extinction  and  also  by  the  separation 
MON  xxxvi 6 


82  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

of  the  black  cross  of  uniaxial  minerals  into  hyperbolfe.  Embayments  of 
groundmass,  and  liquid  inclusions  in  which  a  dancing  bubble  may  be  seen, 
are  in  places  rather  thickly  distributed  through  the  quartz.  The  liquid 
inclusions  have  very  commonly  an  hexagonal  form,  corresponding  to  the 
contours  of  the  inclosing  quartz. 

These  liquid  inclusions  are  certainly  in  some  cases  secondary.  This 
character  is  well  shown  in  some  of  the  crystals,  which  are  broken  across, 
giving  along  the  line  of  fracture  a  very  wavy  extinction.  Along  this  line 
of  fracture  the  greatest  quantity  of  inclusions  are  seen,  both  with  and 
without  bubbles.  As  the  distance  from  a  fracture  increases,  both  the  undu- 
latorA^  extinction  and  the  number  of  inclusions  diminish.  (See  fig.  A, 
PL  XIX.)  These  fractures  in  the  quartzes  are  but  continuations  of  those 
which  extend  in  many  cases  all  the  way  across  the  section.  The  fractures 
have  since  been  healed  by  secondary  quartz.  This  secondary  quartz  has 
also  in  some  cases  healed  the  fractured  quartz  phenocrysts,  and  then  agrees 
with  them  in  orientation.  The  possession  of  an  imperfect  rhombohedral 
parting  is  very  noticeable  in  a  number  of  quartzes,  and  especially  those 
wdiich,  being  on  the  edge  of  the  section,  are  very  thin.  (See  fig.  B,  PI. 
XIX.)  Similar  parting  in  the  quartz  occurs  in  various  rocks  studied  in  this 
district. 

The  phenocrysts  of  the  porphyries  are  traversed  by  fractures,  some  of 
which  are  more  or  less  circular,  and  simulate  very  imperfectly  perlitic  cracks. 
With  the  exception  of  those  in  porphyries  in  two  localities,  the  quartz 
phenocrysts  are  surrounded  by  zones,  largely  of  quartz,  of  varying  widths, 
and  considerably  lighter  than  the  remainder  of  the  groundmass.  Much  of 
the  quartz  of  these  zones  has  the  same  optical  orientation  as  the  phenocrysts. 
In  those  sections  in  which  the  zones  are  observed  they  occur  around  every 
section  of  quartz. 

The  feldspar  phenocrysts  are  orthoclase  and  plagioclase,  the  latter 
apparently  predominating.  They  occur  usually  in  rounded,  badly  corroded 
crystals,  with  indentations  filled  with  groundmass.  They  are  always  altered, 
and  have  associated  with  them  as  secondary  products  calcite,  epidote, 
muscovite,  biotite,  and  chlorite 

No  large  original  ferro-magnesian  phenocrysts  appear  to  have  been 
present  in  the  porphyry.  Their  former  presence  is  at  least  not  indicated 
by  any  aggregates  of  secondary  products.     Whatever  ferro-magnesian  min- 


ACID  VOLOANICS  OF  HEMLOCK  FOliMATlON.  83 

erals  were  preseut  must  have  been  scattered  tlin)Ugli  the  <>Toiin(lniass,  and 
have  been  completely  altered.  The  secondary  minerals  contained  in  the 
groundniass  are  chlorite,  calcite,  epidote,  niuscovite,  and  biotite. 

TEXTCRE   OF   THE   I'OUrilYpRIES. 

The  texture  of  the  dense  groundniass  varies  according  to  the  mode  of 
association  of  the  two  chief  minerals — quartz  and  feldspar.  The  commonest 
variety  is  the  rhyolite-porphyry  with  microgranitic  groundmass  (porphyre 
granulitique  of  Michel  Ldvy).  A  second  variety  is  the  rhyolite-porphyry 
with  micropoikilitic  groundmass.'  The  microgranitic  texture  is  too  well 
known  to  warrant  a  description  of  it  here. 

The  micropoikilitic  texture  presents  certain  characters  which  render  a 
further  description  desirable.  This  peculiar  phase  of  the  micropoikilitic 
textm-e  was  briefly  described  by  the  writer  and  illustrated  by  microphoto- 
graphs  in  1895.^  Shortly  after  the  separates  of  this  article  were  disti-ibuted, 
I  received  from  H.  Hedstrom,  of  the  Swedish  Geological  Survey,  an  article 
published  in  1894  containing  a  description  of  what  appears  to  be  very 
nearly  the  same  texture.^  If  I  have  understood  the  description  correctly, 
however,  there  seems  to  be  one  essential  difference.  In  order  to  explain 
clearly  this  difference,  I  shall  describe  the  textm-e  in  detail. 

In  certain  of  the  rhyolite-porphyries,  as  already  mentioned,  the  quartz 
phenocrysts  are  surrounded  by  certain  zones.  These  zones  in  the  rocks 
having  a  micropoikilitic  texture  possess  exactly  the  same  texture  as  does 
the  groundmass.  The  zones  are  composed  of  minerals  which  are  of  suffi- 
cient size  to  permit  readily  their  determination.  Quartz  and  feldspar  are 
the  essential  components,  with  some  chlorite,  epidote,  muscovite,  and  iron 
oxide.  The  first  two  are  the  important  minerals,  and  will  alone  be  referred 
to  in  the  further  description.  The  chief  peculiarity  of  the  zone  is  in  the 
arrangement  of  the  two  minerals,  and  this  character  is  best  shown  on  the 
accompanying  microphotographs.  This  textm-e  can  be  seen  even  in 
ordinary  light.     It  is  brought  out  better  when  the  field  is  partly  shaded,  so 


'  Eruptive  rocks  of  Electric  Peak  aud  Sepulchre  Mountain,  by  J.  P.  IiUlings :  Twelfth  Anu.  Eeirt. 
U.  S.  Geol.  Survey,  1891,  p.  589.  On  the  use  of  the  terms  poikilitic  aud  micropoikilitic  in  petrography, 
by  G.  H.  Williams:  Jour.  Geol.,  Vol.  1, 1892,  pp.  176-179. 

-  Volcanics  of  the  Michigamme  district  of  Michigan,  by  J.  Morgan  Clements:  Jour.  Geol.,  Vol 
III,  1895,  pp.  814-816,  flgs.  1  and  2. 

» Studier  iifver  Bergarter  fran  Moriin  vid  Visby,  by  H.  Hedstrom :  Geol.  Foren  i  Stockholm 
Forhandl,  Bd.  16,  H.  i,  1894,  pp.  5-9. 


84  THE  OKYSTAL  FALLS  IRONBEAKING  DISTRICT. 

as  to  exhibit  the  difference  in  reHef  of  the  minerals,  and,  best  of  all,  between' 
crossed  nicols.  (See  fig.  A,  PI.  XX.)  The  zones  are  seen  to  be  made  up 
of  reticulating  areas  of  clear  quartz,  in  which  lie  embedded  iiTegular  pieces 
of  feldspar.  Where  two  or  more  of  the  quartz  stringers  or  needles  unite, 
one  sees  broad  ai-eas  of  limpid  quartz.  The  network  of  quartz  is  best  seen 
when  it  exhibits  its  highest  polarization  color,  as  then  the  feldspar  is  for  the 
most  part  dark.  The  pieces  of  feldspar  in  such  a  quartz  area  for  the  most 
part  have  irregular  orientation,  as  is  shown  by  their  varying  extinction, 
although  a  number  extinguish  simultaneously.  This  quartz  net  is  connected 
with  the  quartz  phenocrysts,  as  shown  by  the  continuation  of  the  quartz  of 
the  phenocrysts  and  that  of  the  zone,  and  the  consequent  agreement  in 
orientation.  The  lack  of  a  uniform  optical  orientation  of  the  feldspar  grains 
is  made  especially  apparent  when  the  quartz  is  cut  perpendicular  to  the  c 
axis,  and  consequently  remains  dark  under  crossed  nicols.  Under  the 
above  circumstances  we  see  certain  feldspar  grains  polarizing  in  the  zone 
around  the  quartz,  and  as  the  stage  revolves  other  particles  lighten  as  those 
which  polarized  in  the  previous  position  of  the  stage  become  dark.  From 
this  description  it  is  evident  that  the  texture  is  not  micropegmatitic  according 
to  the  generally  accepted  definition  of  the  term,  but  corresponds  to  the 
micropoikilitic,  as  described  by  Iddings  and  Williams.^ 

A  gradation  toward  a  spherulitic  texture  was  noticed  in  one  instance 
where  a  number  of  long  quartz  stringers  were  arranged  pei-jjendicular  to 
the  periphery  of  the  quartz  phenocryst.  (Fig.  B,  PI.  XX.)  The  texture 
of  this  micropoikilitic  mass,  it  will  be  observed,  is  finer  than  that  before 
described. 

The  groundmass  of  the  porphyries  is  formed  of  irregular  roundish 
areas  having  exactly  the  same  micropoikilitic  texture  as  the  zones  surround- 
ing the  quartz  phenocrysts.  An  explanation  of  the  origin  of  the  zones 
should  therefore  also  explain  the  texture  of  the  groundmass.  Certainly  in 
many  cases,  probably  in  most  cases,  the  groundmass  areas  result  from 
tangential  sections  through  one  of  the  micropoikilitic  zones  surrounding 
the  quartz  phenocrysts. 

The  description  given  by  Hedstrom^  of  this  same  structure  as  observed 
by  him  is  essentially  the  same  as  the  above,  if  I  have  understood  him  cor- 
rectly.    The  following  difference  is,  however,  to  be  noted.     In  speaking  of 

1  Op.  cit.,  pp.  589  and  179.  ^Op.  cit.,  p.  8. 


ACID  VOLCANICS  OF  UEMLOCK  FORMATION.  85 

t\w  stnic'turt'  where  tliu  (iiuutz  i.s  siuTuunded  by  this  luicrojjoikilitic  zone,  he 
calls  it  the  graiiophyric  structure.  As  I  liave  already  emphasized  alcove, 
the  feldspars  in  the  network  of  (piartz  have  varying  orientation,  and  tlie 
structure  is,  strictly  speaking,  micropoikilitic,  and  in  no  sense  granophyric 
(micropegmatitic).  Moreover,  he  describes  in  addition  to  tlie  above  type 
one  in  which  are  found  i)henocr}'sts  of  quartz  lying  in  a  micropoikilitic 
groundmass  with  the  above  reticulating  texture,  but  the  phenocrysts  abut 
sharply  against  the  groundmass,  instead  of  being  connected  with  it  by 
means  of  these  zones. 

The  micropoikilitic  texture  has  been  held  in  some  cases  to  be  of  sec- 
ondary origin  and  the  result  of  devitrification.  While  recognizing  that 
there  may  be  certain  unquestionable  cases  where  a  ixiicropoikilitic  structure 
results  from  the  devitrification  of  a  glassy  groundmass,  I  can  find  no  evi- 
dence in  the  rocks  here  described  that  points  to  this  origin  for  the  micro- 
poikilitic texture  under  discussion.  On  the  other  hand,  there  is  an  absence 
of  evidence  that  indicates  its  unquestionably  primary  character.  Rather 
than  to  regard  the  quartz  as  secondary  and  influenced  in  its  orientation  by 
the  phenocrysts,  as  in  the  enlargements  of  quartz  grains,  it  seems  natural 
to  suppose  that  when  the  lava  was  extruded  after  the  crystallization  of  the 
phenocrysts,  there  began,  consequent  upon  the  diminished  pressure  and 
temperature  and  other  factors,  a  rapid  crystallization  of  the  mineral  elements 
from  the  remaining  magma.  This  resulted  in  the  production  of  the  feldspar 
in  very  imperfect  and  small  crystal  individuals.  At  the  same  time  the  quartz 
of  the  phenocrysts  continued  to  grow,  and  in  so  doing  inclosed  these  small 
feldspars  in  its  meshes. 

In  certain  rhyolite-porphyries  the  micropoikilitic  texture  is  somewhat 
different  from  that  above  described.  In  these  the  quartz  phenocrysts  are 
surrounded  by  zones  which  are  illustrated  in  figs.  A  and  B,  PI.  XXI. 
These  appear  to  correspond  very  closely  to  the  ones  described  by  Michel 
Ldvy^  and  VViUiams,-  and  since  described  by  many  other  writers.  The 
zones  have  a  much  higher  index  of  refraction  than  the  quartz  of  the 
phenocrysts,  and  hence  contrast  strongly  with  it.  Examined  closely, 
they  are  seen  to  be  composed  of  chlorite,  epidote,  and  black   or  reddish 


'Annales  des  Mines,  Vol.  VIII,  1875,  pp.  378,  381. 

=iDie  Eruptivgesteiue  der  Gegeucl  vou  Tiiberg  im  SchwarzwaUl,  by  G.  H.  Williams:  N.  Jahrb. 
fill-  Jliu.,  Bil.  II,  1883,  p.  605. 


86  THE  CRYSTAL  FALLS  lORN-BEARING  DISTRICT. 

ferruginous  grains,  which  He  in  a  white  matrix.  This  matrix  shows  the 
following  characters:  The  greater  part  of  it  extinguishes  and  lightens 
simultaneouslv  with  the  quartz  phenocrysts  which  it  surrounds,  and  is 
consequently  believed  to  be  quartz.  When  the  matrix  and  quartz  pheno- 
crysts  are  dark,  one  sees  scattered  through  the  matrix,  making  up  a  very 
small  proportion  of  the  total  zone,  certain  irregular  areas  which  show 
polarization  effects.  These  are  believed  to  be  feldspar  grains,  though  this 
could  not  be  determined.  With  the  highest  magnification  no  radial 
arrangement  of  the  quartz  and  feldspar  could  be  observed  which  would 
wai-rant  the  inclusion  of  these  aureoles  under  Michel  Levy's  term  "  sjjhero- 
lites  a  quarts  (jlohidaire."^  Where  two  quartz  crystals  with  different  orienta- 
tion are  in  juxtaposition,  each  possesses  its  own  zone  corresponding  with  it 
in  orientation.  The  way  in  which  the  zones  about  the  quartzes  are  confined 
to  the  quartz  is  clearly  shown  in  one  case  in  which  a  very  much  altered 
feldspar  phenocryst  was  found,  one  portion  possessing  a  typical  coarse 
microjDCgmatitic  texture.  In  this  case  where  the  quartz  of  the  micropeg- 
matitic  intergrowth  touches  the  groundmass,  it  grades  into  a  micropoikilitic 
area,  whei'eas  the  feldspar  does  not  do  so. 

The  texture  of  the  zones  about  the  quartzes  is  apparently  but  a  fine- 
grained variety  of  the  micropoikilitic  texture,  the  coarser  phases  of  which 
are  illusti'ated  on  PI.  XX. 

The  groundmass  of  the  rocks  showing  the  texture  is  composed  of 
roundish  areas  of  exactly  the  same  composition  as  the  zones  around  the 
phenocrysts,  with  a  feldspar  of  small  dimensions  here  and  there  between 
these  areas.  (See  fig.  B,  PI.  XXI.)  The  texture  approaches  very  closely 
if  it  does  not  correspond  exactly  to  the  quartz  ^pongeuse  phase  of  the 
quartz-globulaire  texture  of  the  French."  In  one  part  of  a  section  of  rhy- 
olite-porphjTy  the  quartz  phenocrysts  have  aureoles  and  the  groundmass 
has  the  texture  just  described.  In  another  portion  of  the  section  the  quartz 
phenocrysts  have  no  aureoles  and  the  groundmass  possesses  an  imperfect 
microgranitic  texture  (structure  microgranulitique  of  Michel  Ldvy).  This 
shows  the  passage  of  a  micropoikilitic  textured  rock  into  one  with  a  micro- 
granitic  texture.     I   explain  the  aureoles  and  the  roundish  areas  in  the 

'  structures  et  classification  des  rocbes  6rui)tives,  by  A.  Michel  L(?vy,  Paris,  1889,  p.  21. 
=  lt  is  found  to  show  exactly  the  same  texture  as  seen  in  a  section  obtained  from  Paris  and 
labeled  "  Porjihyre  ;i  quartz  globulaire  de  la  Sartbe.'' 


ACID  VOLGANICS  OF  HEMLOCK  FORMATION.  87 

groundmass  as  original,  in  exactly  the  same  way  as  has  been  suggested  hy 
Williams'  tor  those  which  he  described.  This  is  essentially  the  same  expla- 
nation which  I  have  given  on  a  previous  page  for  the  less  common,  coarse 
micropoikilitic  phase.  The  cause  of  the  formation  of  the  microgranitic 
phase  appears,  however,  rather  difficult  to  discern.  Its  development  seems 
to  depend  upon  peculiar  local  conditions. 

APORHYOLITE-PORPHYKY. 

Intimately  associated  with  the  rhyolite-porphyries  are  rocks  very  similar 
to  them  in  mineral  constituents,  both  macroscopically  and  microscopically, 
so  that  the  description  of  the  rhyolite-porphyries  will  largely  answer  for 
the  aporhyolite-porphyries.  Flow  texture,  however,  is  well  shown  by  the 
aporhyolite-porphyries.  A  beautifully  developed  perlitic  parting,  fig.  A,  PI. 
XXII,  is  taken  to  indicate  the  presence  of  an  original  glass;  hence  the  rocks 
are  classed  with  the  aporhyolites.  The  perlitic  cracks  are  well  brought  out 
in  ordinary  light  by  the  chloritic  flakes  along  them.  Between  crossed 
nicols  these  disappear,  and  the  groundmass  resolves  itself  into  a  fine-grained 
mosaic  of  quartz  and  feldspar.  (Fig.  B,  PI.  XXII.)  This  groundmass  has 
all  the  characters  of  that  of  a  microgranite. 

No  evidence  wdiich  would  point  to  the  devitrification  of  a  glass  could 
be  seen  other  than  the  presence  of  a  perlitic  parting,  as  described.  For 
recent  excellent  descriptions  of  similar  devitrified  lavas  in  which  various 
structures  characteristic  of  vitreous  lavas  have  been  identified,  the  reader  is 
referred  to  the  articles  already  mentioned,  and  the  one  by  Dr.  Bascom,-  in 
which  a  moderately  full  bibliography  is  found. 

SCHISTOSE   ACID   LAVAS. 

The  results  of  the  ordinary  alterations  of  the  acid  lavas,  chiefly  meta- 
somatic  in  character,  by  which  the  phenocrysts  and  the  matrix  ha^•e  been 
changed,  have  been  briefly  described.  The  results  produced  by  dynamic 
action  are  more  interesting  and  perhaps  more  striking.  The  mashing,  result- 
ing in  chemical  changes  and  schistose  structure,  has  in  many  cases  almost 
obliterated  the  porphyritic  texture,  and  in  extreme  cases  destroyed  all  inter- 
nal evidence  of  igneous  origin.  Even  the  fluxion  banding,  as  is  well  known, 
at  times  simulates  very  closel}"  sedimentary  bedding,  and  thus  increases 

'Op.  cit.,  p.  607. 

-Acid  volcanic  rocks  of  South  Mouutain,  by  Dr.  Florence  B.ascom:  Bull.  U.  S.  Geol.  Survey  No. 
136, 1896,  p.  87. 


88  THE  CKrSTAL  FALLS  IRON-BEARING  DISTRICT. 

the  difficulty  of  determiniug  the  igneous  character  of  the  rock.  In  the  rocks 
to  be  described  the  phenocrysts  may  still  be  observed,  though  more  or  less 
deformed,  and  the  fluxion  banding  has  been  in  one  case  exceptionally  well 
preserved,  so  that  no  doubt  is  felt  as  to  their  igneous  character.  Dynam- 
ically metamorphosed  rhyolite-porphyry  flows  have  been  found  in  two  areas 
in  the  Hemlock  formation.  In  the  following  each  area  will  be  described 
separately,  the  one  in  which  the  original  character  of  the  porphyry  is  least 
in  doubt  being  considered  first. 

The  Deer  River  schistose  porphyries  are  found  in  the  SE.  J  sec.  36,  T. 
44  N.,  R  32  W.,  beginning  at  400  N.,  250  W.,  and  continuing  to  600  N., 
350  W.,  of  the  southeast  corner  near  the  bridge  on  the  Floodwood  road. 
They  occur  in  several  outcrops  which  are  practically  continuous,  being 
separated  by  very  short  distances,  and  are  so  much  alike  both  macroscopic- 
ally  and  microscopically  that  there  is  sufficient  reason  for  the  conclusion 
that  they  belong  together.  Their  field  relations  to  other  rocks  have  not 
been  observed.  No  data  have  been  found  which  offer  any  clue  as  to  the 
time  of  eruption  of  these  rocks  other  than  the  fact  that  they  are  surrounded 
by  the  basic  volcanics  of  the  Hemlock  formation  and  have  undergone  the 
same  dynamic  action. 

The  porphyries  are  dense,  bluish-black  rocks,  with  porphyritic  crystals 
of  red  feldspar.  A  fluidal  structure  is  not  present  in  them.  The  schistose 
structure  is  apparent  to  the  eye,  especially  upon  the  weathered  surface,  and 
the  cleavage  of  the  rock  also  indicates  it.  The  cleavage  face  of  the  rock 
has  a  silky  luster,  due  to  the  sericite  and  biotite  flakes  parallel  to  it.  The 
rock  breaks  readily  in  various  directions  at  angles  to  the  cleavage,  so  that 
it  is  impossible  to  obtain  well-shaped  hand  specimens.  The  schistosity  in 
these  porphyries  is  clearly  brought  out  by  weathering,  the  weathered  rocks 
showing  perfect  schistosity,  while  fresh  specimens  from  the  same  exposure, 
although  splitting  easiest  in  one  direction,  appear  perfectly  massive  in  hand 
specimens  when  broken  across  the  schistosity.  That  the  dynamic  action 
was  greatest  along  certain  zones  of  the  rock,  other  portions  being  more 
or  less  exempt,  is  shown  by  the  fact  that  of  several  specimens  collected 
with  the  view  of  obtaining  different  stages  of  alteration  from  different 
portions  of  the  same  exposure  some  are  markedly  schistose,  while  the  least 
altered  approach  a  fairly  massive  character. 

I  shall  give  a  brief  description  of  tliis  least  altered  phase,  and  then 


AOID  VOLGANIGS  OF  HEMLOCK  FORMATION.  SB" 

cou.sider  the   cliiuigx's  wliicli  liuve   tukeii   place  and   tlie  character  of  the 
rock  which  has  resulted  in  the  more  altered  phases. 

The  slightly  schistose  rock,  like  all  the  porphyries,  is  very  fine  gi-ained 
and  l)lack,  with  a  more  or  less  silky  luster  on  fresh  fractures  parallel  to 
the  schistosity.  The  porphyritic  character  is  not  very  strongly  marked. 
Maotroscopically,  comparatively  few  small  feldspar  phenocrysts  are  visible. 
Under  the  microscope  the  rock  is  seen  to  be  a  micropegmatitic  rhyolite- 
porphyry  in  which  the  silica  has  not  crystallized  as  quartz  phenocrysts, 
bi<it  has  remained  in  the  groundmass.  The  feldspar  phenocrysts  are  both 
orthoclase  and  plagioclase.  The  latter  shows  its  usual  characters,  but  is 
iK)t  present  in  well-formed  crystals.  The  orthoclase,  on  the  contrary,  is 
well  crystallized,  occurring  iii  Carlsbad  twins.  While  some  of  the  feldspar 
crystals  are  broken,  they  as  a  rule  do  not  show  many  signs  of  pressure 
The  fine-grained  micropegmatitic  groundmass  is  made  up  of  the  quartz  and 
feldspar  intergrowth  and  of  secondary  mica,  both  muscovite  and  biotite, 
and  remnants  of  iron  oxide.  Micropegmatitic  intergrowths  of  quartz  and 
feldspar  occur  in  irregularly  shaped  areas  which  frequently  have  a  fairly 
large  quartz  at  the  center.  Very  similar  irregular  areas  which  seem  to  be 
composed  altogether  of  unstriated  feldspar  also  occur.  These  two  kinds  of 
areas  compose  the  greater  part  of  the  rock.  The  mineral  particles  fre- 
quently show  undulatory  extinction.  Between  the  micropegmatitic  inter- 
growths one  finds  here  and  there  granular  aggregates  of  quartz  and  striated 
and  unstriated  feldspar.  These  feldspar  grains,  and  likewise  the  feldspar 
intergrown  with  the  quartz,  are  considerably  altered.  Sericite  and  biotite 
are  present  in  considerable  quantity.  The  former  possesses  the  better  crys- 
tallographic  outlines,  the  biotite  being  usually  found  in  ragged  fragments. 
The  two  micas  occiu-  in  the  feldspars  and  lie  between  the  quartz  grains,  but 
not  in  them.  They  appear  to  be  secondary  products  from  the  feldspar. 
The  micas  lie  with  their  long  directions  approximately  parallel,  and  impart 
to  the  rock  its  schistose  character.  A  few  automorphic  crystals  of  apatite 
were  found.  There  occur  also  a  few  irregular  grains  of  a  dark  reddish 
brown  mineral  with  high  single  refraction,  but  which  is  isotropic.  This 
mineral  is  presumed  to  be  allanite,  though  conclusive  tests  could  not  be 
made.  Some  crystals  of  zircon  were  also  observed.  The  iron  oxide  is  evi- 
dently titaniferous,  probably  titaniferous  magnetite.  Secondary  calcite  is 
scattered  through  the  rock  in  considerable  quantity. 


90  THE  CEYSTAL  FALLS  IROiSI -BEARING  DISTRICl. 

In  a  more  altered  phase  of  the  porphp-}'  exhibited  in  a  number  of 
specimens,  the  schistose  structure  is  much  better  marked  both  macroscopic- 
ally  and  microscopically.  The  macroscopical  appearance  is  otherwise 
quite  similar  to  the  one  just  described.  Under  the  microscope  the  pheno- 
crysts  show  up  well.  These  are  rounded  and  shattered  orthoclase  and 
plagioclase  feldspars.  They  lie  usually  with  their  long  direction  in  the 
lines  of  marked  schistosity  of  the  rock.  The  larger  crj'stals  have  been 
much  more  generally  fractured  than  have  the  smaller  ones,  and  seem  to 
have  obtained  relief  from  sti-ain  in  that  way,  the  individual  fragments  not 
showing  very  strong  dpiamic  effects.  The  small  crystals  are  more  or  less 
rounded.  The  crushing  to  which  the  rock  has  been  subjected  has  severed 
the  fragments  in  a  number  of  cases.  Triangular  areas  on  two  sides  of  the 
broken  or  unljroken  feldspars  in  the  direction  of  schistosity  are  filled  with 
what  appear  to  be  secondary  quartz  and  flakes  of  biotite.  The  feldspars 
as  a  whole  have  undergone  considerable  chemical  changes,  the  freshest 
being  red  and  very  cloudy.  Those  more  altered  show  secondary  muscovite 
and  biotite  scattered  through  them.  The  character  of  the  triclinic  feldspar 
could  not  be  determined.  It  appears,  however,  to  be  very  rich  in  calcium, 
as  in  some  of  the  badly  weathered  sections  the  feldspar  fragments  may 
almost  be  said  to  lie  in  a  calcite  matrix,  resulting  appai-ently  from  the 
alteration  of  the  feldspar  and  not  from  infiltration.  No  quartz  phenocrysts 
retaining  their  normal  character  are  found.  There  occur  here  and  there, 
however,  small  rounded  mosaics  of  quartz,  the  individual  grains  of  which 
show  undulatory  extinction.  These  are  e^^dently  the  result  of  the  granula- 
tion of  quartz  grains,  such  as  occur  in  the  freshest  specimens.  It  is  well 
known  that  the  quartz  is  more  easily  affected  by  pressure  than  feldspar, 
and  Futterer^  has  shown  that  they  may  be  found  in  a  completelj^  crushed 
condition,  in  the  same  section  with  feldspars  which  still  retain  their  regular 
crystal  contours. 

The  groundmass  of  the  poqihp-y  is  made  up  of  quartz  and  feldspar, 
in  and  between  which  lie  leaflets  of  biotite  and  sericite.  The  holocrystalline 
granular  mixture  of  quartz  and  feldspar  is  very  fine  grained,  and  the  pres- 
ence of  the  feldspar  was  only  determined  by  difference  in  the  refraction  of 
the  two  minerals.     No  striated  feldspar  grains  were  observed.     The  second- 

1  Die  "Ganggranite"  von  Grosssachsen,  und  die  Quartzporpbyre  von  Thai  im  Thiiringerwald, 
by  Karl  Fiitterer,  Heidelberg,  1890,  pp.  31, 126. 


ACID  VOLCANICS  OF  HEMLOCK  FORMATION.  91 

ary  micas  appear  usiuiUy  in  ragged  flakes,  though  the  shglitly  greenish- 
yeUow  sericite  flakes  approach  crystal  nntlines  rather  frequently.  The 
biotite  is  lm>\vnish-green  and  strongly  i)leochroic.  A  few  spots  of  brown 
iron  hydroxide  and  small  heaps  of  grains  of  sphene  probably  indicate  the 
former  presence  of  titaniferous  iron  ore.  The  few  crystals  of  apatite  present 
are  broken  and  separated,  but  otherwise  retain  the  usual  characters  of  this 

mineral. 

The  groundmass  has  a  very  marked  schistose  structure,  brought  out 
especially  well  by  the  i)arallel  arrangement  of  the  mica  flakes.  The  way 
in  which  these  lines  of  schistosity  flow  around  the  mashed  phenocrysts,  one 
line  never  coalescing  with  another,  but  remaining  continuous,  may  be  seen 
with  great  distinctness  where  the  lines  abut  sharply  against  the  crystal  at  a 
very  obtuse  angle.  As  the  angle  becomes  less  and  less  obtuse,  the  ends  of 
these  lines  bend  up  slightly  in  the  direction  which  would  enable  them  to 
pass  the  crystal,  and  then  end,  so  that  along  the  face  of  the  crystal  one  can 
follow  them,  as  it  were,  in  a  series  of  steps  until  those  lines  which  strike  the 
crystal  near  enough  the  edge  to  flow  around  it  bend  slightly,  and  passing 
around  continue  on  the  opposite  side.  The  fact  noted  by  Fiitterer^  that  an 
increased  amount  of  sericite  occurs  on  the  two  sides  of  the  feldspar  crystals 
parallel  to  the  schistosity  is  very  patent  in  these  porphyries.  The  diminu- 
tion in  grain  of  quartz  and  feldspar  seems  to  accompany  the  increase  in  the 
amount  of  the  sericite.  The  slides  are  crossed  by  narrow  fractures  cutting 
the  planes  of  schistosity,  which  are  filled  with  secondary  quartz,  showing 
marked  sti-ain  efi"ects.  Associated  with  the  quartz  were  observed  some 
crystals  of  brown  nitile.  In  one  of  the  more  altered  slides  these  fissures 
have  been  filled  with  calcite,  whether  or  not  as  a  replacement  of  the  quartz 
could  not  be  told. 

Schistose  porphyries  showing  the  extreme  alteration  phases  are  found 
from  N.  300,  W.  300,  to  N.  400,  W.  250,  in  the  SE.  i  sec.  4,  T.  44  N.,  R. 
32  W.  They  form  a  rough  escai-pment  upon  the  southeast  side  of  and  near 
the  base  of  a  large  hill,  and  overlook  McCutcheon's  Lake.  The  exposure  is 
not  continuous  throughout,  though  practically  so,  but  the  unexposed  parts 
are  sufiicient  to  prevent  a  perfect  sequence  being  traced.  The  appearance 
of  the  rock  is  strongly  like  that  of  sedimentary  rocks.  Difterent  bands 
nearly  on  edge  may  be  seen,  dipping  60°-90°  SW.  and  striking  N.  30°  W. 

I  O]).  cit.,  p.  40. 


92  THE  CEYSTAL  FALLS  IRON-BEARING  DISTRICT. 

At  a  point  about  100  feet  higher  and  three-fourths  of  a  mile  distant,  oq 
the  very  northwest  flank  of  the  same  hill,  at  N.  1725,  W.  775,  from  the  south- 
east corner  of  sec.  4,  T.  44  N.,  R.  32  W.,  there  is  a  small  ledge  of  schistose 
jDorphyry,  macroscopically  and  microscopically  similar  to  those  to  the  south- 
east of  it,  and  with  its  schistosity  striking  N.  20°  W.  and  dipping  80°  SW. 
The  striking  agreement  in  strike,  dip,  and  general  character  of  these  two 
separated  outcrops  points  to  their  being  merely  isolated  portions  of  the 
same  mass.  There  seems  to  be  no  discrepancy  between  the  dip  and  strike 
of  the  schistosity  and  that  given  above  for  the  bands. 

The  most  striking  macroscopical  characteristic  of  these  mashed  por- 
phyry flows  is  the  occuiTCUce  of  phenocrysts  in  a  schistose  and  beautifully 
banded  rock.  These  phenocrysts  stand  ont  clearly  from  the  groundmass^ 
in  all  cases.  The  general  appearance  of  the  rocks  is  that  of  the  well-known 
very  dense  banded  halleflintas  of  Elfdalen,  Sweden.  The  bands  vary  in 
color,  ranging  on  the  weathered  surface  from  light  creamy  white,  tln'ough 
light  greenish,  to  red  and  almost  black.  Tlie  rocks  which  have  very  light 
colored  weathered  surfaces  are  always  bluish  black  on  a  fresh  fracture,  and 
very  dense,  and  those  weathering  red  are  usually  cream  colored  on  freshly 
fractured  faces;  Many  of  the  areas  which  appear  macroscopically  to  be 
single  phenocrysts  are  resolved  under  the  microscope  into  tangled  groups 
of  individuals,  though  in  rare  cases  the  individuals  show  the  imperfect 
radial  arrangement  rather  frequent  in  medium-grained  micropegmatitic 
rhyolite-porphyries. 

The  feldspar  has  iindergone  considerable  alteration.  In  the  least- 
changed  grains  there  is  a  cloudiness  caused  by  numerous  indeterminable 
specks,  probably  of  iron  oxide,  which  give  a  reddish  tinge  to  the  mineral. 
Further  changes  result  in  the  production  of  muscovite  and  epidote,  with 
biotite  in  rare  cases,  accompanied  by  the  obliteration  of  the  twinning 
lamellse.  The  greater  part  of  the  phenocrysts  seem  to  be  orthoclase, 
though  associated  with  them  are  found  pieces  which  show  indistinct  traces 
of  the  polysynthetic  twinning  of  plagioclase  feldspar.  The  feldspars 
exhibit  marked  strain  eff'ects,  especially  in  their  flattening  into  long  oval 
and  spindle-shaped  areas.  Some  crystals  have  been  broken  and  separated 
perpendicular  to  the  direction  of  the  schistosity.  The  spaces  between  the 
fragments  are  filled  with  secondarj-  muscovite,  quartz,  and  feldspar.  Sur- 
rounding the  phenocrysts — that  is,  between  the  phenocrysts  und  the  ground- 


ACID  VOLOANICS  OF  HEMLOGK  FOKMATION.  93 

mass  proper — we  find  a  mass  of  small  angular,  finely  striated,  limpid  grains 
of  feldspar,  associated  with  similar  grains  of  quartz,  tlie  two  lumng  in 
places  between  tliein  sericitic  fiakes.  In  one  especiall}-  clear  case,  this 
secondary  aggregate  fills  half  the  space  formerly  occupied  by  an  individual 
feldspar,  the  other  half  being  still  occupied  In^  the  remnant  of  the  appar- 
ently simply  twinned  feldspar  from  which  it  was  derived.  (Fig.  A,  PI. 
XXIII.) 

While  no  large  quartz  phenocrysts  were  observed,  a  mosaic  of  qviartz 
is  found  in  small  round  or  oval  areas  in  various  sections.  The  individual 
fragments  exhibit  the  usual  strain  effects  of  crushed  minerals.  (Figs.  A,  B, 
PI.  XXIV.) 

The  groundmass  consists  of  the  same  preponderant  minerals  as  the 
scliistose  porphyries,  which  have  been  previously  described.  The  accessory 
minerals  are  apatite,  which  is  present  in  very  small  quantity,  and  rutile, 
which  in  one  of  the  slides  is  present  in  verj'  considerable  quantity  in  the 
form  known  as  "thonshiefer-nadelchen."  Calcite  is  found  in  all  of  the 
slides,  the  amount  varying  very  much.  Those  which  contain  a  great  deal 
have  a  scoriaceous-looking  surface,  due  to  weathering  out  of  the  calcite. 

The  flow  structure  mentioned  as  having  been  observed  in  the  schistose 
porphyries  of  the  Hemlock  formation  is  perhaps  of  sufficient  general  interest 
to  warrant  a  few  comments.  This  is  well  marked  only  on  one  hand  speci- 
men. In  this  there  is  an  alternation  of  pink  and  dark  grayish-blue  bands 
which  are  rarely  more  than  a  fraction  of  an  inch  thick.  Some,  especially 
the  thicker  bands,  are  remai-kably  persistent.  Even  macroscopically  on  the 
weathered  surface  the  pinkish  bands  can  be  distinctly  seen  to  wrap  around 
the  pink  feldspar  phenocrysts  and  oval  areas  of  the  grayish-blue  part  of  the 
rock.  Under  the  microscope  the  bands  which  macroscopically  are  the 
darkest  are  clear  and  transparent,  while  the  pink  bands  are  much  less  trans- 
parent. The  microscope  shows  the  diff"ereuce  in  the  color  of  the  bands  to 
be  due  chiefly  to  tlie  fineness  of  grain,  and  brings  out  the  flow  structure 
even  more  clearly  than  the  weathered  surface.  (Figs.  A  and  B,  PI.  XXIV.) 
Accompanying  this  variation  of  grain  there  is  also  a  diff"erence  in  mineral- 
ogical  composition.  The  dark  bands  are  composed  essentiall}-  of  quartz 
grains,  with  feldspar,  sericite,  some  magnetite,  considerable  calcite,  and 
rare  crystals  of  apatite  and  rutile,  the  quartz  including-  many  black  and 
indeterminable  specks.      The  pink  bands  are  very  fine  grained,  so  much 


94  THE  CRYSTAL  FALLS  IRO:J^-BEARING  DISTEICT. 

so  that  tlie  clear  white  mmeral  grains  composing  it  can  uot  be  determined, 
though  probably  both  quartz  and  feldspar  are  present.  These  bands  are 
darkened  by  innumerable  black  indeterminable  specks  and  long  rutile 
needles,  with  a  small  amount  of  biotite.  It  is  possible  that  some  of  the 
minute  biotite  flakes  have  been  mistaken  for  rutile  needles  when  viewed  on 
edge,  but  it  is  certain  that(  these  bands  contain  a  great  deal  more  rutile 
than  do  the  others.  Whether  or  not  there  is  a  still  more  essential  chemical 
difference  between  the  bands  than  that  indicated  by  the  increased  quantity 
of  rutile,  was  not  determined. 

It  has  become  more  or  less  common  of  late  to  attribute  the  banding 
found  in  metamorphosed  eruptives  altogether  to  the  pressure  to  which  they 
have  been  subjected.  In  the  present  instance  I  can  not  but  consider  the 
banding  as  being  an  original  fluxion  structure,  with  the  slight  original 
difterences  between  the  bands  emphasized,  as  it  were,  by  subsequent 
pressure.  It  appears  highly  probable  that  the  rock  was  originally  more 
or  less  glassy  and  showed  a  flowage  structure,  and  that  the  present  miner- 
aloo-ical  character  of  the  groundmass  is  due  to  the  process  of  devitrification 
wliicli  did  not  destroy  the  banding  of  the  original  glass. 

ACID     PYROCLASTICS. 

The  only  acid  pyi-oclastic  rock  found  was  formed  from  the  aporhyolite- 
porphyry.  This  is  a  true  eruptive  breccia.  The  fragments  are  angular  to 
rounded  in  shape,  weather  to  a  pure  white  color,  and  have  an  exceedingly 
rough  surface.  This  roughness  is  due  to  a  great  extent  to  perhtic  partings, 
which  are  macroscopically  visible,  and  give  the  rock  an  almost  scoriaceous 
appearance.  Other  inequalities  on  the  surface  adding  to  its  rouglmess  are 
caused  by  the  leacliing  out  of  feldspars,  and  by  the  fact  that  many  of  the 
quartz  phenocrysts  have  fallen  out  of  the  inclosing  matrix  The  fragments 
are  all  aporhyolite-porphyry,  containing  a  very  large  proportion  of  quartz 
and  feldspar  phenocrysts.  The  cement  of  the  breccia  is  aporhyolite.  This 
contains  far  less  numerous  phenocrysts,  and  is,  therefore,  on  the  whole  much 
finer  grained  than  the  fragments.  The  weathered  surface  of  the  cementing 
aporhyolite  appears  a  bluish  gray,  and  is  very  smooth  compared  to  the 
scoriaceous  appearing  surface  of  the  fragments  ah-eady  described.  Tliis  dif- 
ference in  weathering  shows  the  brecciated  character  admirably,  as  the  finer- 
grained  matrix  stands  out  sharply  and  delimits  the  contours  of  the  fragments. 


BASIC  VOLCANICS  OF  HEMLOCK  FORMATION.  95 

Movi'iiK'Hts  of  the  inayina  are  shown  by  a  flowage  sti'ucture  in  the 
matrix  and  by  the  fracturing  of  the  quartz  and  feldspar  phenocrysts  and 
separation  of  the  pieces  in  ))oth  the  cement  and  fragments.  (Fig.  B,  PI. 
XXIII.) 

This  eruptive  breccia  can  be  seen  in  its  best  development  in  the  NW.- 
SK.  trending  ridge,  just  west  of  the  small  lake,  crossed  by  the  Chicago, 
Milwaukee  and  St.  Paul  track  in  sec.  32,  T.  44  N.,  R.  32  W. 

BASIC  VOLiCANICS. 

The  basic  volcanics  are  considered  under  the  main  headings  of  lavas, 
pyroclastics,  and  Bone  Lake  crystalline  schists. 

BASIC  LAVAS. 
GENERAL  CHARACTERS. 

The  basic  lavas  are  so  very  characteristically  developed  that  no  one 
could  for  a  moment  doubt  their  trae  natm-e,  even  upon  the  most  superficial 
examination.  One  of  the  nearly  general  characters  is  the  presence  of  a 
well-marked  amygdaldidal  texture.  (Figs.  A^  B,  Pis.  XXV  and  XXVI, 
and  fig.  A,  PL  XXVII.)  Some  of  the  lavas  are  so  full  of  amygdules  that 
they  may  be  correctly  said  to  have  been  scoriaceous.  The  amygdaloidal 
portions  of  the  rock  masses — which  may  be  considered  the  surface  parts — 
grade  over  into  other  portions,  the  interiors  of  the  lava  flows,  which  are, 
macroscopically  at  least,  nonamygdaloidal.  Owing  to  the  homogeneous 
character  of  the  basic  magmas,  a  fluxion  structure  is  rarely  shown  macro- 
scopically, though  microscopically  it  may  be  more  or  less  well  developed. 
Columnar  jointing  was  nowhere  observed.  An  ellipsoidal  parting,  on  the 
other  hand,  is  common. 

NOMENCLATURE. 

In  a  preliminary  article  on  the  Hemlock  volcanics,^  I  made  a  brief 
mention  of  the  occun-ence  on  the  Upper  Peninsula  of  Michigan  of  the 
basic  pre-Tertiary  equivalents  of  the  post-Tertiary  and  Recent  family  of 
basalts.  Following  the  Danas,  Wadsworth,  Williams,  Iddiugs,  Kemp, 
Darton,  and  Diller,  some  of  the  most  influential  of  the  men  who,  in  the 

'  The  volcanics  of  thj  Michigamme  district  of  Michigan,  by  J.  Morgan  Clements:  Jour.  Geol., 
Vol.  Ill,  1895,  pp.  801-822. 


96  THE  CRYSTAL  FALLS  IRON-BEAEING  DISTRICT. 

United  States,  have  advocated  the  simplification  of  petrogTaphical  nomen- 
chxture,  I  used  the  term  basah,  now  ordinarily  used  for  the  Tertiary  or  post- 
Tertiary  basic  rocks.  This  term  was,  however,  modified  by  the  prefix 
"apo,"  as  indicating  their  altered  condition  and  the  presumed  presence  of  a 
glassy  base.^  This  was  a  logical  continuation  of  the  use  of  the  prefix  as 
proposed  by  Dr.  Bascom'  for  devitrified  acid  lavas. 

More  detailed  studies  upon  the  Hemlock  volcanics  have  shown  the 
presence  of  rocks  which  were  apparently  originally  holocrystalline,  and 
therefore  do  uot  belong  with  the  altered  vitreous  basalts,  the  apobasalts,  and 
others  in  which  some  of  the  glass  is  apparently  unaltered.  Consequently, 
since  the  apobasalts  comprise  only  a  portion  of  the  Hemlock  volcanics,  the 
replacement  of  that  term  as  a  general  heading  by  the  older,  more  general, 
one  of  basalt  was  considered. 

The  use  of  this  term  is,  however,  not  altogether  satisfactory,  for  the 
rocks,  while  clearly  recognizable  as  basalt  derivatives,  do  not  possess  the 
mineralogical  composition  of  the  basalts.  The  term  "apo"  having  been 
restricted,  as  above  indicated,  can  not  be  applied  to  them,  for  their  altera- 
tion is  in  many  cases  metasomatic  and  dpiamic,  "and  in  most  cases  not 
devitrification.  If  we  adopt  the  prefix  "meta"  to  indicate  alteration  of  all 
kinds,  then  these  rocks  could  be  called  "metabasalts." 

The  terms  "metadolerite,"  "metadiabase,"  etc.,  were  proposed  by  Dana^ 
for  metamorphic  dolerites,  diabases,  etc.,  and  first  used  by  Hawes*  in  the 
description  of  the  altered  rocks  around  New  Haven.  Recently  these  tenns 
have  been  revived,  but  with  a  very  diff"erent  significance  from  that  with 
which  they  were  first  used.  It  is  proposed  to  designate  bj^  such  terms 
"rocks  now  similar  in  mineralogical  composition  and  structm-e  to  certain 
igneous  rocks,  but  derived  by  metamorphism  from  something  else."^  Fol- 
lowing this  suggestion,  an  uralitized  dolerite  (diabase)  would  be  called  a 
metadiorite.     Such  a  use  of  the  term  does  not  seem  justified,  and  the 

1  Loc.  cit.,  p.  805. 

'^The  structures,  origin,  and  nomenclature  of  the  acid  volcanic  rocks  of  South  Mountain,  by 
Florence  Bascom.     Jour.  GeoL,  Vol.  1, 1893,  p.  82s. 

3  Chloritic  formation  of  New  Haven,  CoDuecticut,  by  J.  D.  Dana :  Am.  Jour.  Sci.,  3d  ser.,  Vol.  XI, 
1876,  pp.  119-122. 

••The  rocks  of  the  "chloritic  formation"  on  thevresteru  border  of  the  New  Haven  region,  byG.  W. 
Hawes :  Am.  Jour.  Sci.,  3d  ser.,  Vol.  XI,  1876,  pp.  122-126. 

'■  On  a  series  of  peculiar  schists  near  Salida,  Colorado,  by  Whitman  Cross :  Proc.  Colo.  Sci.  Soc, 
g).  6,  footnote.     Paper  read  Jan.  2, 1893. 


BASIC  VOLOANIOS  OF  HEMLOCK  FORMATION.  97 

ol))i'C'ti(iii  to  it  (•iiu  not  he  g'wvn  butter  tliau  by  (juotiug  the  words  wliicli 
Zirki'l  uses  in  the  disoussion  (tf  the  uietamorphisni  of  rocks:' 

Bei  solcheu  nietaraorphiscli  veriiiulerten  Gesteinen  ist  es  nicht  zweckmilssig,  sie 
niit  dem  Nameu  desjeiiigeu  Typus  zu  belegen,  dein  sie  durcb  die  Veriiriderung  iilinlich, 
oft  bios  scheinbar  iihiilicli  gewordeu  siiid.  Eiiie  solclie  Bezeichnuiig  werde  iiur  za 
inissverstiiiidliclien  Autt'assuug  der  von  dem  (iestein  gespielten  geologisclieu  Eolle 
fiihren,  welcbe  niemals  ausser  Acht  gelasseii  werden  darf.  Und  so  ist  es  deiin  ent- 
scUicdenvorziizielien,  der  Beneiiiimig  soldier  Gesteineeiue  Form  zu  gebeu,  in  wlielcber 
zuviirderst  auch  zuin  Ausdruck  koiiimt,  was  sie  friiber  gewesen  sind,  and  nicbt  eiueu 
Nauieu  zu  wiibleu,  der  sie  iu  erster  Linie  zu  etwas  stem  pelt,  mit  welcbem  sie  genetiscb 
keine  Gemeiuscbaft  baben. 

Using  these  terms  in  the  way  suggested  by  Cross,  attention  is  most 
pointedly  directed  to  that  variety  of  rock  which  the  secondary  product  now 
resembles  mineralogically,  rather  than  to  the  type  from  which  it  was  derived, 
and  which  in  all  likelihood  it  still  resembles  most  closely  in  its  chemical 
constitution.  Whether  or  not  a  petrographer  will  use  the  term  "metadio- 
rite"  or  the  term  "metadolerite"  (diabase)  for  a  metamorphosed  dolerite 
will  depend  on  whether  or  not  he  prefers  to  emphasize  the  present  miner- 
alogical  composition  of  the  rock,  or  its  original  characters,  and  thereby  its 
chemical  constitution  and  genetical  relations.  In  the  present  report  the 
terms  "metabasalt"  and  "metadolerite"  are  used  as  including  all  those 
altered  rocks  which  demonstrably  were  originally  basalts  and  dolerites. 

These  same  strictures  hold  good  in  the  case  of  Giimbel's  term  "epidior- 
ite,"  when  used,  as  it  is  very  commonly,  in  the  literature  of  the  Lake 
Superior  region  and  elsewhere,  for  rocks  avowedly  derived  from  dolerites 
(diabases),  and  characterized  by  the  presence  of  fibrous  secondary  amplii- 
bole.  It  is  preferred,  in  accordance  with  the  above  statement,  to  use  the 
term  "epidolerite"  (epidiabase)  instead  of  "epidiorite"  for  such  altered 
dolerites.  None  of  these  rocks,  unless  extremely  changed,  would  resemble 
chemically  a  diorite,  and  we  have  come  of  late  years  to  rely  more  and  more 
upon  the  chemical  composition,  combined  of  course  with  the  mineralogical 
composition  and  textures  of  the  rocks,  to  separate  the  vanous  kinds  from 
one  another.  As  stated  above,  the  term  "epi,"  associated  with  the  rock 
name,  has  come  more  and  more  to  be  restricted  in  its  use  solely  to  a  rock, 
the  epidiorite,  characterized  by  a  specific  alteration  product,  the  amphibole. 
In  respect  to  this  restriction  to  specific  alteration,  the  term  corresponds  to 

'  LehrbucU  der  Petrographie,  F.  Zirkel:  2(1  ed.,  Vol.  f,  \>.  573. 

MON  xxxvr 7 


98  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

"apo,"  and  it  is  iiufortuuate  that  these  two  terms  should  have  been  so  nar- 
rowly confined.  As  it  is,  the  epi-  and  apo-basalts  would  be  subordinate  to 
and  therefore  included  under  the  metabasalts,  as  this  term  is  used  in  this 
report.  In  the  first  the  production  of  secondary  hornblende  is  character- 
istic; in  the  second  the  process  of  devitrification,  and  hence  the  original 
presence  of  a  vitreous  base,  is  the  chief  characteristic.^ 

METABASALTS. 

All  of  the  basalts  belong  to  the  plagioclase  type.  They  may  be  most 
conveniently  divided  into  nonjjorjjhyritic  and  porphyritic  kinds,  according 
to  their  most  obvious  macroscopical  characters.  There  has  also  been  found 
a  single  occurrence  of  a  spherulitic  basalt,  which  will  be  described  under  the 
head  "  variolite." 

NONPORPHVUITIC   METABASALT. 

The  nonporphyritic  rocks  possess  a  fine-grained  or  aphanitic  structure 
and  are  amygdaloidal  or  nonamygdaloidal.  There  are  included  under 
this  general  name  the  microophitic-textured  fine-grained  pre-Cambrian 
basalts  (diabases  in  part),  the  very  amygdaloidal  forms  of  tlie  basalts 
(spilites),^  and  the  melaphyres  in  part.  In  these  rocks  the  former  presence 
of  a  considerable  amount  of  original  glass  is  probable,  and  they  show  the 
various  textures  known  as  navitic,  intersertal,  pilotaxitic,  and  hyalopilitic. 

With  the  nonporphyritic  basalts  there  have  been  included  some  rocks 
which  are  to  a  considerable  extent  devitrified  glasses,  and  others  in  which 
only  a  few  microlites  have  developed.  These  last  two  vitreous  types  occur 
more  especially  in  fragments  in  the  tuff's,  and  are  quantitatively  unimportant. 

petrographicai  characters. — 111  color  the  uouporphyritic  basalts  on  fresh  fracture 
show  various  uniform  shades  of  green,  dark  olive  green  usually  prevailing. 
Much  less  common  are  purplish-black  rocks,  and  these  are  much  more  vari- 
able in  color.  In  one  of  them  is  seen  lighter-colored  schlieren,  which  pass 
over  into  the  ordinary  dark  colors.  The  lighter-colored  portions  are  seen 
on  microscopical  examination  to  be  due  to  a  smaller  quantity  of  the  iron  in 
them  and  to  a  greater  quantity  of  chlorite  than  occurs  in  the  rest  of  the  rock 


'  The  above  discussion  was  written  and  the  determination  to  use  the  terms  porphyry — without 
textural  significauce,  as  in  rhyolite-porphyry — metabasalt,  etc.,  was  reached,  in  1896,  before  the  com- 
mittee on  petrographic  nomenclature  of  geologic  folios  was  appointed  by  the  Director  of  the  United 
States  Geological  Survey. 

'^  Microscopic  characters  of  rocks  and  minerals  of  Michigan,  by  A.  C.  Lane:  Kept.  State  Board  of 
Geol.  Survey  for  1891-92, 1893,  p.  182. 


BASIC  VOLCANIOS  OF  HP:ML()CK  FORMATION.  99 

mass.     Where  weathered,  the  rock.s  are  usually  covered  by  a  thin  crust,  in 
which  ^rav,  brown,  and  pinkish  tints  prevail. 

The  rocks  vary  in  texture  very  much,  from  the  dense  aphanitic  kinds 
to  medium  fiue-grained  varieties.  The  latter  are  usually  less  amygdaloidal 
than  are  the  aphanitic  forms,  and  approach  in  ajjpearance  both  macroscopic- 
ally  and  microscopically  the  coarser-grained  basalts  or  dolerites  represented 
m  the  Michigamme  district  by  the  coarse-grained  intrusives.  0\<'ing  to  the 
basic  nature  of  the  rocks,  they  have  generally  suifered  mucli  alteration,  and 
as  a  result  the  original  texture  is  in  many  cases  poorly  preserved.  On  the 
whole,  however,  it  is  remarkable,  considering  their  age  and  basic  character, 
how  well  preserved  it  is.  Where  it  is  preserved  it  varies  from  the  micro- 
ophitic  to  the  various  microlitic  textures,  such  as  intersertal,  navitic,  pilo- 
taxitic,  and  hyalopilitic,  and  lastly  glassy.  In  places  a  flowage  structure  is 
beautifulh"  brought  out  by  the  jjosition  of  the  feldspar  microlites,  especially 
around  the  amygdules. 

The  constituents  present  are  plagioclase,  light-green  fibrous  hornblende 
epidote-zoisite,  chlorite,  calcite,  muscovite,  apatite,  sphene,  quartz,  mao-net- 
ite,  and  pyrite.     Of  these  the  feldspar,  apatite,  and  iron  oxide  alone  are 
original.      In  some  places  the  hornblende   is  wanting,  the   chlorite   then 
appearing  in  correspondingly  greater  quantity. 

The  feldspar  ordinarily  occurs  in  lath-shaped  crystals  showing  twins  of 
the  albite  type,  but  where  the  texture  is  fine  the  feldspars  are  microlitic,  and, 
while  showing  their  prominent  long  extension,  the  edges  of  the  various  crys- 
tals interfere,  and  the  outlines  consequently  are  less  sharp. 

In  some  of  the  rocks  which  appear  to  have  been  vitreous  the  feldspar 
forms  feather  and  sheaf  like  aggregates  (figs.  A,  B,  PI.  XXVI),  apparently 
quite  similar  to  those  described  by  Ransome  in  rocks  from  Point  Bonita, 
California.^  No  reliable  measurements  could  be  made  upon  the  microlites, 
and  consequently  their  character  could  not  be  determined.  The  feldsj)ar  is 
more  or  less  completely  altered  to  aggregates  of  epidote-zoisite  which  have 
chlorite  associated  with  them  or  are  altered  to  sericite.  In  a  number  of 
places  minute  limpid  spots  of  secondary  qviartz  and  albite  are  present.  The 
very  small  quantity  of  apatite  present  shows  its  usual  characters.  Titauif- 
erous  magnetite  ore  is  apparently  the  only  oxide  present.  It  occurs  in  crys- 
tals and  in  irregular  grains,  which  in  a  few  cases  are  not  entirely  altered, 


'  Eruptive  rocks  of  Point  Bonita,  by  F.  Leslie  Ransome :  Bull.  Univ.  of  Cal.,  ^'ol.  1, 1893,  p.  84,  fig.  6. 


100  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

thoug'h  iu  most  cases  they  are  replaced  by  spheue.  In  some  cases  the 
alteration  product  is  not  well  enough  individualized  for  one  to  diag-nose  it 
as  sphene,  and  it  should  perhaps  be  called  leucoxene.  In  some  of  the  fine- 
grained rocks  the  matei'ial  in  the  angles  between  the  feldspars  consists  jjre- 
dominately  of  grains  of  magnetite.  This  abundant  magnetite  renders  the 
rock  very  dark,  giving  the  rare  purplish-black  lavas. 

The  most  of  the  hornblende  has  a  light-green  color.  A  lesser  portion 
shows  a  decided  bluish  tinge,  and  gives  fairly  strong  pleochroism.  This 
resembles  the  hornblende,  which  in  the  coarse  dolerites  is  undoubtedly  sec- 
ondary after  the  augite  and  it  is  considered  to  be  secondary  after  the  origi- 
nal augite  in  these  rocks. 

The  original  augite  was  presumably  in  most  cases  pi-esent  in  wedge- 
shaped  pieces  filling  the  spaces  between  the  feldspars,  and  consequently  the 
hornblende  pseudomorphs  never  show  augite  outlines.  No  unaltered  augite 
was  observed  amongst  the  hornblende  fibers.  The  fine  fibers  frequently 
form  a  fringe  beyond  the  original  boundaries  of  the  pieces  and  penetrate 
the  adjacent  felds])ar.  Quite  frequently  the  secondary  hornblende  shows 
partial  alteration  to  chlorite  and  epidote. 

Though  careful  search  was  made  for  olivine  or  indications  of  its  pres- 
ence, no  traces  of  it  were  found,  and  I  have  concluded  that  these  basic  vol- 
canics  were  essentially  nonolivine  bearing,  though  it  would  be  rash  to  state 
that  such  rocks  did'  not  contain  some  olivine. 

The  calcite  is  usually  found  in  irregular  secondary  granular  aggregates 
scattered  tlirough  the  rock,  and  evidently  replaces  the  other  minerals.  Less 
commonly  it  is  seen  as  an  infiltration  product  along  fissures. 

A  second  form  of  the  occurrence  of  calcite  in  the  nonporphyritic  meta- 
basalts,  and  one  not  so  conimon  as  the  granular  aggregate,  is  that  of  large 
porphyritic  atitomorphic  rhombohedra  and  scalenohedra  which  lie  embedded 
in  the  eruptive  groundmass.  Such  a  rock,  as,  for  instance,  Sp.  32472,  shows 
macroscopically  large  rhombohedral  phenocrysts  in  a  green  groundmass. 
On  the  weathered  surface  are  ferruginous  rhombohedral  cavities,  once  occu 
]3ied  by  the  carbonates.  The  groundmass  consists  of  rather  fresh  plagioclase 
microlites,  between  which  are  observed  some  quartz,  fresh  magnetite  crys- 
tals, and  lastly  chlorite  flakes  as  alteration  products  of  originally  present 
bisilicates  or  glass,  or  l)oth.  The  texture  is  undoubtedly  that  of  an  eruptive. 
The  carbonate  is  more  or  less  ferruginous,  brown  iron  hydroxide  resulting 


BASIC  VOU'ANICS  OF  HEMLOCK  FORMATION.  101 

from  its  alteration,  and  as  it  ctiervesces  quite  readily  with  cold  IICl,  it  is 
supposed  to  l)t'  ferruginous  falcite. 

Sericite  is  found  in  minute  flakes  replacing  the  feldspars,  and  it  is  also 
found  in  large  porphyritic  plates  occurring  in  the  eruptive  grounihiiass  asso- 
ciated witii  the  porphyritic  carbonate  al)ove  descrilied.  In  some  cases  we 
find  epidote  in  these  altered  basalts,  in  others  zoisite.  In  a  great  number 
of  instances  the  same  individual  exhibits  the  high  interference  colors  of 
epidote  and  the  low  l)lue  interference  color  of  zoisite  in  different  parts. 
These  different  portions,  formed  respectively  of  the  epidote  and  zoisite  mole- 
cules, are  most  closely  intergrown,  and  I  have  therefore  used  the  compound 
term  "epidote-zoisite,"  indicating  this  fact.  Associated  with  this,  one  finds 
m  many  of  the  specimens  small  mineral  aggregates  which  merit  somewhat 
further  notice.  These  aggregates  have  a  brownish-yellow  color  and  possess 
a  verv  high  single  and  also  a  high  double  refraction.  In  these  masses  the 
single  and  double  refractions  of  the  granules  composing  the  aggregates 
appear  to  be  higher  than  that  of  epidote.  In  shape  the  aggregates  vary 
from  perfectly  round,  zonally  arranged  spheres  and  irregular,  elongated, 
rounded  aggregates  to  forms  giving  oblique  quadratic  sections.  All  of  these 
ao-o-reo-ates  are  found  at  times  included  in  the  epidote-zoisite  crystals.  In  a 
few  cases  the  oblique  quadratic  sections  were  seen  to  occupy  the  centers  of  the 
epidote-zoisite  crystals,  having  exactly  identical  outlines.  It  is  believed  that 
they  are  composed  of  an  epidote  much  richer  in  iron  than  the  common  variety 
with  which  they  are  associated.  This  increase  in  iron  explains  the  darker 
color  and  the  increase  in  single  and  double  refraction,  as  shown  by  Forbes.' 
Why  it  should  appear,  especially  in  the  aggregates,  can  not  be  explained. 

The  chlorite  does  not  appear  to  be  entirely  an  alteration  product  of  the 
secondary  hornblende  with  which  it  is  associated.  There  is  usually  rather 
more  chlorite  than  it  would  seem  could  pos.sibly  have  been  formed  from  the 
alteration  of  the  hornblende  alone.  In  some  of  the  rocks  the  larger  angles, 
as  well  as  the  extremely  fine  areas  between  adjacent  feldspars,  are  occupied 
by  a  very  fine  felt-like  chloritic  mass.  The  chlorite  which  is  not  secondary 
after  hornblende  is  considered  as  the  product  of  an  altered  glassy  base. 

No  glass  was  observed  in  the  nonporphyritic  basalts  occurring  in  large 
masses,  but  in  one  of  the  fragments  of  basalt  in  a  tuff  a  dark  chocolate- 
brown  glass  forms  the  matrix  in  which  are  lying  well-developed  plagioclase 

'Epidote  ami  its  optical  proiierties,  by  E.  H.  Forbes :  Am.  Jonr.  Sci.,  4th  ser.,  Vol.  1, 1896,  p.  30. 


102  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

microlites.  The  glass  where  thick  appears  isotropic,  but  where  thin  appears 
to  be  full  of  globulitic  de^'itrification  products,  which  show  slight  polariza- 
tion effects  between  crossed  nicols. 

The  original  ])resence  of  glass  in  other  basalts  is  considered  to  be  indi- 
cated bv  the  occurrence  of  amvgdaloidal  cavities,  with  very  sharply  defined 
walls  marked  by  accumulations  of  magnetite. 

The  character  of  one  basalt  points  strongly  toward  its  glassy  condition. 
It  is  amygdaloidal,  the  amygdaloidal  cavities  being  sharply  defined.  The 
groundmass  contains  at  present  no  indication  of  the  existence  of  any  orig- 
inally crystalline  elements  whatever.  It  is  now  a  dense  mass  of  felty 
chlorite  and  minute  epidote  grains.  Through  this  mass  and  around  the 
amyg'daloidal  cavities  wind  lines  which  are  somewhat  difterently  colored 
from  the  rest  of  the  matrix,  and  seem  to  indicate  the  direction  of  flowage. 
The  amygdules  are  not  all  elongated,  though  some  are,  and  these  agree  in 
direction  of  elongatiou.  It  is  really  impossible  to  describe  the  groundmass 
so  as  to  do  justice  to  its  appearance  and  convince  one  who  has  not  seen  it 
of  its  devitrified  character.  The  general  impression  it  makes  is  that  of  a 
devitrified  glass,  and  the  photomicrograph  (fig.  B,  PI.  XXV)  gives  a 
fairly  good  idea  of  its  appearance  under  the  microscojje,  and  will  j^robably 
prove  more  convincing  than  any  description  that  might  be  given.  Fig.  A, 
PI.  XXVII,  represents  a  polished  face  of  the  specimen  in  its  natural  size. 

Another  kind  of  glassy  basalt  is  represented  in  this  district.  This  rock 
resembles  the  one  just  described,  but  differs  from  it  in  that  it  was  not  alto- 
gether glassy.  In  it  one  sees  long,  slender,  much-altered  feldspar  microlites 
scattered  through  the  matrix.  These  feldspars  occur  in  needles,  which 
fringe  out  at  the  ends.  They  do  not  give  the  groundmass  textures  usually 
found  in  the  basalts,  but  occur  in  sheaves  and  imperfect  spherulitic  forms; 
the  rock  thus  approaches  in  texture  the  variolites.  The  base  in  which  the 
feldspars  lie  is  brownish  gray,  and  consists  of  recognizable  chlorite,  epidote, 
some  clear  mineral  in  minute  particles,  probably  quartz  or  feldspar,  or  both, 
and  aggregates  of  yellowish  granules,  which  are  apparently  of  a  single 
kind  and  are  so  minute  as  not  to  permit  of  determination.  The  grannies  show 
very  slight  polarization  effects  mider  crossed  nicols,  and  the  groundmass  in 
many  places  where  they  occur  in  great  quantity  appears  almost  isotropic. 
It  seems  highly  probable  that  a  large  portion,  if  not  all,  of  the  ground- 
mass  was   originally  a  glass.     Further  evidence  of  the  originally  glassy 


BASIC  VOLCANICS  OF  HEMLOCK  FORMATION. 


103 


iiiitiire  ot"  the  gTouudiuas.s  is  afforded  by  tlie  grouudinass,  wliicli,  under  the 
microscope,  shows  variable  li<»-hter  and  darker  shades  of  brown,  and  these 
poi-tions  interpenetrate,  forminj^-  an  imperfect  eutaxitic  structure.  Such 
structures  are  especially  common  in  the  glasses.  The  photomicrographs 
(figs.  A,  Jl,  PI.  XXVI)  show  veiy  well  the  general  microscopical  characters 
of  this  rock. 

Chemical  composition. — Tile  followiug  couiplcte  aiialvsis,  for  wliicli  I  am 
indebted  to  Dr.  Henry  Stokes,  of  the  United  States  Geological  Survey, 
shows  the  chemical  composition  of  one  of  these  pre-Cambrian  nonporphy- 
ritic  metabasalts.  The  rock  is  very  fine  grained  and  microophitic,  with  a 
marked  amygdaloidal  character.  The  amygdaloidal  cavities  are  filled  with 
chlorite,  into  which  project  crystals  of  epidote-zoisite  and  calcite.  The 
altered  condition  of  the  basalt  is  very  clearly  shown  by  the  high  percent- 
ages of  water  and  carbon  dioxide.  The  other  oxides  show  nothing 
remarkable  except  that  the  percentage  of  titanium  oxide  is  quite  high.  On 
the  whole,  however,  the  analysis  is  very  similar  to  those  of  recent  fresh 
rocks  of  the  same  character. 

Analysis  of  pre-Cambrian  nonporphyritic  metabasalt. 


Constituent. 

Per  cent. 

Constituent. 

Per  cent. 

SiO,              

46.47 
1.28 

16.  28 
3.15 
8.96 
.09 
6.56 
7.90 
3.64 

K5O           

0.21 

.13 

1.26 

TiO> 

P2O, 

CO. 

AliOi 

FeOi 

CI 

FeO 

SO, 

MnO 

H.O  at  110° 

.28 
3.89 

MgO 

H;0  above  IIC^ 

Total    .     ... 

CaO 

100. 11 

Na,0 

PORPHYKITIC   METABASALT. 


The  porphyritic  rocks  are  fine  grained,  and  may  be  or  may  not  be 
amygdaloidal.  They  include  diabase  porphyrites  and  porphyritic  forms  of 
the  melaphyres.  These  last  in  the  textures  of  their  groundmass  correspond 
to  the  labradorite-porphyrites,  the  equivalents  of  the  andesite-porphyries, 
though  more  basic  than  they.  The  phenocrysts  lie  in  a  fine  groundmass 
which  shows  the  same  kinds  of  texture  already  mentioned  as  having  been 


104  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

observed  in  the  corresponding  nonporphyritic  basalts,  the  microophitic,  inter- 
sertal,  navitic,  pilotaxitic,  and  hyalopilitic.  The  various  basalts  are  connected 
by  transition  phases.  The  close  connection  between  the  different  varieties 
is  well  shown  where  one  passes  from  the  fine-grained  amygdaloidal  rock 
through  the  fine-grained  nonamygdaloidal  over  to  the  porphyritic  macro- 
scopicallv  nonamygdaloidal  type. 

petrographicai  characters. — As  statcd  lu  tlic  gcueral  descriptlou,  these  rocks  do 
not  differ  essentially  from  the  nonporphyritic  basalts  just  described.  The 
most  important  difference  is  in  the  presence  of  the  feldspar  phenocrysts, 
giving  them  a  porphyritic  texture. 

Measurements  upon  the  phenocrysts,  made  against  the  albite  twinning 
plane  on  zone  _L  to  010,  according  to  the  Michel  Ltivy  method,  give  an 
average  extinction  angle  of  about  18°,  which  points  toward  its  character 
as  labradorite.  However,  angles  obtained  lower  than  this  indicate  the 
possibility  of  the  association  of  andesine  with  the  predominant  labradorite. 
The  feldspars  show  the  usual  alteration  products.  One  very  infrequently 
finds  augite  phenocrysts  which  have  been  completely  uralitized  associated 
with  the  feldspars.  Other  phenocrysts  are  now  represented  by  masses  of 
chlorite,  with  or  without  epidote,  evidently  pointing  toward  the  basic  and 
maonesian  nature  of  the  original  mineral.  As  uralite  is  the  common  sec- 
ondary  product  of  pyroxene  in  these  volcanics,  the  altered  phenocrysts 
represented  b^s'  chlorite  masses  are  not  believed  to  have  been  pyroxene. 
The  original  mineral  was  perhaps  olivine.  The  very  noticeable  scarcity  of 
augite  phenocrysts  in  the  basalts  stamps  them  as  difterent  from  the  great 
majority  of  basaltic  rocks  and  as  being  very  similar  to  the  basalts  described 
by  Judd,^  from  the  Brito-Icelandic  petrographicai  province,  in  which  por- 
phyritic crystals  of  augite  are  seldom  if  ever  seen  and  in  which  the  pheno- 
crysts are  feldspar  and  sometimes  olivine. 

The  groundmass  in  which  the  phenocrysts  lie  have  generally  the  same 
mineralogical  composition  and  texture  as  the  nonporphyritic  rocks  already 
described,  and  the  two  kinds  are  supposed  to  have  been  originally  similar.^ 

'  On  the  gabbros,  dolerites,  and  basalts  of  Tertiary  age  in  Scotland  and  Ireljind,  by  J.  W.  Judd: 
Quart.  Jour.  Geol.  Soc,  Vol.  XLII,  1886,  p.  79. 

-The  groundmass  of  one  of  these  porphyritic  forms  dift'ers  somewhat  in  one  important  respect. 
In  it  were  observed  numerous  round  areas  of  small  size  occupied  by  a  clear  white  aggregate,  polariz- 
ing in  low  gray  colors.  The  centers  of  some  of  the  areas  were  occupied  by  clumps  of  yellow  grains, 
with  here  and  there  a  minute  flake  of  chlorite.  Others  contain  only  the  white  material,  which  is 
apparently  secondary.  The  round  areas  are  not  sharply  delimited,  and  hence  are  most  probably  not 
microamygdules.     Their  general  appearance  is  strikingly  like  that  of  leucite  in  those  plagioelase 


BASIC  VOLCANIOS  OF  IlKMLOCK  FOKMATION.  105 

Measurements  made  on  the  feldspar  microlites  (»f  the  g-roundmass  gave  17° 
as  tlie  maxinmm  extinetiou  in  zone  perpendicular  to  010.  This  angle  points 
toward  tlie  microlites  being  acid  labradorite.  The  microlites  thus  seem  to 
agree  essentially  with  the  phenocrysts  in  composition.  Flowage  structure 
around  the  phenocrysts  is  most  distinctly  shown  by  the  arrangement  of  the 
feldspar  microlites.  In  one  case  in  which  the  porphyritic  texture  and  the 
ilowage  structure  are  very  good,  secondary  actinolite  crystals  have  devel- 
oped parallel  to  one  another  and  parallel  to  the  flowaee  direction,  o-ivino- 
the  rock  under  the  microscope  a  distinctly  schistose  appearance. 

Chemical  composition. — In  the  preliminary  article  upon  these  Hemlock  vol- 
cauics  published  in  1895/  the  occurrence  of  andesites  as  well  as  basalts  was 
mentioned.  This  determination  was  based  solely  on  the  microscopical  study 
of  the  rocks,  and  the  rocks  which  were  presumed  to  be  andesites  were  those 
porphyritic  forms  which  have  just  been  described.  Since  the  publication  of 
that  article  the  following  analyses  (Nos.  1  and  2)  have  been  obtained  of 
the  porphyritic  rocks.  The  rocks  selected  for  analysis  were  those  which 
appeared  to  be  especially  rich  in  feldspar,  and,  having  a  rather  lighter  color 
than  the  others,  seem  to  be  somewhat  more  acid  than  the  average.  These, 
it  was  thought,  might  have  the  composition  of  andesite. 

The  comparison  of  series  of  rocks  derived  presumabl}-  from  the  same 
magma  is  more  profitable  than  the  study  of  single  analyses.  This  line  of 
investigation,  as  followed  by  Rosenbusch,"  Iddings,^  Lang,^  Broegger,' 
Becke,*  and  Michel  Li^vy,'  has  been  very  fruitful. 

basalts  iu  which  it  is  present  in  very  small  quantity,  filling  irregular  but  iu  general  rounded  areas 
between  the  other  constituents.  It  would,  of  course,  be  impossible  to  base  the  determination  of  the 
former  presence  of  leucite  in  these  pre-Cambrian  rocks  upon  such  scant  evidence  as  has  been  obtained. 
Still  it  is  worth  while  to  notice  even  such  a  doubtful  indication  of  its  former  presence  as  has  been 
mentioned  above. 

'  Jour.  Geol.,  cit.  pp.  805-806. 

'Ueber  die  chemischen  Beziehungen  der  Eruptivgesteine,  bv  H.  Rosenbusch:  Tsch.  Min  u  Pet 
Mitt.,  Vol.  II,  1889,  pp.  144-178. 

3  Origin  of  igneous  rocks,  by  J.  P.  Iddings :  Bull.  Phil.  Soc.  Wash.,  Vol.  XII,  1892,  pp.  88-214.  Tlie 
eruptive  rocks  of  Electric  Peak  and  Sepulchre  Mountain,  by  J.  P.  Iddings:  Twelfth  Ann.  Kept  U  S 
Geol.  Survey,  1892,  pp.  .571-664. 

<  Ordnung  der  Eruptivgesteine  uach  ihrem  chemischen  Bestand,  by  H.  Otto  Lang:  Tsch.  Min.  u. 
Pet.  Mitt.,  Vol.  XII,  pp.  199-252.  Beitrage  zur  Systematic  der  Eruptivgesteine,  bv  H.  Otto  Lang-  Tsch 
Min.  u.  Pet.  Mitt.,  Vol.  XIII,  1892,  pp.  115-169. 

■•  Die  Eruptivgesteine  des  Kristianiagebietes,  by  W.  C.  Broegger.  I.  Die  Gesteine  der  Grorudit- 
Tinguait  serie.  II.  Die  Eruptiousfolge  der  triadischen  Eruptivgesteine  bei  Predazzo  in  Siidtyrol. 
Videnskabsselskabets  Skrifter,  I  Matheuiatiek-naturv.  klasse.     No.  4, 1894;  No.  7, 1895. 

<i  Gesteine  der  Columbretes,  by  F.  Becke :  Tsch.  Min.  u.  Pet.  Mitt.,  Vol.  XVI,  1896,  pp.  308-336. 

'Note  eur  la  Classification  des  Magmas  des  Roches  Eruptives,  by  A.  Michel  L^vy;  Bull.de  la 
Soc.  G6ol.  de  France,  .Sd  ser.,  Vol.  XX  V,  No.  4.  .Inly.  1897,  pp.  326-377. 


106 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 


The  A-alue  of  such  an  investigation  largely  depends  on  the  freshness  of 
the  rocks  examined  and  the  amount  of  variation.  The  Hemlock  volcanics 
are  all  more  or  less  altered,  and  the  variation  in  character  is  slight.  I  wish, 
however,  to  call  attention  to  the  close  relationship  exhibited  by  the  types  of 
which  analyses  were  made,  and  to  that  end  the  analysis  of  the  nonpor- 
phyritic  very  basic  appearing  basalt  (No.  3  of  Table  I)  is  repeated  and  is 
placed  by  the  side  of  the  analyses  of  the  porphyritic  ones.  The  complete 
analyses  made  by  Dr.  Henry  M.  Stokes,  of  the  United  States  Geological 
Survey,  are  found  in  the  first  table.  In  the  second  table  there  is  given  the 
molecular  proportion  of  the  chief  oxides,  those  which  are  not  here  repre- 
sented having  been  first  proportioned  among  them.  From  this  table, 
following  Rosenbusch,'  there  were  obtained  the  figures  in  the  tliird  table, 
showing  the  atomic  proportions  of  the  metals  present." 

The  analyses  are  arranged  according  to  the  increasing  percentage  of 

calcium. 

Analyses  of  porphyritic  nietabasalt. 

TABLE  I.— COMPLETE  ANALYSES. 


Oonstitnont. 

1. 

2. 

a  3. 



SiO,                           

47.20 
3.30 

15.  36 
3.06 
8.87 
.20 
5.05 
4.20 
4.72 
1.40 
.36 

52.59 

1.36 

15. 93 

6.12 

3.96 

.25 

5.55 

5.04 

.5.79 

.67 

.15 

46.47 

■       1.28 

16.28 

3.15 

8.96 

.09 

7.90 

6.56 

3.64 

.21 

.13 

TiO.                        

AI,Ot                

FeOi        

FeO                

MnO       

CaO            

MjrO                                

NaiO                          

K.;0 

P2O, 

CI 

SO.                            - . ' 

COi                       

3.34 

.16 

3.04 

None. 

.16 

2.16 

1.26 

.28 
3.89 

H,O:itll0>;                

H2O  above  IIC^ 

Total 

100.26 

99. 73 

100.  11 

aNo.  3  is  the  analysis  by  Dr.  Stokes  of  the  nonporphyritic  basalt,  and  is  given  for  comparison. 


I  Uber  (lie  cliemischen  Beziehungen  der  Eniptivgesteine,  by  H.  Rosenbusch :  Tsch.  Min.  n.  Pet. 

Mitt.,  Vol.  XI,  1890,  p.  144. 

2 Tables  Xos.  II  and  III  were  calcuLated  for  me  by  Mr.  Victor  H.  Bassett,  assistant  in  cbeniistry 

ill  the  University  of  Wisconsin. 


BASIC  VOLCANICS  OF  HEMLOCK  FORMATION. 


107 


TAUI.E  II.— MOLKCULAK  rKdl'dRTloN  Ol'  THE  CIIIKB-  OXIDES. 


SiO.                           

50.55 
3.54 

16. 45 
3.28 
9.72 
5.41 
4.50 
5.05 
1.50 

54.07 
1.40 

16.  38 
6.29 
4.33 
5.71 
5.18 
5.95 
.69 

49.15 
1.35 

17.22 
3.33 
9.57 
8.36 
6.94 
3.  85 
.22 

TiO; 

Al.Oi                     

Fe,0 1                     

FeO                  

CaO                     

MgO 

Na^O 

K  0                        

Total  

100.00 

100.00 

100. 00 

TAHLE  III.— ATOMIC  PROPOKTION  OF  METALS. 


Si       

46.97 
2.48 

18.07 
9.87 
5.42 
6.25 
9.16 
1.78 

49.49 

.97 

17.73 

7.67 

5.63 

7.09 

10.61 

.81 

45.41 

.94 

18.80 

9.74 

8.33 

9.58 

6.93 

.27 

Ti          

Al              

Fe        

Ca       

JI"     

Na         

K       - 

Total              

lOO:  00 

100. 00 

100. 00 

1 

As  tne  calcium  increases  there  is  a  corresponding  increase  of  magne- 
sium and  a  diminution  in  potassium.  A  decrease  in  sodium  is  also  shown 
if  Nos.  1  and  3  alone  are  compared.  The  percentage  of  sodium  present  is 
rather  high  and  with  the  potassium  indicates  a  magma  family  rich  in  alkalies. 
The  magnesium  is  notably  high;  such  high  percentages  as  we  have  here 
usually  accompanying  much  lower  percentages  of  alkali.  It  may  also  be 
noted  here  that  the  presence  of  the  magnesium  in  such  amounts  indicates 
the  former  presence  of  olivine  or  the  presence  still  of  its  alteration  products, 
a  point  to  which  attention  was  directed  in  the  microscopic  description  of  the 
rocks.  No.  1  is  remarkable  for  its  percentage  of  titanium,  which  is  very 
high,  even  when  compared  with  that  contained  in  the  others,  which  are 
themselves  considerably  above  the  average.  All  of  the  rocks  contain  a 
large  amount  of  water  of  hydration.  The  percentage  of  CO2  contained  in 
Nos.  1  and  2  indicates  also  that  they  are  much  altered. 

These  analyses  show  that  the  rocks  can  not  be  classed  with  the  typical 
andesites.     Should  they  be  called  andesites  at  all,  they  must  be  classed  with 


108  THE  CRYSTAL  FALLS  IRON  BEARING  DISTRICT. 

the  augite-andesites,  and  placed  on  the  border  Hne  between  them  and  the 
plagioclase  basahs.  It  is  preferred  to  hichide  them  under  the  basalts,  though 
it  can  not  be  doubted  but  that  if  analyses  of  perfectly  fresh  rocks  could  be 
obtained,  there  would  be  found  some  which  would  incline  more  decidedly 
toward  andesites  than  do  the  above  specimens. 

VAKIOLITIC    METAUASALTS. 

Variolites  are  spherulitic  basalts,  usually  very  vitreous.  Since  the  tend- 
ency to  crystallization  is  so  much  stronger  in  the  basic  than  in  the  acid 
rocks,  it  is  not  surprising  that  they  should  be  far  less  common  than  the  cor- 
responding acid  kind.  Moreover,  the  basic  glasses  are  very  susceptible  to 
alteration,  which  naturally  obscures  the  original  characters  of  the  rocks. 
This  probably  partly  accounts  for  the  fact  that  they  are  very  nifrequently 
observed.  This  spherulitic  phase  of  the  basalts  is  well  known  in  Europe, 
but  there  has  thus  far  been  found  only  one  reference  to  its  occurrence  in 
the  United  States.  Ransome^  has  described  a  variolite  from  Point  Bonita, 
California.  To  this  there  may  now  be  added  a  single  occurrence  in  the 
Crystal  Falls  district  of  Michigan.  This  variolite  exposure  occurs  at  N.375, 
W.  900,  sec.  4,  T.  44  N.,  R.  33  W.,  in  close  proximity  to  the  remnant  of  a 
basalt  sti'eam  which  shows  well-marked  flowage  structure.  The  relations 
of  the  two  rocks  are  not  determinable  from  the  exposures. 

The  rock  presents  a  very  rough  manimillated  surface,  due  to  diiferential 
weathering.  The  varioles,  being  more  resistant  than  the  groundmass  sur- 
rounding them,  form  the  protuberances.  These  protuberances  vary  in  shape 
from  round  to  oval,  and  very  rarely  are  irregular.  The  varioles  vary  also 
in  size  from  minute  ones  to  those  about  one-half  inch  in  diameter,  and  con- 
stitute by  far  the  greater  part  of  the  rock.  These  general  characters  may 
be  seen  on  the  photogi'aph,  fig.  A,  PI.  X,  taken  from  the  hand  specimen.  The 
color  of  the  weathered  surface  of  the  rock  is  gray  or  light  brown,  while  the 
fresh  surface  is  in  general  a  dark  green.  Upon  the  polished  surface  of  a 
fresh  rock  the  varioles  have  an  olive-green  color,  with,  in  the  majority  of 
cases,  a  distinctly  darker  center  of  purplish  color.  Less  frequently  this 
center  is  lighter  green  than  the  remainder  of  the  variole.  The  varioles  are 
usually  separated  from  each  other  by  narrow  areas  of  groundmass,  darker 
than  the  varioles  themselves,  with  a  purplish  or  very  dark    olive-green 

•  The  eruptive  rocks  of  Point  Bonita,  California,  by  F.  Leslie  Ransome:  Bull.  Dept.  Geol.  Univ. 
of  Cal.,  Vol.  1, 1893,  p.  90. 


PLATE   X. 


109 


platp:  X. 

Fig.  .4. 
(Sp.  No.  32273.     Natural  size.) 

Photographic  reproductiou  of  the  weathered  surface  of  a  variolite.  This  brings  out  very  clearly 
the  uiaumiillated  surface  of  the  rock,  which  is  due  to  the  diftereutial  weathering  of  the  varioles  and 
of  the  groundmass  between  them.  The  rounded  character  of  the  varioles,  and  their  gradation  irom 
those  of  very  small  to  those  of  much  larger  size  can  readily  be  seen.     (Desc.,  p.  108.) 

Fig.  B. 

(Sp.  No.  32273.     Natural  size.) 

Reproduction  of  the  polished  surface  of  a  variolite.  This  is  designed  to  show  the  circular 
character  of  the  varioles,  and  the  fact  that  each  is  separate  and  distinct  from  the  one  adjoining  it. 
It  can  be  seen  that  some  of  the  varioles  have  very  dark  and  others  much  lighter  centers.  (Desc, 
p.  108.) 

110 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PL.    X 


(-A)    Weathered  surface  of  variolite. 
iS)    Polished  surface  of  variolite. 


BASIC  VOLCANIOS  OF  HEMLOCK  FORMATION.  1 1 1 

color.     In  places  the  varioles  are  in  juxtapositiou.     However,  they  do  nut 
coalesce,  but  each  is  separate  and  distinct  (tig.  B,  PI.  X). 

The  rock  when  examined  under  the  microscope  is  seen  to  be  extremely 
altered.  The  only  ori<;-iual  minerals  present  are  feldspar,  apatite,  and  pos- 
sibly some  magnetite. 

The  groundmass  consists  of  a  finely  crystalline  secondary  aggregate 
of  flakes  of  chlonte,  associated  with  minute  limpid  grains,  some  of  which 
are  probably  qua/tz  and  others  feldspar.  Scattered  tlu-ough  this  aggregate 
are  grains  of  epidote,  calcite,  a  few  crystals  of  original  apatite,  and  mag- 
netite, and  numerous  dark  reddish  brown  and  black  fen-uginous  specks. 

The  varioles  are  readily  distinguishable  from  the  matrix.  From  this, 
as  well  as  from  each  other,  they  are  invariably  separated  by  a  crack,  along 
which  reddish-brown  ferruginous  matter  has  been  infiltrated.  The  varioles 
are  in  o-eneral  much  finer  grained  than  the  groundmass,  and  at  times 
exhibit  phenocrysts  of  feldspar.  The  composition  of  the  varioles  is  the 
same  as  that  of  the  groundmass,  except  that  apatite  is  more  common  in 
them,  and  that  in  addition  to  the  minerals  mentioned  as  occurring  in  the 
groundmass  a  small  quantity  of  original  feldspar  may  be  recognized,  both 
as  phenocrysts  and  as  part  of  the  groundmass  of  the  varioles.  Where 
these  feldspars  occur,  they  are  to  a  great  extent  replaced  by  a  mass  of  epi- 
dote, chlorite,  sericite,  quartz,  and  feldspar.  The  phenocrysts  are  found 
near  the  center  of  the  varioles,  and  the  occasional  light-colored  centers 
which  were  observed  macroscopically  are  due  to  the  presence  of  these 
altered  feldspar  phenocrysts.  The  more  frequent  dark  centers  are  due  to 
an  acctunulation  of  the  dark  ferruginous  specks  in  varioles  in  which  the 
phenocrysts  are  wanting. 

No  textures  could  be  determined  from  the  remnants  of  the  original 
minerals.  In  one  variole  aggregates  of  secondary  epidote  grains  and  fer- 
ruginous specks  lie  in  such  a  position  as  to  produce  a  distinct  radial 
arrano-ement.  With  advancing  alteration,  spherulites  in  acid  rocks  are 
frequently  found  to  have  between  their  radial  fibers  secondary  deposits  of 
epidote  and  ferruginous  matter,  which  mark  very  clearly  their  radial 
arrangement.  The  similar  radial  arrangement  in  these  varioles  of  epidote 
and  ferruginous  matter  seems  to  point  to  the  varioles  having  possessed  the 
spherulitic  character,  though  it  is  now  impossible  to  determine  the  nature 
of  the  fibers  forming  the  spherulites. 


112 


THE  CRYSTAL  FALLS  IKON  BEARING  DISTRICT. 


TIIK    ELLIPSOIDAL    STRL'CTUUE    IN    THE    iMETAIS ASALTS. 

Upon  examining  the  flat  surfaces  of  many  of  the  lavas  one  is  immedi- 
ately struck  by  their  resemblance  to  a  conglomerate  formed  of  round 
bowlders,  all  of  the  same  kind  of  rock,  lying  in  a  matrix  of  very  small 
quantit}'  and  of  very  different  color. 

Fig.  7  is  a  sketch  showing  a  portion  of  such  a  lava  flow.  I  find  that  these 
ellipsoidally  parted  rocks  have  been  called  "massive  conglomerates,"  and  the 
blocks  have  been  spoken  of  as  "bombs"  in  the  manuscript  notes  of  some  of 
the  men  who  have  worked  among  them.  The  latter  term  was  undoubtedly 
due  to  the  resemblance  of  the  ellipsoids  to  the  spindle-shaped  pieces  of  lava 


m 


^    -  H 


■^fmm'^:rA 


Fig.  7.— Skitch  of  ihe  siirliice  dl'  Uic  «Hitci'ui»  of  au  ellip.suidal  basalt,  aliuwiufi  the  gtin^ral  rliaiaiter  of  tlit-  ellipsoids  find 

matrix- 

which  one  finds  around  the  modern  volcanoes.  Ellipsoidal  basalt  is  very 
common  throughout  the  Hemlock  volcanic  area.  It  is  found  most  frequently 
in  isolated  ledges.  However,  it  is  also  associated  with  and  grades  into  non- 
ellipsoidal  varieties.  In  one  good  exposure  it  is  overlain  by  a  fragmental 
scoriaceous  mass  which  separates  it  from  another  mass  of  similar  ellipsoidal 
basalt.  While  the  scoriaceous  portion  may  represent  the  brecciated  surface 
of  a  lava  flow,  it  is  not  so  considered,  biit  is  presumed  to  be  a  tuff"  deposited 
upon  the  flow  represented  by  the  ellipsoidal  basalt.  According  to  this  view 
the  ellipsoidal  basalt  is  on  the  surface.  In  another  exposure  an  ellipsoidal 
basalt  overlies  a  bed  of  water-deposited  clastic  I'ock.  There  is  no  passage 
between  the  two  kinds  of  rock.  The  contact  between  the  two  is  an  undu- 
lating one,  and  is  marked  by  a  mass  of  schistose  material  about  2  inches 
thick  and  similar  to  that  which  is  between  the  ellipsoids.     This  particular 


BASIC  VOLOANICS  OI'"   HEMLOCK   FOKMATIOX. 


113 


biisiilt  is  \cr\-  iK'iisi.',  with  ouh'  otHMsioiiuUy  small  cliloritc-lilU'd  \c.sicli's  in 
it,  iiiul  tlicic  is  no  true  flow  structure  observable.  The  facts  cited  seem  to 
show  thai  this  elli[)soiihil  [lortiou  was  tlic  surface  of  a  lava  flow,  whether 
the  top  or  the  bottom  is  immaterial  In  c(n-taiu  cases  the  (■lli|)soi(hil  tiu-ies 
ma\'  constitute  an  entire  How.  Where  tlie  direction  of  How  coidd  with'  anv 
de"Tee  of  certuint\'  be  deterniinc^d,  it  w;is  seen  that  the  two  hiu"er  axe.s  of 
the  ellipsoids  are  in  the  })laue  of  tlie  flow. 

Tlie  ellipsoids  vary  in  size  from  a  few  inches  to  G  or  8  feet  in  diameter, 
and  are  usually  spoken  of  as  spheroids.  Attention  has  already  l^eeu  called 
to  tlie  incorrect  usaye  of  this  term  by  F.  Leslie  Ransoiue,  in  lii.s  iuterestinu- 
paper  on  "The  eruptive  rocks 
of  Point  Bonita,  California."' 
The  outlines  of  the  Ixtdies  are 
circular  onlv  in  exceptional 
cases.  Un  the  other  hand, 
sections  in  all  directions 
through  them  give  almost  in- 
variably ellipses,  aiid  there- 
fore they  are  more  })roperly 
ellipsoids  than  spheroids.  On 
the  surfaces  exposed  the  long- 
axes  of  the  ellipses  lie  in  the 
same  general  direction. 

The  ellipsoids  are  formed 
of  a  very  fine-grained  porphy- 
ritic  or  nonporphvritic  rock. 
This  is  amygdaloidal  or  non- 
amygdaloidal.  Where  amygdaloidal,  the  amygdules  are  as  a  rule  dis- 
tributed throughout  the  ellipsoids,  though  on  the  whole  the  masses  are  more 
scoriaceous  on  the  periphery  than  near  the  center.  In  exceptional  cases, 
the  amygdules  are  much  more  numerous  on  the  west  side  of  the  ellipsoids 
than  on  the  east  side  (fig.  8).  In  such  cases  the  west  sides  are  toward  the 
tops  of  the  lava  flows.  The  ellipsoids  are  very  commonly  split  up  by 
cracks.  Some  of  them  have  a  roughly  radiate  arrangement.  These  may  be 
due  to  the  effects  of  contraction  in  the  early  stages  of  the  existence  of  the 


Fig.  8. — Sketch  showing  the  couceDtr.^tion  of  the  ain.vgdak)idal  cavities 
ou  one  aide  of  au  ellipsoid,  thia  side  probably  representing  the  side 
nearest  tlie  surface  of  the  How. 


MON    XXXVI- 


'  Op.  cit.,  p.  75. 


114  THE  CRYSTAL  FALLS  IRON-BEARmG  DISTRICT. 

ellipsoids.  Others,  and  by  far  the  greater  number,  liave  one  set  of  lines  par- 
allel and  another  parallel  set  which  in  different  cases  cut  the  first  set  at  dif- 
ferent angles,  very  rareh'  at  a  right  angle  (figs.  8,  9).  One  of  these  sets  is 
usually  tranverse  to  the  long  axes  of  the  ellipsoids  (figs.  7,  8). 

The  blocks  are  separated  from  one  another  by  a  thin  layer  of  a  schistose 
matrix,  rarely  more  than  3  inches  in  thickness,  though  exceptionally  nearly 
8  inches  thick.      (Of  figs.  7  and  8  of  this  pajjcr,  and  fig.  1  by  Ransome.^) 

Since  the  above  description  of  the  Crystal  Falls  ellipsoidal  lavas  was 
written  in  1896,  there  has  appeared  Sir  Archibald  Greikie's  valuable  work 
on  the  Ancient  Volcanoes  of  Great  Britain,-  in  which  several  similar  occur- 
rences are  mentioned.     His  illustration  of  this  structure  on  page  184,  as  can 
rcadih'  be  seen  on  comparison,  would  answer,  but  for  the  absence  of  a  well- 
defined  schistose  matrix  between  the  ellipsoids,  very  well  for 
>^^^^C        ^  sketch  of  a  Michigan  pre-Cambrian  ellipsoidal  lava. 
ImI 4x1/1 'I  ll  '^riw  schistose  matrix  between  the  ellijjsoids  upon   the 

i|i''Tj+l/ll^/.  weathered  surface  is  seen  to  be  made  up  of  layers  concentric 
'/^^^f  with  the  ellipsoids.  It  is  possible  that  these  layers  are  not 
EiG.  9.-Eiiipsuids    absolutelv  concentric  in  the  third  dimension.     However,  no 

■with  sets  of  parallel  •  i        r      l  l  •  •  c      ^   •  •  f^i 

lines  cutting  each  exposui'e  ])ermitted  01  the  determination  ot  this  pomt.  iTe- 
quenth'  certain  layers  seem  to  grade  off  into  others  of  a  some- 
what different  character.  The  matrix  between  any  two  ellipsoids  usually 
separates  near  the  center;  where  apparently  the  greatest  movement  having 
occurred  the  schistosity  is  most  developed.  One  can  often  as  easily  knock 
an  ellijjsoid  out  of  its  encircling  matrix  as  one  can  the  kernel  out  of  a  nut. 
In  some  cases  there  is  no  absolutely  sharp  line  of  demarcation  l)etween 
matrix  and  ellipsoid,  but  a  gradation  from  one  into  the  other.  At  the  ^^laces 
■where  three  blocks  are  in  juxtaposition  one  frequently  finds,  instead  of  a 
triangular  space  entirely  filled  by  the  matrix,  in  the  center  of  the  matrix  a 
triangular  area  of  infiltrated  vein  quartz  (figs.  7,  8). 

In  certain  cases  the  minerals  which  compose  the  schistose  matrix  are 
not  thoroughlv  cemented  and  give  it  a  somewhat  friable  character,  causing 
it  on  weathered  surface  to  appear  granular. 

In  very  rare  cases  a  matrix  with  a  distinctly  brecciated  character  was 
observed,  but  in  this  as  well  as  in  the  cases  above  described  a  certain  degree 


'  Op.  cit.,  p.  76. 

=  Ancieut  Volcanoes  of  Great  Britain,  by  Sir  Archibald'Geikie,  Vol.  I,  1897,  pp.  26,  184,  193. 


PLATE    XI. 


115 


PLATE    XI. 

(Sp.  Xii.  2367.").     Xatiinil  si.e.) 

Thi.s  colmed  plate  rej>re.seiits  tlic  pnlisUeil  surface  of  iiu  ellipsoiil  with  a  portion  of  the  matrix 
which  surrounds  it  and  .separates  it  from  the  adjacent  ellipsoids.  The  dense  character  of  llie  center 
is  nicely  .shown.  Aroiiud  this  oval  area  we  get  narrow  concentric  zones  of  alternatiug  light-green 
and  dark-green  material.  The  light-green  material  corresponds  to  that  in  the  center,  and  represents 
the  least-altered  basalt  of  the  ellipsoids.  The  dark-green  areas  are  the  cliloritized  basalt.  Beyond 
this,  forming  the  outermost  greeuish-gray  zone,  one  linds  the  matrix,  which  possesses  distinctly  frag- 
mental  cdiaracters,  though  in  spite  of  this  with  a  marked  schistose  character.  The  schistosity  of  the 
matrix  conforms  to  the  contours  of  the  ellipsoid. 
116 


us  GEOLOGICAL  SURVEY 


MONOGRAPH    XXXVI  PL  XI 


JULIUS  eiENaco  lith  n  v 


BASALT  ELLIPSOID  WITH  MATRIX. 


BASIC   VOLCANICS  OK   IlKMLOCK  FORMATION.  117 

(it'  scliistositA'  is  iiotici'iiblc.       Tlif  iiiaTrix  lictwccii  llic  ellipsoids  viirics  very 
iniicli  in  (l('<>Tee  of"  srhistosity,  color,  aiul  coiniiositioii. 

Tlic  most  schistose,  and  1)\'  far  tlic  most  comuKin  variety,  is  tlie  dark 
tiTcen  matrix,  whicli  consists  essentially  of  chlorite,  ei)i<lote,  and  zoisite. 
This  material  is  clearl\-  the  resnlt  of  the  chloritization  and  the  e[)idotization 
of  the  original  basalt  constitnting  the  ellii)soids,  for  we  see  it  alternating 
with  hands  of  and  grading  into  the  less  altered  basalt.      (PI.  XI.) 

A  second  facies  of  the  matrix  is  that  which  possesses  only  a  moderate 
deo-ree  of  schistosity,  and  appears  at  times  almost  massive.  This  matrix  may 
l)e  lio-ht  colored,  almost  white  or  greenish,  or  a  dark  bluish-blax-k.  It  is 
cdimn  o-rained  or  aphanitic.  The  light-coktred  matrix  consists  essentially 
lit  (piartz  and  calcite.  When  a  little  chlorite  or  epidote  is  present,  it  has  a 
greenish  tinge.  The  very  dark  variety  consists  of  (piartz  and  siderite,  col- 
ored with  minute  particles  of  iron  oxide.  The  quartz-calcite  or  qnartz-siderite 
aggregates  owe  their  origin  to  essentially  the  same  processes,  calcification,  or 
sideritizatiou,  respectively,  followed  by  silicification  of  the  original  basaltic 
material.  They  are  therefore  briefly  described  together  here.  Their  charac- 
ters and  origin  will  be  found  discussed  in  detail  on  page  130  et  seq.  Some 
of  the  peculiar  characters  of  this  matrix  are  illustrated  in  fig.  B,  PI.  XXVII. 

The  least  common  variety  of  matrix  found  between  the  ellipsoids  is  of  a 
light  greenish-gray  or  brownish  color,  and  possesses  a  noticeably  brecciated 
character  (fig.  B,  PI.  XXXIV,  and  PI.  XI),  but  with  at  the  same  time  a  certain 
degree  of  schistosity.  Its  characters  are  best  seen  under  the  microscope  by 
moderate  magnification.     Its  brecciated  character  is  then  well  shown. 

The  frag-ments  of  such  a  matrix  are  of  all  sizes  and  are  angular.  Thev 
show  quite  commonly  a  separation  into  zones.  The  fragments  now  consist  of 
chlorite  and  epidote,  and  in  the  fragments  with  zonal  ari-angement  chlorite  in 
exceedingly  fine  flaky  aggregates  occupies  the  center  and  ejoidote  the  out- 
side. Now^  and  then  there  may  be  several  alternating  zones  -of  chlorite  and 
epidote.  In  all  cases  both  epidote  and  chlorite  are  present  in  the  zones,  but 
the  one  mentioned  is  in  great  quantity,  while  the  other  is  very  subordinate. 
The  epidote  is  very  commonly  the  dark  ferruginous  kind  mentioned  on 
l)age  101,  and  marks  otf  the  outer  limits  of  the  fragments.  Now  and 
tlien  the  limits  are  outlined  by  a  zone  of  brownish  ferruginous  material, 
whose  exact  character  could  not  be  determined.  The  fragments  of  the 
breccia  show  now  neither  original  minerals  nor  textures.     To  judge  from 


118  THE  CRYSTAL  FALLS  IROX-BEARIXG  DISTRICT. 

the  unitovni  character  of  the  zoues  in  the  fragments,  the  original  material 
was  very  homogeneous,  most  probably  a  basalt  glass. 

The  spaces  between  the  fragments  are  occnpied  by  a  iinely  crystalline 
aggregate  of  quartz  and  chlorite,. with  a  small  amount  ( )f  epidote.  This  aggre- 
gate outlining  the  original  fragments  owes  its  origin  most  probably  to  the 
process  of  iniiltration.  The  long  a.xes  of  the  quartz  grains  and  chlorite  flakes 
in  this  aggregate  usually  show  a  general  parallel  arrangement.  Moreover,  tlie 
long  directions  of  the  fragments  are  in  general  parallel  with  each  other,  and 
vvnth  tlie  (juartz-chlorite  aggregate  between  them.  This  parallelism  results  in 
giving  an  imperfect  schistosity  to  the  matrix.  The  schistosity  of  the  matrix 
is  in  general  parallel  to  tlie  contours  of  the  ellipsoids  which  it  surrounds. 

Origin  of  the  ellipsoidal  structure. — ElHpsoidal  structurcs  Similar  to  those  just 
considered  have  been  described  by  various  authox's.^ 

'  On  colnmnnr,  fi.ssile,  nnil  spheroidal  stnictnre,  by  T.  G.  Boiiney :  Quart.  Jour.  Geol.  Soc,  Vol. 
XXXII.  1S76,  pp.  140-1.">4. 

Uelier  niechiiuische  Gcsteiusuniwandlungeu  bet  Hainii--heu  in  Snrbsen,  by  A.  Rotbpletz  :  Zi:-itschr. 
(lent.  (ieol.  Gesell.,  Vol.  XXXI,  1879,  pp.  374-397;   Vol.  XXXII,  18S0.  p.  447. 

Report  (lu  tlie  jjeology  of  iiortbein  New  Brunswick,  by  R.  W.  Ells:  .\un.  Rept.  Geol.  and  Xat. 
Hist.  Survey  of  Canada,  1879-80,  D,  p.  24. 

E.  Datlie:  Jarb.  K.  pieiiss.  geol.  Laudesanstalt,  1883,  p.  432. 

K.  Dalmer:   C'f.  Zirkel  Pet.,  Vol.  IT,  p.  lioO. 

Report  on  the  geology  of  the  Lake  of  the  Woods  region,  by  A.  C.  Lawson :  Geol.  and  Nat.  Hist. 
Survey  of  Canada,  1885,  CC,  pp.  51-.53. 

The  greenstone-schist  areas  of  the  Meiioiniuee  and  Marquette  regions,  by  G.  H.  Williams:  Bull. 
U.  S.  Geol.  Survey,  No.  62,  1890,  pp.  137,  166-168,  173,  and  203. 

Ou  the  variolitic  rocks  of  Mont  Genevre,  by  G.  A.  J.  Cole  and  J.  W.  Gregory  ;  Quart.  .Jour.  Geol. 
Soc,  Vol.  XLVI,  1890,  pp.  29.5-332. 

On  a  variolitic  diabase  of  the  Fichtelgebirge,  by  .J.  W.  Gregory:  Quart.  .Jour.  Geid.  Soc.,  Vol. 
XLVII,  1891,  i.p.  45-62. 

The  Kawishiwin  agglomerate  at  Ely,  Minnesota,  by  N.  H.  Winchell:  Am.  Geol.,  Vol.  IX,  1892, 
pp.  359-368. 

The  eruptive  rocks  of  Point  Honita,  California,  l)y  F.  L.  R.ansome:  Bull.  Dept.  of  Gecd.  I'uiv. 
of  Cal.,  Vol.  I,  1893,  pp.  71-114. 

Editorial  note  on  the  above  paper,  by  N.  H.  Wiuchell :  Am.  Geol.,  Vol.  XIV,  1894,  p.  321. 

The  geology  of  Angel  Island,  by  F.  L.  Ransome:  Bull.  Dept.  Geol.  Univ.  of  Cal.,  No.  7,  1894, 
p.  202. 

Variolite  of  the  Lleyn  and  associated  volcanic  rocks,  by  C.  Raisin:  Quart.  .Jour.  Geol.  Soc,  Vol. 
XLIX,  1893,  pp.  145-165. 

Ou  a  radiolarian  chert  from  Mullion  Island,  by  H.  Fox  and  .J.  .J.  H.  Teall:  Quart.  .lour.  Geol. 
Soc,  Vol.  XLIX,  1893,  p.  211. 

Ou  greenstone  associated  with  radiolarian  chert,  by  J.  J.  H.  Teall:  Trans.  Roy.  Geol.  Soc.  of 
Cornwall,  1894:  C'f.  Rosenbusch,  Mikroskopische  Physiographie,  3d  ed.,  p.  1064. 

The  volcanic  rocks  of  the  Michigamme  district,  by  J.  M.  Clements:  .Jour.  Geol.,  Vol.  Ill,  1895, 
p.  808. 

The  geology  of  Point  Sal,  by  H.  W.  Fairbanks:  Bull.  Uept.  Geo!.  Univ.  of  Cal.,  Vol.  II,  1896, 
p.  40. 

Geology  of  the  Fox  Islands,  Maine,  by  G.  O.  Smith,  1896,  pp.  16-18. 

The  Ancient  Volcanoes  of  Great  Britain,  by  Sir  Archibald  Geikie,  London  and  New  York,  1897, 
|.p.  26,  184,  .and  193. 


BASIC  VOLOANIOS  OF  HEMLOCK  FORMATION.  119 

^'il^illus  attcnijirs  liavc  liccn  made  to  explain  tliis  peculiar  structure 
Honnev,  hatlii-,  au<l  ivaisiu  I'eg-ard  contraction  as  tlie  f(tr('(_'  wliicli  proilucctl 
the  rounded  masses. 

Datlu'  and  Dalmer  show  l)y  the  presence  of  the  concentrically  arrang-ed 
amyydules  that  the  ellipsoids  were  units,  and  were  formed  l)efore  solidi- 
tication  of  the  rock.  This  arrangement  of  the  amygdules,  as  well  as  the 
arrangement  illustrated  in  fig.  H  on  page  113,  precludes  at  once  the  idea,  that 
the  structure  owes  its  origin  to  the  well-known  weathering  process  which 
by  exfoliation  produces  spheroidal  blocks. 

Rothpletz  and  Williams  look  upon  the  ellipsoids  as  due  to  mechanical 
forces  which  ground  down  the  angles  and  edges  of  a  fractured  la^'a  flow, 
the  idea  of  both  authors  apparently  being  that  the  fractures  were  h)ng  sub- 
sequent to  the  movement  of  the  flow. 

Ells  ami  Lawson  mention  the  structure  as  concretionary. 

Winchell  considers  the  cases  described  by  him  as  agglomeratic 
accumulations. 

Cole  and  Grregory  see  in  the  masses  evidence  of  lavas  rolling  over 
among  themselves.  In  the  later  paper,  published  alone,  Gregory  detinitely 
states  that  the  lava  first  contracted  into  spheroids,  which  then  rolled  over 
one  another. 

Ransome  explained  the  Point  Bonita  occurrence  as  a  basalt  which 
flowed  "as  a  viscous  pahoehoe,  one  sluggish  outwelling  of  lava  being  piled 
upon  another  to  form  the  whole  mass  of  the  flow."^  In  the  description  of 
the  basalts,  he  writes:  "A  certain  amount  of  crushed  and  sheared  material 
fills  the  interstices  between  the  spheroids  and  seems  to  be  made  up  of  com- 
minuted fragments  of  the  same  rock.  It  is,  however,  too  crumbling  and 
too  full  of  secondary  products  for  a  satisfactory  determination."^  In  the 
second  occurrence,  in  a  fourchite  (augitite  ?),  the  relations  of  the  rocks  are 
such  as  to  prove  "conclusively  that  such  structure  can  not  be  rigidly 
restricted  to  surface  flows,  although  it  is  still  believed  that  lavas  exhiljiting 
it  must  have  been  erupted  under  very  nearly  surface  conditions."^ 

Teall  agrees  with  Ransome  in  comparing  the  ellipsoidally  parted 
masses  of  basalt  to  pahoehoe  lava.  Teall  concludes  that  such  ellipsoidally 
parted  basalts  are  submarine  flows. 

In  a  recent  paper  Smith  has  described  from  certain  volcanics  bodies 

'  Op.  cit.,p.  112.  "Op.  cit.,  p  78.  » Angellslaiu;,  op.  cit.,  p.  202. 


120 


THli  CRYSTAL  FALLS  IROX-KEARING  DISTRICT. 


which  ill  cross  section  give  elHptical  iigures,  l>ut  whose  iudetenninate  down- 
ward extension  sliows  them  to  be  cohimns.  The  rounding  of  the  cokimns, 
which  were  presumably  originally  prismatic,  he  ascribes  to  dynamic  action- 
He  also  sug'srests  that  elliijsoidal  masses  could  result  from  a  similar  dynamic 
modification  of  a  mass  of  lava  parted  into  shorter  prisms,  or  even 
ellipsoids. 

In  the  description  of  the  eruption  at  >>antorin,  Fouque'  mentions  a 
viscous  lava  exuded  in  the  form  of  a  mass  of  blocks.  These  blocks,  tum- 
bling o^'er  one  another  as  the  mass  is  pushed  from  l^ehind,  have  accumulated 
in  a  rough  pile,  PI.  XII.  Fouque  climbed  these  piles  of  block  lava  shortly 
after  their  production,  and  noticed  the  breaking  off  of  pieces  from  the  sides, 

due  to  the  cooling  and  contraction  of 
the  individual  blocks.^ 

In  general  this  character  agrees 
well  v.'ith  that  of  the  aa  lava  of  Hawaii, 
as  descriljed  by  tlie  late  Prof.  J.  D. 
Dana.'  He  describes  the  formation  of 
the  blocks  as  due  to  the  slow  for- 
ward movement  and  contemporaneous 
breaking  up  of  the  viscous  lava.  The  surface  contrasts  with  the  ropy 
surface  of  the  more  liquid  pahoehoe.  The  aa  is  as  a  rule  compact  as 
compared  with  the  })alioehoe,  though  the  exterior  "is  roughly  cavernous, 
horribly  jagged,  with  projections  often  a  foot  or  more  long  that  are  bristled 
all  over  with  points  and  angles."  From  the  illustrations  of  this  lava  (see 
fig.  10,  taken  fi-om  Dana)  the  blocks  may  be  seen  to  be,  while  irregular,  still 
in  general  distinctly  rounded.  This  is  the  shape  which  viscous  material 
would  naturally  tend  to  take  when  sitbjected  to  the  rolling  action  attendant 
upon  the  onward  motion  of  the  stream  of  which  they  form  an  outer  portion, 
or  in  certain  cases  the  entire  thickness.  This  is  clearly  shown  from  the 
following  quotation  from  Dana's  description  of  the  constitution  and  concli- 

'  Santorin  t-t  ties  truptions,  Ijy  F.  Foiujue:  Paris,  1879.  Clia]).  II.  Compare  especially  Pis,  VIII 
and  XIII. 

2  Op.  cit.,  p.  54. 

'Characteristics  of  Volcanoes,  by  .I.D.Dana:  New  York,  1890,  pp.  9,  241,  and  Am.  Jour.  Sci.,  'ii\ 
ser..  Vol.  XXXIV,  1887,  p.  362. 

"An  aa  or  arate  lava  stream  consists  of  detached  masses  of  lava  as  far  as  is  visible  from  the 
outside.  The  masses  are  of  very  irregular  shapes  and  confusedly  piled  up  to  nearly  a  comuion  level, 
altliough  often  coveriuj;  areas  mauy  miles  long  aud  half  a  mile  to  a  mile  or  more  wide.  The  size  of 
the  masses  in  the  coarser  kind  varies  from  a  tew  inches  across  to  several  vards." 


Fig.  10. — Rt-produetion  of  illustr.4tion  of  aa  lav.i,   after 
Daua  (Characteristics  of  VoleaDoes). 


S     G 


BASIC  VOLCANICS  OF  HEMLOCK  FOh'MATION.  121 

tidii  of  the  ;i;i  strciini  wlu-ii  in  motion:'  "(1)  A  mass  of  rouyii  blocks  outside, 
jii-fciscly  like  the  cooled  aa  stream;  (2)  tlie  motion  extremelv  slow,  indi- 
catiiifi-  a  semifluid  condition  heneath:  .  .  .  (."))  the  Idocks  of  the  upper  part 
of  tlie  front,  as  the  stream  creeps  on,  tund)lin»'  down  the  liigh  slope,  owinj^- 
to  retardation  at  bottom  from  friction,  and  tlnis  a  rollinj^-  action  in  tlie  front 
part." 

Dana  describes  the  gradation  of  pahoehoe  into  aa  lava.  He  writes,  "a 
lava  stream  may  chano'e  from  the  smooth-flowing  or  pahoehoe  condition  to 
the  aa  and  back  again  to  the  smooth-flowing."- 

Platania'  describes  from  Aci-Trezza  and  Aci-Castello  basalts  with 
globular  structure.  The  interspaces  between  the  globes  are  filled  with  silt, 
or  silt  and  tufF,  and  the  exterior  of  some  of  these  globes  presents  a  thin  vit- 
reous cracked  crust  (cf.  p.  117).  These  globular  basalts  are  apparentlv  but 
a  modification  of  the  block  or  aa  lavas  described  by  Fouque'  and  Dana,  in 
which  the  separate  portions  of  the  lava  have  assumed  a  sufficiently  rounded 
character  to  be  called  globes.  However,  Platania's  fui-ther  descriptions  show 
this  term  to  be  clearly  inapplicable  unless  the  word  "globe"  is  used  with 
considerable  latitude. 

The  Santorin  block  lava,  the  Hawaiian  aa  lava,  and  the  Aci-Castello 
globular  lava  are  all  products  of  a  slowly-flowing  comparatively  viscous 
mass.  They  will  in  the  further  description  be  included  under  the  general 
term  "  aa  lavas,"  as  this  is  the  most  common  form  of  occurrence  of  such 
viscous  lavas. 

The  elhpsoidal  basalts  of  the  Crystal  Falls  district  appear  to  be  com- 
parable to  the  Hawaiian  aa  lava  and  block  lavas  of  the  kind  described  b}" 
Fouqu^.  The  lavas  have  subsequent]}-  been  exposed  to  great  pressure  and 
are  considerably  altered.  The  most  obvious  character  of  these  masses,  their 
rounded  outline,  is  believed  to  be  due  to  considerable  extent  to  the  onw^ard 
motion  of  the  stream  as  desci'ibed  by  1  )ana. 

Contraction  caused  by  cooling,  accompanied  by  falling  off  of  fragments 
from  the  outside,  as  observed  b}-  Fouque''  in  the  Santorin  block  lava,  would 
also  tend  to  round  blocks  which  were  originally  angular.     (PI.  XI.)     In 

'Characteristics  of  Volcanoes,  liy  J.  D.  Dana,  New  York,  1890,  p.  242;  and  Am.  .lour.  Sci.,  3tl  ser., 
Vol.  XXVI,  p.  100. 

2Am.  Jour.  Sci.,  3d  ser.,  Vol.  XXXIV,  p.  363. 

^Geological  notes  of  Acireale,  by  Gaetano  Platauia:  The  Southern  Italian  Volcanoes,  H.  J. 
Johuston-Lavis,  editor,  Naples,  1S91,  Chap.  II.,  p.  41. 

<0p.  cit.,  p.  54. 


122  THE  CRYSTAL  FALLS  IRON  BEARING  DISTRICT. 

some  cases  the  separate  portions  of  the  lava  may  have  been  originally 
nearly  globular,  similar  t(i  the  ones  described  by  Platania.  The  ellip- 
soidal basalts,  however,  are  so  common  in  the  Crystal  Falls  district  and 
such  globular  basalts  are  so  rare  that  this  peculiar  form  is  not  considered 
worthy-  of  much  consideration  in  the  further  discussion,  the  first  two  kinds 
being  chiefly  the  forms  from  which  these  were  derived. 

The  lava  blocks  rolling  over  one  another  as  the  lava  stream  advanced, 
would  lie  witli  their  axes  in  all  positions,  but  pressure  and  the  onward 
movement  of  the  flow  would,  in  the  lower  portion  of  the  stream  at  least, 
be  sure  to  produce  from  the  blocks  ellipsoidal  bodies  with  their  two  longest 
axes  corresponding — the  one  to  the  direction  of  flow  and  the  other  to  the 
lateral  extension  of  the  stream.  After  the  stream  ceased  to  flow  and  the 
lava  solidified,  there  would  be  a  gradation  from  the  ellipsoidal  into  the  non- 
ellipsoidal  portion  of  the  flow. 

An  aa  stream,  such  as  described  and  shown  in  fig.  10,  when  subjected 
to  great  pressure  subsequent  to  burial  beneath  thick  deposits,  would  be 
compacted  bv  the  breaking  up  of  the  jagged  outer  portions,  which,  falling 
down,  would  fill  the  spaces  between  the  blocks.  This  broken  material 
filling  the  spaces  would  be  most  exposed  to  movement  and  to  the  action  of 
percolating  waters.  It  would  consequently  be  very  much  altered,  as  in  the 
material  described  above  (p.  119)  by  Ransome.  Such  alterations  would 
result  in  producing  a  matrix  of  exactly  the  same  general  composition  as  the 
altered  ellipsoids.  It  is  the  common  case  of  inetamorphic  action  producing 
from  rock  masses  of  essentially  the  same  chemical  composition,  but  of 
difterent  character,  similar  end  products.  This  brecciated  character  of  parts 
of  this  matrix  is  well  shown  in  parts  of  PL  XI,  and  fig.  B,  PI.  XXXIV.  In 
this  case  silica  has  been  introduced,  filling  the  spaces  and  marking  out  the 
outlines  of  the  fragments.  Where  mashing  has  been  excessive,  the  outlines 
of  the  fragments  are  obliterated  and  the  matrix  rendered  schistose.  There 
may  even  be  a  gradation  fi-oni  the  schistose  matrix  into  the  altered  basalt  of 
the  ellipsoid,  which  at  the  center  is  massive. 

Let  me  recall  the  statement  made  on  previous  pages  concerning  the 
distribution  of  the  amygdaloidal  cavities  in  the  ellipsoids.  This  is  one  of 
the  characteristic  features  of  the  lavas.  We  have  (1)  amygdaloidal  cavities 
distributed  about  evenly  throughout  the  ellipsoids,  the  cavities  being  some- 
what smaller  in  the  center  than  upon  the  periphery ;   (2)  the  cavities  are 


BASIC  VOLCANICS  OF  HEMLOCK  FORMATION.  123 

(■(luct'iitrittctl  upon  the  |ifri})lu'ry  witli  tV-\v  (ir  only  iiiicntscopical  ciivitics  in 
the  center;  (3)  tlle^'  ;ire  concentrated  on  one  side  of  the  ellipsoid,  this  side 
rei)re.senting'  apparentl\-  that  side  of  the  ellipsoid  tiirucd  towanl  tlie  upper 
surface  of  the  lavii  stream.  'I'he  following-  explanation  is  offered  for  this 
difference  in  occurrence.  The  distribution  of  cavities  is  determined  by 
three  factors:  The  viscositv  of  the  lava;  the  difference  in  specific  gravity 
between  the  l)ubl)les  tilling'  the  cavities  and  the  lava;  and  the  expansive 
action  of  the  gas.  In  the  case  of  (1)  the  ellipsoids  are  considered  to  have 
consisted  of  lava  in  a  viscous  condition  through  wliich  the  gas  [)ores  formed, 
but  in  which,  owing-  to  the  high  degree  of  viscosity,  they  remained  nearly 
or  quite  in  the  positions  in  which  they  were  formed.  Here  viscosity  was 
the  determining  factor.  In  case  (2)  the  gas  pores,  influenced  chiefl}-  by  the 
expansion  of  the  gas,  collected  upon  the  periphery — ^^just  as,  for  instance,  in 
the  steel  ingot  while  the  center  is  compact  the  outer  surface  is  porous. 
The  lava  in  this  case  was  prolialily  less  viscous  than  in  the  former.  In  the 
last  described  condition  of  distril)ution  (3),  where  the  gas  cavities  are  on 
one  side,  which  is  the  upper  surface,  the  lava  was  still  less  viscous  than  in 
the  preceding  cases.  Here  specific  gravity  was  the  controlling  factor,  and, 
as  a  result  of  the  speciflc  gravity  and  the  less  viscous  nature  of  the  lava, 
the  gas  bubbles  rose  and  collected  upon  the  upper  surface. 

The  explanation  of  the  ellipsoidal  basalts  which  has  been  offered — viz, 
that  thev  are  comparable  with  aa  or  block  lava — seems  to  offer  a  ready 
explanation  for  all  of  the  observed  characters.  On  the  whole,  the  e\\i\)- 
soids  owe  their  origin  and  certain  peculiarities  to  the  viscous  nature  of  the 
lava.  Thev  possess  also  characters  which  are  due  to  contraction,  others 
which  are  due  to  original  flowage,  and  still  others  which  are  the  result  of 
subsequent  orogenic  movements. 

In  certain  places  we  may  find  the  ellipsoids  only  half  formed — that  is, 
attached  by  one  side  to  the  main  unbroken  part  of  tlie  lava  flow,  the  other 
side  showing  a  rounded  outline.  This  probably  represents  a  place  where 
the  aa  grades  into  a  pahoehoe  or  smooth-flowing  form.  Such  an  instance 
is  possibly  that  illustrated  by  Ransome.' 

Both  Ransome  and  Teall  compare  the  ellipsoidal  basalts  studied  by 
them  with  pahoehoe  lava.  The  latter  also  suggests  a  submarine  origin  for 
the  basalts  studied  by  him.     It  should  be  noted  that  pahoehoe  lava  in  its 

1  Point  Boiiita,  op.  cit.,  fig.  2.  p.  77. 


124  THE  CKYSTAL  FALLS  lEON-BEAKlNG  DISTRICT. 

typical  occuiTeiK-e  in  Hawaii  is  fuuiid  only  in  dry  places,  whereas  tlie  aa  is 
confined  to  those  jjarts  of  the  lava  stream — which  in  other  portions  of  its 
course  is  perhaps  developed  as  pahoehoe — where  it  crosses  moist  valleys  or 
other  depressions  presumed  to  have  contained  a  considerable  amount  of 
moisture.^ 

In  the  case  of  some  of  the  block  lava  of  Santorin  described  by  Fouque," 
with  which  this  may  be  compared,  the  conditions  were  such  that  the  lava 
practically  welled  up  through  the  water. 

From  Dana's  description  it  appears  that  lava  in  the  pahoehoe  form  can 
not  exist  in  the  presence  of  moisture,  being  changed  to  the  aa  form.  It 
would  thus  seem  that  Teall's  statement  of  a  submarhie  origin  for  the 
pahoehoe  lava  is  untenable. 

Wherever  the  ellipsoids  have  been  studied  in  the  Cr^ystal  Falls  district, 
they  have  been  found  to  exist  as  separate  units,  thus  indicating  the  extremely 
viscous  character  of  the  lava.  It  would  seem  that  the  analogy  between 
these  basalts  and  the  aa  or  block  lava  is  much  greater  tlian  that  which 
exists  between  them  and  the  pahoehoe  or  smooth-flowing  lava. 

AMYGDALOIDAL   STUUCTURK. 

The  amygdules  in  the  basalts  are  composed  of  nearly  the  same  min- 
erals as  those  which  occur  secondarily  in  the  rock  mass  itself  Arranged 
in  order  of  frequence  of  occurrence,  they  are  as  follows:  Chlorite,  ei)idote- 
zoi.site,  quartz,  calcite,  feldspar,  iron  oxide,  and  biotite.  An  araygdule  may 
consist  entirely  of  one  of  the  above  minerals,  or,  as  is  most  commondy  the 
case,  of  two  or  more  of  them.  In  the  latter  case  tlie  minerals  are  usually 
arranged  in  concentric  layei's.  The  nonoccurrence  of  zeolites  is  very 
noticeable.  Their  al)sence  from  these  Huronian  volcanics  is  especially 
striking  since  they  are  so  common  in  their  altered  modern  equivalents, 
and  also  occur  in  basalts  as  old  as  those  of  the  Keweenawan  of  Lake 
Superior'  and  of  the  South  Mountain  of  Pennsylvania.^ 


'  Cf.  Characteristics  of  Volcanoes,  by  J.  D.  Dana  :  Ne-sv  York,  1890,  p.  243. 

=  0p.  cit.,  Chap.  II. 

'Faragenesis  and  derivation  of  copper  and  its  associates  on  Lake  Superior,  by  Raphael  I'lim- 
pcUy  :  Am.  Jour.  Sci.,  3d  ser.,  Vol.  II,  l.STl,  ji.  1K8;  also  Cieol.  Survey,  Michigan,  Vol.  I,  part  2,  1873,  pp. 
19-46;  Gcol.  of  Wisconsin,  Vol.  Ill,  l.-SO,  p.  31. 

The  copper-bearing  rocks  of  Lake  Superior,  by  R.  D.  Irving:  Mon.  U.S.  Geol    Survey,  A'ol.  V. 

1883,  p.  89. 

■iTbe  volcanic  rooks  of  South  Mouutain  m  Pennsylvania  and  Jlaryland,  by  G.  H.  Williams:  Aiu 
Jour.  Sci.,  3d  scr.,  \ol.  XLl  V.  1892,  p.  491. 


BASIC  VOI.GANICS  OF  HEMLOCK  FOIIMATIOX.  125 

It  is  also  ot'  iiitcrcsr  to  notice  tliat  there  is  a  total  absence  of  indica- 
tions ot"  copper  in  these  Huroiiian  volcanics,as  well  as  in  tliosi;  ot"  the  Peiiokee- 
Gofebic,  althoiiiih  it  is  associated  with  similar  rocks  in  the  nix-ita  above 
referretl  to  as  well  as  in  many  others. 

The  aniN-^'dn.les,  with  the  exception  of  those  of  chlorite  and  of  l)iotite, 
are  of  nnich  lighter  color  than  the  body  of  the  rock,  and  from  a  short  dis- 
tance gWe  the  rock  the  appearance  of  a  porphyry.  AVeathering-  gives  tlie 
rock  a  different  appearance  according  to  the  materials  filling  the  vesicles. 
Where  these  weather  readily  they  are  removed  and  the  rocks  become 
scoi-iaceous.  Where,  on  the  other  hand,  as  frequently  happens,  the  vesicles 
are  tilled  with  (juartz,  the  matrix  weathers  more  rapidly  and  the  rounded 
quartz  cores  stand  out  on  the  face  of  the  rock  like  the  rjuartz  })ebbles  from 
the  softer  matrix  of  a  conglomerate. 

In  a  few  cases  hematite  is  disseminated  tlii'ovigh  the  quartz  of  the 
amvgdules,  giving  it  the  bright-red  color  of  jasper,  and  by  some  these 
amvgdaloidal  fillings  have  lieen  taken  for  included  jasper  pebbles. 

Careful  study  was  made  of  the  filling  of  the  vesicles  with  the  object  of 
determining  the  order  of  deposition  of  the  minerals.  However,  it  was  found 
that  the  aniA'gdules  in  a  single  slide  contain  very  different  fillings,  one 
chlorite,  another  calcite,  a  third  epidote,  and  so  on;  and  that  even  in  the 
same  slide  the  relations  are  not  always  the  same,  a  mineral  which  here 
occupied  the  center  of  an  amygdule  being  found  there  on  the  periphery. 
]kIoreover,  the  same  mineral  species  was  found  at  times  occupying  tlie  out- 
side and  the  center  of  the  same  amygdule. 

It  is  clear  that  the  fillings  are  not  the  result  of  a  solution  connnon  to 
all  the  lavas,  but  that  the  same  kinds  of  solutions  were  active  in  the  various 
lavas  at  different  times  and  even  in  the  same  la\a.  at  different  times.  How- 
ever, the  conclusions  reached  were  that  the  chlorite  was  generally  the  first 
product  deposited  an<l  the  quartz  usually  the  last.  From  the  stud}'  of  the 
related  amygdaloids  upon  Keweenawan  Point,  Pumpelly^  long  ago  reached 
the  conclusion  that  chlorite  was  the  earliest  product  of  alteration — hence  we 
may  conclude  the  first  to  be  deposited  in  the  amygdaloidal  cavities;  and 
that  the   latest  mineral  deposited  in   the    cavities,    omitting  copper  from 


'  The  paiageuesis  and  derivation  of  copper  and  its  associates  on  Lake  Superior,  liy  Raphael 
Pumpelly:  Am.  .lour.  Sci.,  3d  ser.,  Vol.  II,  1871,  ]>.  29. 

Jletasiiuuitio  development  of  the  copper-beariuj;  rocks  of  Lake  Superior,  liy  Raphael  Pumpelly  : 
Proc.  Am.  Acad.  Arts  and  Sci,,  Vol.  XIII,  1878,  p.  307. 


126  THE  CRYSTAL  FALLS  IRON-BEAKING  DISTRKJT. 

consideration,  was  quartz,  the  tendency  naturally  being  to  replace  more 
alterable  witli  less  alterable  minerals. 

Flattening    of  amygdaloidal   cavities. 111      SOlllC      of     the      amVgdaloids    (fig.    B,     PI. 

XXV)  the  cavities  retain  their  circular  shape,  as  though  the  rock  had  not 
flowed  to  any  great  extent.  ]\Iore  commonly  the  ca^'ities  are  di-awn  out 
into  irregular  (fig.  A,  PI.  XXV)  or  lenticular  shapes,  the  long  axes  agreeing 
with  the  direction  of  flowage  in  case  their  deformation  resulted  from  this,  or 
with  the  direction  of  schistosity  in  those  cases  where  the  rocks  have  been 
extensively  mashed.  In  some  cases  the  cavities  have  been  so  extremel}' 
flattened  that  the  amygdules  appear  almost  the  shai)e  of  a  melon  seed, 
showing  a  mere  streak  of  chlorite  in  the  sections  cut  perpendicular  to  the 
schistosity,  and  in  the  planes  of  schistosity  large  lustrous  oval  areas. 

In  some  few  of  the  basalts  the  groundmass  immediately  surrounding 
the  amygdules  is  characterized  by  an  accumulation  of  ferruginous  matter. 
In  most  cases,  however,  this  part  of  the  groundmass  does  not  diff"er  in  any 
respect  from  the  rest  of  the  groundmass  of  the  basalts  and  points  to  a  very  _ 
gradual  cooling. 

Al.TEKATION   OK   THK   BASALTS. 

The  descriptions  given  are  of  the  freshest  and  most  characteristic 
basalts.  As  already  explained,  the  mineral  constituents  in  even  these 
freshest  ones  have  undergone  a  veiy  far-reaching  alteration.  The  rocks 
which  show  a  more  advanced  stage  of  alteration  exhibit  merely  a  difference 
in  degree  rather  than  in  kind,  and  the  minerals  which  result  are  in  all  cases 
the  same.  They  are  uralite,  actinolite,  epidote-zoisite,  chlorite,  white  and 
brown  mica,  calcite,  sphene,  quartz,  and  feldspar. 

The  amount  of  these  secondary  minerals  varies  greatly,  showing  that 
the  alteration  products  resulting  from  the  same  kind  of  original  rock  may 
differ  very  materially  according  to  the  process  of  metaniorphism. 

In  a  general  way  the  alteration  of  the  basalts,  as  observed  under  the 
microscope,  has  taken  the  following  course:  Even  in  the  rocks  nearest  their 
original  condition  the  augite  has  largely  changed  to  uralite.  The  vitreous 
base,  if  any  was  present,  has  become  devitrified.  Rocks  in  this  stage  of 
change  still  show  the  more  important  external  characters  of  igneous  rocks, 
including  in  many  cases  those  which  are  characteristic  of  glass.  Some  of 
the  rocks  at  this  stage  are  light  gray  to  green  and  exceedingly  tough. 
Many  of  these  break  with  a  ringing  sound  almost  like  phouolites.     At  a 


'     BASIC  VOLCANICS  OF   HEMLOCK  FiJUMATlON.  127 

furtliLT  stage  of  (.•luingL-  thu  t'cld-spars  are  partly  altered  to  a  graiuilar 
■io(ire<'-ate  of  A'arious  niinerals.  In  ordillar^'  liylit  the  textures  of  iyueous 
rocks  are  still  preserved,  Imt  in  polarized  light  none  are  seen,  with  the 
exception  of  aniygdules  which  may  be  present.  In  some  cases  even  these 
are  obliterated,  and  the  original  nature  of  the  rock  can  only  be  determined 
from  its  mode  of  ttccurrence  and  its  association. 

Further  changes  may  produce  rocks  which  consist  practically  of  calcite, 
and  may  be  nearly  white. 

Ag-ahi,  from  these  basic  rocks  there  may  be  produced  in  extreme  cases, 
bv  a  process  of  silicification,  a  rock  which  consists  practically  of  ])ure  silica. 

Description  of  some  phases  of  alteration. As     iUuStratiug    SOme     CaSBS     iu     wlucll     the 

same  alteration  products,  but  in  different  proportions  and  arrangement,  give 
rocks  dift'ering  very  essentially,  there  are  given  the  following  Ijrief  descrip- 
tions of  some  of  the  rocks  studied. 

The  flow  structure  was  noted  as  being  exceedingly  well  develoj^ed  in 
the  microlitic  rocks,  and  in  some  of  them  the  production  of  amphibole 
needles  and  cldorite  flakes  has  taken  place  parallel  with  the  long  direction 
of  the  feldspar  microlites  (the  flowage  direction),  thus  develo})ing,  in  com- 
bination with  the  unaltered  microlites,  a  well-marked  schistosity.  The 
feldspars  are  still  fairly  well  preserved. 

In  another  case  the  feldspar  microlites  have  become  completely  sericit- 
ized,  the  interspaces  between  them  being  occujjied  by  epidote,  chlorite, 
and  iron  oxide.  The  preservation  of  the  feldspar  shapes,  showing  in  ordi- 
nary light  the  igneous  texture  of  the  rock,  gives  the  only  clue  to  its  original 
nature.  (Figs.  A  and  B,  PI.  XXVIII.)  In  some  of  the  basalts  the  feldspar 
is  replaced  chiefly  by  epidote-zoisite,  and,  as  in  the  above  case,  such  rocks 
show  their  igneous  character  only  when  examined  in  ordinar}-  light  or  by 
uncrossed  nicols.     (Figs.  A  and  B,  PI.  XXIX.) 

In  still  other  rocks  calcite  is  very  abundant.  Its  occun-ence  in  jjor- 
phyritic  rhombohedra  and  scalenohedra  was  mentioned  in  the  description  of 
some  of  the  rocks.  These  porjjhvritic  calcites  have  thus  far  been  found  only 
in  the  fine-grained  microlitic  types  of  groundmass,  the  coarser  ophitic  rocks 
having  it  only  in  the  usual  granular  aggregates.  Muscovite,  occurring  in 
large  porphyritic  ^^lates,  conforms  in  occurrence  to  the  calcite.  When 
muscovite  is  present,  calcite  is  found  associated  with  it  in  every  case, 
though  the  calcite  may  occur  alone,  and  this  latter  is  also  b}*  far  the  more 


128  THE  CEYSTAL  FALLS  IKON  BEARING  DISTRICT.    ' 

common.  These  crystals  give  a  secondary  porpliyritic  character  to  the  havas, 
and  the  microscopical  appearance  of  the  rocks  varies  somewhat  according 
to  the  occurrence  of  the  calcite.  Such  rocks,  for  instance  where  the 
rhombohedra  occur,  look  on  fresh  surface  l^y  rapid  examination  like 
por^jhyrites  in  which  the  feldspar  sections  are  all  c[uadratic.  In  the  others 
the  scalenohedral  sections  resemble  in  general  lath-shaped  feldspar  pheno- 
crysts  Iving  scattered  in  all  directions  on  the  surface  of  the  rock. 

Another  case  of  extreme  alteration  is  shown  in  a  light  greenish-gray, 
much-altered  schistose  rock  from  sec.  21,  T.  46  N.,  R.  32  W.  Upon  the 
weathered  surface  long  grooves  are  noticed — one  measuring  60  mm.  long 
bv  5  nmi.  wide — which  on  the  fresh  sm-face  are  tilled  with  calcite.  On 
faces  perpendicular  t(^  the  long  extension  of  such  grooves  they  appear  as 
narrow  slits,  with  the  long  direction  of  the  slit,  that  is,  the  width  of  the 
groove,  agreeing  with  the  schistosity.  These  are  clearly  flattened  amygda- 
loiilal  pores,  and  but  for  them  the  igneous  nature  of  the  original  rock  could 
not  have  been  determined.  The  extreme  flattening  of  these  amygdaloidal 
cavities  and  the  schistose  nature  of  this  rock  produced  from  an  original 
volcanic,  points  toward  mashing  as  one  of  the  causes,  if  not  the  main  cause, 
of  its  present  characters.  It  is  now  composed  of  fairly  large  automorphic 
actinolite  individuals,  a  very  small  amount  of  biotite  and  chlorite  flakes,  and 
masses  of  grains  of  cpiartz,  calcite,  epidote-zoisite,  magnetite,  with  ilmenite 
and  hematite  in  thick  plates  filling  in  the  spaces  between  the  actinolites. 
If  any  feldspar  was  originally  present,  it  is  now  entirely  concealed  hv  tlie 
calcite  and  epidote-zoisite. 

The  calcite  phenocrysts  are  found  in  the  ftiirlv  fresh  lavas.  They  are 
beautifully  automorphic  and  are  certainly  not  replacement  pseudomorphs  of 
some  original  phenocrysts,  hnt  replace  the  various  minerals  of  the  fine- 
grained mass.  Moreover,  it  is  clear  that  they  were  formed  subsequent  to 
all  dynamic  action,  as  their  crystal  Qutlines  are  perfect  and  the}'  never 
show  any  evidence  of  pressure.  This  is  so  even  in  those  cases  where  the 
amygdules  which  have  been  markedly  elongated  are  filled  with  calcite 
The  process  of  replacement  could  not  be  followed,  but  it  is  evidently  con- 
nected with  the  development  of  chlorite,  those  rocks  in  which  a  great  deal 
of  the  calcite  occurs  having  chlorite  developed  instead  of  actinolite. 

In  other  sections  in  which  the  amount  of  porpliyritic  calcite  or  calcite 
and  museovite  is  much  greater  than  in  the  rocks  just  described,  the  amount 


BASIC  VOLCANIOS  OF  HEMLOCK  FORMATION.  129 

of  i-lilorite,  iron  oxidt',  rutik',  and  quartz  is  also  greater.  Tlie  quartz  is  in 
very  fine  graius.  The  jn-esence  of  the  feklspar  cau  only  be  d(itermiued 
witli  (hffiouhy,  and  usually  only  on  the  edges  of  the  sections,  as  the  laro-e 
amount  of  chlorite  in  the  center  conceals  it.  The  textures  caused  by  the 
feldspar  and  the  amygdules  still  indicate  the  original  character  of  such 
extremely  altered  stages.  Figs.  A  and  B,  PI.  XXX,  illustrate  such  a  rock, 
showing  the  secondary  porphyritic  muscovite  and  calcite,  and  also  the 
original  amygdaloidal  character. 

A  still  further  stage  of  alteration  gives  a  rock  whose  groundmass  is 
composed  of  the  finest-gTained  quartz  and  of  grains  and  needles  of  brown 
rutile  (auatasef).  In  this  lie  rhombohedra  of  ferruginous  calcite,  plates  of 
muscovite,  and  irregular  flakes  of  chlorite.  The  rock  is  macroscopically 
gi-ay,  hard,  and  quartzitic,  has  a  ferruginous,  brown,  weathered  crust, 
effervesces  with  cold  HCl,  and  yet  shows  its  volcanic  character  by  the 
numerous  beautiful  amygdules.  These  stand  out  on  the  surface  like 
pebbles  in  a  conglomerate.  In  some  cases  the  weathering  bnngs  out  the 
concentric  character  of  the  filling  very  nicely.  For  example,  some  may 
be  seen  in  which  the  core  is  quartzitic,  and  is  standing  suiTounded  by  a 
ring-like  depression,  showing  by  difference  in  the  weathering  the  diff"erent 
character  of  the  mineral  filling.  Under  the  microscope  the  only  amyg- 
dules which  happened  to  be  cut  by  the  section  were  found  to  be  filled  with 
fine-grained  quartz,  with  chlorite  in  automorphic  flakes  at  the  center  of  the 
amygdules,  and  lying  in  the  quartzitic  mass.  The  macroscopical  appear- 
ance of  some  of  the  amygdules  shows  that  just  the  reverse  condition  also 
exists,  that  is,  that  quartz  forms  the  centers  and  chlorite  surrounds  it. 

The  extreme  stage  of  such  an  alteration  is  a  rock  which  shows  no 
amygdules  macroscopically  or  microscopically,  but  is  otherwise  like  the 
groundmass  of  the  above  last-described  rock.  It  would  be  impossible  to 
determine  the  original  character  of  such  a  rock  except  by  its  association. 

The  extremes  of  texture  obtained  in  the  alteration  ^w'ocesses  are,  on 
the  one  hand,  a  porphyry  with  eruptive  groundmass  and  secondary  pheno- 
crysts;  on  the  other,  a  porphyritic  schist,  in  which  all  elements  are  secondary. 
These  extremes  are  connected  by  gradation  varieties,  in  some  of  which  the 
calcite  and  muscovite  approach  more  closely  to  the  size  of  the  elements 
composing  the  groundmass,  and  which  consequently  approacli  the  ordinary 
schists  in  structure. 

>I0N  xxxvi 9 


130  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT, 

111  these  rocks  the  porphyritic  characters  are  unquestionably  due  to 
the  itroductioii  of  secondary  pheuocrysts  of  mica  (muscovite)  and  calcite, 
not  by  contact  metaniorphisni  Ijut  by  dynamic  action.^ 

It  has  not  been  found  possible  to  determine  definitely  from  a  study  of 
the  specimens,  in  many  cases  from  widely  separated  exposures,  on  which 
the  above  observations  were  made,  whether  the  process  which  has  taken 
place  in  the  production  of  such  rocks  has  been  a  combination  of  calcification 
and  silicification,  or  a  process  by  which  carbonate  is  being  replaced  by 
silica  or  the  reverse.  The  replacement  of  carbonate  by  silica,  as  shown  by 
Irving  and  Van  Hise,^  has  taken  place  extensively  in  the  case  of  the  ferru- 
ginous carbonates  of  the  Penokee-Gogebic  and  Marquette  iron  ranges  of 
Wisconsin  and  Micliigan.  The  automorphic  character  of  the  carbonate 
would  seem  to  point  toward  calcification  as  the  controlling  process  in  the 
Crystal  Falls  rocks. 

Though  the  presence  of  quartz  as  the  last  filling  of  the  amygdaloidal 
cavities  points  toward  silicification  as  being  the  process  which  would 
eventually  predominate,  it  is  most  prol^able  that  both  processes  of  calcifi- 
cation and  silicification  are  active;  but  whether  the  one  or  the  other  is  the 
controlling  one  depends  upon  the  depth  of  burial  of  the  rocks  which  are 
altering. 

This  statement  appears  to  be  supported  by  the  facts  to  be  described  in 
the  following  pages.  The  following  oljservations,  which  were  made  upon 
sections  taken  fi-oiu  an  ellipsoidally-parted  basalt  occurring  on  top  of  the 
bills  to  the  west  of  and  overlooking  Mansfield,  illustrate  the  changes  which 
take  place  in  the  passage  from  the  massive  rock  of  the  ellipsoids  into  the 
schistose  material  of  the  mati-ix.  •  The  change  is  one  of  increasing  altera- 
tion.    This  alteration  is  largely  one  of  carbonation  followed  by  silicifica- 

'  Metamorpbism  of  clastic  feldspar  in  conglomerate  schist,  by  J.  E.  Wolff:  Bull.  Mvis.  Couip.  Zool., 
Vol.  XVI,  1891,  pp.  173-183.  Pis.  I-XI.  Cf.  also  Wolff  on  Green  Mountains,  Mon.  U.  S.  Geol.  Survey,  Vol. 
XXIII. 

Principles  of  North  American  pre-Cambrlan  geology,  by  C.  R.  Van  Hiso:  Sixteenth  Ann.  Rept. 
U.  S.  Geol.  Survey,  Pt.  1, 1896,  p.  692. 

Phases  in  the  metamorpbism  of  the  schists  of  Southern  Berkshire,  by  W.  H.  Hobbs:  Bull.  Geol. 
Soc.  Am.,  Vol.  IV,  1894,  pp.  169-177. 

'^  Origin  of  the  ferruginous  schists  and  iron  ores  of  the  Lake  Superior  region,  by  R.  D.  Irving: 
Am.  Jour.  Sci.,  3il  ser.,  Vol.  XXXII,  1886,  pp.  255-272. 

The  iron  ores  of  the  Penokee-Gogebic  series  of  Michigan  and  Wisconsin,  by  C.  R.  Van  Hise: 
Am.  Jour.  Sci.,  3a  ser.,  Vol.  XXXVII,  1889,  pp.  32-48. 

The  Penokee  iron-bearing  series  of  Michigan  and  Wisconsin,  by  R.  D.  Irving  and  C.  R.  Van  Hise : 
Tenth  Ann.  Rept.  U.  S.  Geol.  Survey,  1889,  pp.  341-507 ;  Men.,  Vol.  XIX,  1892,  pp.  254-257. 


BASIC  VOLOANICS  OF  BEMLOGK  FORMATION,  131 

tioii.  It  may  be  characteristic  nho  of  basalts  with  uo  ellii)S()idal  parting, 
but  it  has  l)eeu  possible  to  follow  the  siiccessive  changes  only  in  the  ellip- 
soidal basalts.  This  is  due  to  the  fact  that  each  ellipsoid  shows  all  stages 
from  the  comparatively  fresh  material  of  the  center  to  the  much  altered 
material  on  the  periphery,  and  to  the  most  altered  basaltic  material  forming 
the  so-called  matrix  sun-ounding  the  ellipsoidal  bodies  (p.  114). 

The  freshest  part  of  the  interior  of  an  ellipsoid  from  this  occurrence  is 
a  very  fine-grained  micro-amj-gdaloidal  basalt,  in  which  in  ordinary  light 
lath-shaped  feldspar  microlites  can  be  readily  distinguished.  Upon  close 
examination  the  feldspars  are  found  to  be  nmch  altered,  and  in  many  cases 
their  crystal  outlines  are  almost  completely  filled  out  by  grains  of  calcite 
and  flakes  of  sericite  and  chlorite  in  a  quartz-albite  (!)  aggregate.  The 
spaces  between  the  feldspar  laths  are  now  occupied  by  large  crystals  of 
epidote-zoisite,  grains  of  iron  oxide,  a  few  flakes  of  chlorite,  and  innumera- 
able  small  round  yellowish-brown  and  greenish  indeterminable  Ijodies.  The 
epidote-zoisite  crystals  also  include  large  quantities  of  the  brown  and  green 
globular  bodies,  showing  that  they  were  produced  previous  to  the  epidote-. 
zoisite.  The  substance  in  which  this  aggregate  is  embedded  could  not  be 
determined,  as  the  aggregate  is  either  so  dense  that  nothing  could  be  dis- 
cerned or  else  underlain  by  feldspar.  In  the  last  case  the  substance  is  r.oen 
to  be  clear  white.  The  minerals  mentioned,  with  the  exception  possibly  of 
the  iron  oxide,  have  evidently  been  produced  secondarily  from  the  sub- 
stance or  substances  originally  filling  the  spaces  between  the  feldspars. 
Nothing  points  toward  the  original  substance  or  substances  having  been 
crystallized,  and  I  am  inclined  to  believe  that  it  was  glass. 

Toward  the  exterior  of  the  ellipsoid  the  rock  is  more  altered.  The 
zoisite  and  calcite  are  more  abundant.  The  calcite  occurs  in  the  spaces 
between  the  feldspars,  as  well  as  occupying  parts  of  their  outlines.  (Figs. 
A  and  B,  PI.  XXXI.)  All  of  the  other  products  drop  into  the  back- 
ground, owing  to  the  fact  of  nonproduction,  or  concealment  by  the  zoisite- 
calcite  aggregates. 

Still  nearer  the  exterior  of  the  ellipsoid  the  calcite  frequently  fills  the 
spaces  once  occupied  by  the  feldspars  with  long  scalenohedi-al  crystals, 
which  in  a  way  maintain  the  original  igneous  structure.  The  calcite  is, 
however,  not  confined  to  these  feldspar  areas  alone,  but,  as  stated  above, 
also  occurs  between  them. 


132  THE  CRYSTAL  FALLS  lEON-BEAEOG  DISTRICT. 

The  matrix,  representiug  the  most  altered  phase,  is  a  granular  aggre- 
gate of  calcite,  in  which  one  may  here  and  there  discern  small  clear  limpid 
grains  of  secondary  quartz  and  feldspar  (?)  and  flakes  of  chlorite.  The 
calcite  includes  in  considerable  quantity  the  globular  bodies  mentioned. 
These  are  found  also  in  the  spaces  between  the  calcite  grains,  as  though 
pushed  away  from  the  gi-ains  as  thej^  crystallized. 

Some  of  the  calcite  in  the  first  stages  of  the  alteration  of  the  rock  may 
have  been  derived  from  a  basic  feldspai;  It  is  clear,  however,  that  the 
o-reat  mass  can  not  owe  its  origin  to  this  process,  but  nmst  be  the  residt  of 
infiltration.  The  calcite  grains  derived  from,  and  lying  in,  the  feldspar 
acted  as  nuclei,  around  which  the  infiltrated  calcite  was  gradually  col- 
lected, producing  pseudomorphs  after  the  feldspar  laths.  Quite  recently 
Dr.  W.  S.  Bayley^  has  noted  in  the  Clarksburg  submarine  volcanic  forma- 
tion of  the  JMarquette  district,  Michigan,  the  occurrence  of  tuffs,  in  which 
calcite  has  been  introduced  in  such  quantity  that  they  may  almost  be  called 
limestones. 

In  another  case  in  which  the  alteration  of  the  ellipsoid  (PI.  XI)  appar- 
ently proceeded  along  the  lines  of  shearing,  and  produced  the  kind  of 
aggregates  of  chlorite,  including  crystals  and  aggregates  of  epidote- 
zoisite,  which  were  described  (p.  117)  as  the  usual  matrix  of  such  elhpsoids, 
one  can  see  in  thin  section  the  calcite  entering  the  chlorite  aggregate  along 
minute  fissure  lines.  The  calcite  literally  eats  its  way  into  the  chlorite, 
and  produces  by  an  interchange  of  elements  a  mass  of  calcite  (magnesian  1) 
and  epidote,  besides  including  epidote  which  originally  occurred  scattered 
through  the  chlorite  aggregate. 

The  carbonation  of  the  original  basalt  or  of  the  secondary  chlorite 
mass  results  in  producing  a  mass  of  carbonate  which  has  associated  with 
it  some  secondary  quartz,  chlorite,  and  epidote.  This  carbonate  mass  may 
be  almost  massive  or  it  may  be  decidedly  schistose.  When  schistose,  the 
grains  of  calcite  and  quartz  have  a  uniform  elongation,  and  the  schistosity 
is  matei-ially  enhanced  by  flakes  of  chlorite,  which  are  not  uncommonly 
found  in  thin  streamers  or  thick  masses  in  the  carbonate  aggregate,  at  times 
in  sufficient  quantity  to  give  it  macroscopically  a  decided  green  tinge. 

I  have  used  the  term  "carbonate,"  although  having  described  in  detail 
above  the  calcification  of  the  basalt,  for  the  reason  that  at  times,  and  for  no 


'  Mod.  U.  S.  Geol  Survey,  Vol.  XXVIII,  p.  473. 


BASIC  VOLCANICS  OF  HEMLOCK  FORMATION.  133 

discernible  re.ason,  the  iron  carlionate  (siderito)  may  replace  the  ealcite,  in 
which  case  we  get  a  dark  bluish-black  variety  of"  matrix  (p.  117).  The 
siderite  masse.s  do  not  ditier  essentially  from  the  ealcite,  tliough  in  some  of 
them  a  very  small  quantity  of  actinolite  is  found  associated  with  the  chlorite. 
As  illustrating  the  purely  local  development  of  these  two  carbonates,  I 
would  mention  having  observed  in  one  section  a  band  of  siderite  separated 
from  a  band  of  carbonate,  which  from  its  color  appeared  to  be  quite  pure 
ealcite.  One  may  also  see  commonly  in  exposures  areas  of  pure  white 
ealcite,  almost  in  juxtaposition  with  areas  of  siderite. 

It  is  a  fact  generally  recognized  that  carbonation  is  a  process  confined 
to  the  outer  crust  of  the  earth,  so  that  we  may  perhaps  best  explain  the 
local  occurrence  of  these  carbonates  replacing  the  basalt  as  products  of 
carlDonate-bearing  waters.  That  such  carljonation  of  the  igneous  rocks 
through  which  these  waters  percolate  is  now  taking  place  seems  certain. 
The  carbonate  grains  in  the  rocks  described  are  shattered  and  elongated, 
or  at  least  show  undulatory  extinction.  They  thus  give  evidence  of  having 
been  more  or  less  mashed  since  their  production,  and  this  mashing  probably 
took  place  after  they  had  been  more  or  less  deeply  buried,  and  was,  as  a 
matter  of  fact,  to  some  extent  due  to  the  pressure  of  the  superincumbent  rocks. 

The  probability  that  these  rocks  have  been  thus  deeply  buried  subse- 
quent to  their  formation  is  to  be  borne  in  mind  with  special  reference  to  the 
next  process  to  which  they  have  been  subjected,  that  of  silicificatiou.  This 
process  is  most  clearly  shown  in  the  siderites,  and  the  phases  of  alteration 
noted  in  their  study  will  be  briefly  described. 

The  microscope  shows  the  siderite  matrix  to  be  a  coarsely  granular 
aggregate  composed  essentially  of  crushed  siderite  grains.  Between  these 
grains  in  a  few  places  are  small  grains  of  quartz,  flakes  of  chlorite,  and  very 
rarely  needles  of  actinolite.  Large  quantities  of  black  ferruginous  specks 
are  included  in  and  also  lie  between  the  quartz  grains,  and  such  specks  are 
also  to  be  seen  included  in  siderite  areas,  but  close  inspection  shows  that 
they  are  also  associated  with  blebs  of  quartz.  The  chlorite  flakes  and 
quartz  grains  are  generally  elongated  in  the  same  direction,  and  the  quartz 
shows  wavy  extinction. 

A  more  advanced  stage  in  the  process  of  silicificatiou  was  studied  in 
the  case  of  a  rock  which  is  bluish-black  in  color,  exceedingly  fine  grained, 
and  minutely  schistose,  the  schistosity  agreeing  with  the  contours  of  the 


134  THE  CRYSTAL  FALLS  IKON-BEARING  DISTRICT. 

ellipsoid  from  around  which  it  was  broken.  This  is  essentially  an  exceed- 
ingly fine-grained  quartz  rock,  with  chlorite  flakes  and  black  ferruginous 
specks  scattered  through  it,  and  here  and  there  an  irregular  oval  siderite 
o-rain  remaining.  Very  few  and  unimportant  grains  of  epidote  were  also 
noticed.  This  rock  represents  nearly  the  last  stage  in  the  process  of  silici- 
fication  by  which  the  siderite  has  been  replaced,  and  a  part,  probably  the 
greater  part,  of  its  iron  content  oxidized.  Some  chlorite  and  epidote  has 
been  produced,  clearly  from  the  lime  and  magnesian  impurities  in  the 
siderite.  Essentially  the  same  process  of  siliciflcation  has  been  described 
bv  Van  Hise  in  his  various  articles  on  the  Penokee-Gogebic  and  Marquette 
iron  ranges,  to  which  refei'ences  have  been  so  frequently  made.  I  have 
desired  especially  to  call  attention  to  it  here,  however,  on  account  of  the 
fact  that  it  shows  the  possibility  of  the  production  of  an  iron  ore  from  an 
original  eruptive  rock  by  the  combined  processes  of  earbonation  and  silici- 
flcation. It  is  true  that  the  end  product  in  the  case  described  does  not 
contain  enough  iron  to  be  an  ore  deposit,  but  that  is  a  mere  detail.  May 
not  this  serve  also  to  some  extent  to  explain  the  numerous  clearly  marked 
belts  of  magnetic  attraction  which  occur  throughout  this  area  of  altered 
basalts,  in  Avliich  little  of  the  original  magnetite  remains  i;naltered  to  exert 
an  influence  npon  the  magnetic  needle"?  To  explain  this  we  must  suppose 
the  influence  to  be  exerted  by  secondarj^  magnetite  accunudated  along  cer- 
tain lines.  The  magnetic  lines  traced  out  agree  in  a  very  marked  way 
with  what  has  been  determined  to  be  the  trend  of  the  lava  flows  and  tuff 
beds.  The  condition  which  would  determine  the  presence  of  such  a  line 
of  earbonation,  if  we  may  so  put  it,  may  be  the  presence  of  a  scoriaceous 
lava  flow  or  a  bed  of  tuff,  which  offers  exceptional  facilities  for  the  passage 
of  carbonate-bearing  waters.  It  is  thus  intimated  that  there  is  possibility 
of  flnding  purely  local  ore  bodies  of  small  size  even  in  the  midst  of  this 
volcanic  area. 

The  process  of  siliciflcation  is  generally  considered  as  a  deep-seated 
one,  occurring  far  below^  the  outer  weathering  zone. 

When  the  rocks  exhibiting  these  various  phases  of  siliciflcation  are 
exposed  in  the  zone  of  weathering,  certain  interesting  re.sults  are  obtained 
which  are  worth  noticing.  Rocks  are  produced  from  these  which  upon  the 
surface  strongly  resemble  amygdaloids,  but  in  which  the  pseudo-amygda- 
loidal  cavities  are  of  purely  secondary  origin.  For  instance,  when  the 
siderite  mass  has    become   onlj'  partially  replaced  by   silica,   weathering 


BASIC  VOLCANICS  OF  HEMLOCK  FOKMATION.  135 

afcncies  It'iu-li  out  the  reinaiuiug  siderite  areas  and  leave  the  tliiu  lihns  of 
silica  which  lie  between  them  standing  up,  thus  giving  the  rock  the  appear- 
ance of  a  ver}'  dark  pumice.  As  the  silicificatiou  progresses  the  siderite  is 
very  much  reduced  in  (puuitity,  the  intervening  siliceous  areas  increasing 
correspondingh'.  The  pressure  exerted  upon  the  rock  has  caused  the 
isolated  siderite  areas  to  take  on  an  oval  cliaracter,  the  longer  axes  in 
general  agreeing  and  being  perpendicular  to  the  ])ressure.  When  such 
siderite  areas  are  leached  out,  the  silica  bands  remain,  and  pseudo-amyg- 
daloidal  cavities  are  produced,  giving  a  very  perfect  jiseudo-amygdaloidal 
structure  to  the  hand  specimen.  This  is  the  origin  of  that  character  of 
matrix  which  some  of  the  geologists  have  described  in  their  field  notes 
as  like  rotten,  worm-eaten  wood  (tig.  B,  PI.  XXVII). 

Although  at  present  the  material  between  the  ellipsoids  differs  so 
markedly  from  the  rock  forming  the  ellipsoids  themselves,  nevertheless 
there  is  no  reason  for  supposing  the  original  composition  of  that  part  of 
the  rock  mass  to  have  been  essentially  different.  The  change  in  the 
character  of  the  basalt  in  passing  from  the  ellipsoids  toward  the  schistose 
matrix  is  in  mineralogical  character  much  as  has  been  described  for  other 
basalts  from  this  same  district.  The  reason  for  the  more  complete  degree 
of  the  replacement  process  in  passing  away  from  the  ellipsoids  may  be 
readily  understood  from  the  discussion  of  the  origin  of  the  ellipsoidal 
parting  of  the  basalts,  where  the  conclusion  was  reached  that  the  matrix 
between  the  ellipsoids  resulted  from  the  comminution  of  basaltic  material 
of  the  same  general  character  as  that  of  the  ellipsoids.  This  matrix  was  ot 
course  more  porous  and  probably  more  vitreous  than  the  basalt,  and  hence 
more  liable  to  be  altered. 

PYROCLASTICS. 

The  majoi-ity  of  the  clastic  rocks  have  been  derived  from  the  basic 
volcanic  rocks  already  described.  These  elastics  are  veiy  characteristic  of 
the  Hemlock  formation  and  constitute  the  greater  part  of  it.  They 
comprise  several  classes,  the  more  important  of  which  are  the  eruptive 
breccias,  volcanic  sedimentary  rocks,  and  schistose  pyroclastics. 

ERUPTIVE   BRECCIA. 

The  term  "eruptive  breccia"  is  here  used  to  include  those  clastic  rocks 
in  which  angular  fragments  of  an  igneous  rock  are  surrounded  by  a  matrix 


136  THE  CRYSTAL  FALLS  lEON-BEARlNG  DISTRICT. 

also  of  igneous  origin.  In  an  eruptive  breccia  the  fragments  may  be  like 
or  unlike.  Likewise  the  matrix  may  be  hke  or  unlike  the  fragments. 
Where  the  fragments  have  been  rounded  during  the  movement  of  the  erup- 
tive magma  surrounding  them,  the  resulting  rock  may  be  called  an  eruptive 
pseudo-conglomerate. 

Eruptive  breccias  are  not  very  common  in  the  Crystal  Falls  district. 
"Where  they  do  occur,  the  fragments,  while  predominantly  angular,  are  to 
some  extent  more  or  less  rounded,  and  are  similar  in  nature  to  the  matrix 
in  which  they  lie.  Since  the  rocks  which  form  them  preserve  the  main 
characters  of  the  massive  lava  flows  which  have  just  been  described,  they 
will  not  be  discussed  in  detail.  The  exact  method  of  the  formation  of  these 
breccias  could  not  be  told. 

In  one  case,  in  which  both  fragments  and  matrix  are  amygdaloidal, 
it  appears  probable  that  the  occurrence  represents  a  true  flow  breccia  in 
which  the  broken  surface  of  a  lava  flow  had  been  recemented  by  a  later 
lava  flow  of  the  same  kind  of  rock,  or  that  it  represents  a  very  possible 
case  in  wdiich  the  lava  welled  up  through  and  floAved  over  portions  of  its 
own  crust,  cementing  the  fragments.  In  such  breccias  a  flow  structure 
around  the  fragments  is  quite  plainly  shown  and  the  matrix  possesses  a 
peculiar  ropy  appearance.  In  one  instance,  in  which  both  the  fragments 
and  matrix  were  niacroscopically  nonamygdaloidal,  it  is  probable  that  they 
were  formed  under  considerable  pressure,  and  that  this  was  a  case  in  wdiich 
lava  was  forced  up  through  a  previously  consolidated  mass  of  rock  of  like 
character,  and  in  its  passage  carried  with  it  various  fragments,  forming 
an  eruptive  "reibimffs-hreccia"  or  friction  breccia. 

VOLCANIC  SEDIMENTARY  ROCKS. 

Under  the  term  "tuffs"  have  been  very  generally  included  all  kinds  of 
volcanic  clastic  rocks.^  This  is  probably  due  to  the  fact  that  there  is  fre- 
quently considerable  difficulty  in  discriminating  between  eolian  deposits 
and  those  which  have  been  deposited  in  water.  It  seems  desirable,  wdierever 
it  is  possible,  to  make  this  discrimination.  To  that  end  I  shall  in  the  fol- 
lowing pages  restrict  the  term  "tuff"  to  eolian  deposits.  The  term  "volcanic 
conglomerate,"  or,  for  the  sake  of  brevity,  simply  "conglomerate,"  will  be 
used  for  those  coarse  deposits  which  have  l^een  sorted  by  and  deposited 


'  Text-book  of  Geology,  by  Sir  Archibald  Cieikic  :  3d  ed.,  p.  135. 


PYROCLASTICS  OF  HEMLOCK  FORMATION.  137 

in   wutor,   and   wlio.se   t'niynientsj   show   a  rounded    character.      Slntuld  the 
fragiueut!:)  be  aiiguhir,  the  rocks  may  be  called  "volcanic  breccias." 

It  has  been  found  practicable  to  maintain  this  distinction  in  earlier 
studies  on  Tertiary  volcanics,^  and  it  is  also  maintained  in  the  ^wesent  study 
of  pre-Cambrian  volcanics.  I  am  confident  the  same  distinction  could  he 
made  more  generally  tlian  it  is,  and  would  in  that  case  tend  to  a  greater 
precision  in  the  separation  of  rocks  of  diiferent  characters.  However,  it  is 
rather  difficult  to  separate  true  eolian  deposits  of  volcanic  fragmentary 
materials  from  those  in  which  tlie  fragments  have  been  deposited  rapidly 
through  water  without  liaAing  eml^edded  organic  remains  and.  without 
having  undergone  sufficient  attrition  to  he  much  rounded.  More  or  less 
rounding,  it  is  well  iinderstood,  results  from  the  attrition  of  the  volcanic 
ejectamenta  during  their  ascent  and  descent  through  the  air,  so  that  they 
may  in  this  respect  resemble  many  of  the  sedimentaries.  The  exact  mode 
of  origin  of  many  of  the  volcanic  fragmental  deposits  of  the  Michigamme 
district  is  not  clear.  The  greater  portion  appear  to  be  of  true  eolian  origin, 
and  where  the  origin  of  any  is  in  doulit  it  has  been  put  with  those  of  eolian 
origin. 

COARSE   TUFFS. 

The  coarse  tuffs  include  rocks  composed  of  fragments  of  all  sizes,  from 
the  large  volcanic  blocks  to  the  fine-grained  particles  of  sand  and  dust 
which  fill  in  the  interstices.  The  ejectamenta  may  be  uKire  or  less  rounded 
by  attrition  during  their  progress  through  the  air,  so  tliat  if  a  refinement  of 
the  nomenclature  should  be  needed  one  might  very  properlv  be  justified  in 
speaking  of  tuft'  breccias  and  tuff  conglomerates. 

Tuffs  are  very  common  and  characteristic  for  the  district.  The  char- 
acters of  the  beds  is  best  shown  on  the  weathered  surfaces.  Here  the 
scoriaceous  and  dense  light-green  fragments  stand  out  well  from  the 
brownish-red  matnx  of  more  altered,  finer  fragments  and  cement.  On  a 
fresh  surface  the  interstitial  material  usually  has  a  darker  green  color  than 
the  fragments.  The  fragments  have  a  prevailing  green  color,  but  many, 
especially  in  sections,  are  brown,  much  darker  than  anv  of  the  rocks 
forming  the  lava  flows.  The  larger  fragments  are  usually  sharply  angular, 
but  in  many  cases  are  more  or  less  rounded  because  of  attrition  durino- 


•  Die  Gesteine  des  Duppauer  Gebirges  in  Nord-Bohmen,  by  J.  Morgan  Clements :  Jahrbucli  K.-k. 
geol.  Relchaanstalt,  Vol.  XL,  1890,  p.  324. 


138  THE  CRYSTAL  FALLS  IRON-BE ARIXG  DISTRICT. 

their  j^rogress  tlu'ough  the  air.  (PI.  XIII.)  They  are  for  the  most  part 
not  .scoriaceous,  though  rather  commonly  amygdaloidal.  The  macroscopi- 
cally  dense  fragments  seem  to  predominate,  though  the  amygdaloidal  ones 
do  occur  in  some  specimens  in  nearly  equal  quantity. 

The  fragments  of  the  tuffs  are  derived  from  the  various  kinds  of  basalt 
already  described  as  forming  the  lava  flows. 

Among  the  fragments  some  of  the  most  typical  of  these  rocks  liave 
been  found,  and  remarkable  as  it  may  seem,  some  of  the  thin  sections  from 
them  show  the  least-altered  basalts. 

In  addition  to  the  kinds  mentioned  under  the  basalts  there  are  a  number 
which  differ  slightly  from  them,  and  ajjparently  represent  more  glassy  modi- 
fications of  the  basalt  magma.  In  one  of  these  the  amygdules  are  more 
sharply  outlined  by  the  accumulation  of  iron  oxide  around  the  edges  of  the 
amygdule  than  is  the  ease  in  the  crystalline  flow  rocks.  An  especially 
well-preserved  fragment  shows  perfectly  fresh  plagioclase  microlites  exhib- 
iting well-developed  fluidal  structure  lying  in  a  dark-brown  apparently 
isotropic  glassy  base.  Where  the  section  is  thin,  globulitic  devitrification 
products  can  be  seen,  and  tliere  also  the  base  no  longer  appears  isotropic, 
but  very  feebly  double  refracting.  There  is  very  frequently  found  among 
these  tuffs  amygdaloidal  fragments  which  appear  to  have  been  derived  from 
what  was  originally  a  completely  glassy  rock,  no  indication  of  the  presence 
of  any  original  crystals  having  been  preserved.  The  background  of  these 
fragments  consists  of  a  fine  felt  of  a  green  chloritic  mineral,  dotted  with 
innumerable  grains  of  epidote,  in  which  one  may  distinctly  discern  concen- 
tric circles  and  arcs  of  circles  outlined  liy  aggi'egates  of  epidote  grains. 
These  circles  probably  rejjresent  perlitic  partings.  (Fig.  A,  PI.  XXXII.) 
The  dark-ljrown  fragments  mentioned  as  occurring  with  the  prevailing  green 
ones  are  very  dense,  appear  to  be  very  rich  in  iron,  and  may  possibly 
represent  a  very  basic  devitrified  glass.  Should  accumulations  composed 
essentialh'  of  such  glassy  fragments  be  found,  they  could  j^roperly  be 
called  "palagonite  tufts." 

In  addition  to  the  rock  fragments,  a  few  rare  ones  of  large  plagioclase 
crystals  were  found,  and  also  in  one  case  a  fragment  of  a  violet-brown 
augite,  the  only  specimen  of  fresh  pyroxene  thus  far  found  in  any  of  the 
volcanics. 

The  tuffs  show  in  places  fairly  well-developed  banding,  caused  by  the 


PLATE   XIII. 


139 


PLATE    XIII. 

(Sp.  No.  23644.     Natural  size.) 

This  illustration  is  a  faitliful  representation  of  the  appearance  of  the  polished  surface  of  a 
pyroclastic  from  the  Hemlock  formation.  It  is  somewhat  doubtful  whether  or  not  the  fragments 
composing  the  rock  have  been  deposited  through  the  mediation  of  water  or  air  alone.  The  larger 
fragments  are  rather  dense.  Vesicular  fragments  are  more  common  among  the  smaller  particles. 
Pyroclastics  similar  in  appearance  to  this  are  of  very  common  occurrence  in  the  Crystal  Falls  district, 
and  huge  clift's  of  it  are  readily  accessible  from  the  railroad. 
140 


U  S  GEOLOGICAL  SURVEY 


MONOGRAPH    XXXVI  PL    XIII 


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I'YUOOLASTICS  OF  HEMLOCK  FORMATION.  141 

iiitoi-bcHl(liii<i'  of  layeris  in  which  course   and  liiicr  tVayinent.s   prevail,  illus- 
trating well  the  varying  intensity  of  the  volcanic  discharges. 

Owing  to  the  fragmental  nature  of  the  exposures,  it  is  impossible  to  get 
a  correct  idea  of  the  maximum  thickness  of  any  of  the  tuflf  deposits. 
Exposures  were  seen  in  the  north  half  of  sec.  5,  T.  43  N.,  R.  32  W.,  which 
o-ave  a  thickness  of  over  500  feet  for  some  of  these  deposits,  but  as  their 
fiirther  continuation  had  l:)een  cut  otf  by  valleys,  most  probably  eroded  in 
the  tuffs,  no  means  was  afforded  of  determining  their  total  thickness. 

It  is  almost  needless  to  state  that  the  most  of  the  tuffs  have  undergone 
a  great  amount  of  alteration.  The  alterations  were  apparently  due  to  an 
interchange  of  the  various  elements  without  any  essential  variation  in  the 
chemical  nature  of  the  rock  as  a  whole.  Since  water  is  the  chief  agent 
through  which  alterations  occur,  these  always  begin  along  the  interstices. 
In  the  case  of  the  fragments  the  alteration  accordingly  proceeds  from  the 
outside  inward,  and  ordinarily  at  an  equal  rate  all  around  the  fragments, 
following  its  contours.  In  this  way  zones  of  somewhat  different  mineralogical 
composition  are  formed,  surrounding  the  less  altered  part  of  the  fragment. 
This  secondary  zonal  structure  may  be  observed  more  or  less  imperfectly 
in  almost  all  of  the  sections  made  from  the  Ijreccias,  but  is  much  better 
shown  in  the  field,  where  the  concentric  zones  are  well  brought  out  on  the 
larsre  weathered  surfaces  of  the  bowlders. 

In  each  case  the  outside,  hghter-colored  zone  is  chiefly  made  up  of 
chlorite,  from  which  project  light-green  hornblende  needles  into  the  matrix 
beyond.  Less  commonly  we  find  it  composed  of  epidote  grains  and  chlo- 
rite. Inside  of  this  zone  the  mineral  elements  composing  the  fragments 
sometimes  can  not  be  determined  with  any  great  degree  of  certainty. 
Where  determinable,  the  alteration  products  are  found  to  be  the  same  as  are 
produced  from  the  corresponding  rocks  in  the  lava  flows.  As  also  in  the 
lava  flows,  some  of  the  fragments  of  the  denser  rocks  have  become  almost 
opaque  from  the  quantity  of  minute  secondary  epidote  and  sphene  grains. 
These  have  a  lighter  green  color  than  the  less  altered  fragments. 

In  examining  many  of  the  tuffs,  one  is  repeatedly  struck  by  the  large 
amount  of  space  occupied  by  the  cementing  material.  In  some  cases  cavi- 
ties of  very  considerable  size  were  left  between  the  fragments.  It  appears 
that  the  fragments  must  have  been  lying  very  loosely.  This  fact  tends  to 
confirm  the  eolian  origin  of  the  rocks,  since  ^vater  deposition  tends  to  bring 


142  THE  CEYSTAL  FALLS  IRON-BEARING  DISTRICT. 

the  fragments  in  close  contact,  and  also  to  fill  the  intervening  spaces  with 
fine  detritus.  Those  cases  must  of  course  be  excepted  where  the  material 
fell  upon  water  so  deep  that  after  sinking  to  the  bottom  the  action  of  the 
waves  was  not  felt.  Under  such  circumstances  one  can  imagine  the  blocks, 
being  partly  supported  by  the  water,  coming  to  rest  in  a  more  unstable 
position  than  they  would  in  the  air. 

The  cement  diff'ers  in  diff'erent  specimens.  The  minerals  constituting 
it  are  quartz,  feldspar,  calcite,  chlorite,  epidote,  and  hornblende.  The 
minerals  are  found  including  one  another  in  such  a  way  as  to  make  it  prob- 
able that  they  usually  formed  simultaneously.  The  calcite  is  an  exception, 
as  it  is  usually  pi-esent  in  greater  quantity  near  the  surface  of  the  exposures, 
and  is  therefore  a  Aveathering-  product.  It  was  noted  above  that  the  horn- 
blende and  chlorite  frequently  extend  from  the  rock  fragments  into  the 
clearer  elements  composing  the  cement.  Hornblende  needles  in  many 
cases  constitute  a  large  part  of  the  cement.  Where  two  fragments  are  very 
close  together,  a  perfect  network  of  needles  may  extend  from  the  one  across 
the  intervening  space  and  penetrate  the  other,  and  the  fragments  thus  prac- 
tically grow  together.  The  cavities — especially  the  large  ones  mentioned 
above — have  quite  frequently  been  filled  with  concentric  growths  of  vari- 
ous minerals.  In  general,  chlorite  seems  to  be  the  first  mineral  deposited, 
and  quartz  the  last,  but  in  weathered  specimens  calcite  is  the  last. 

FINE   TUFFS    OR   ASH    (DUST)    BEDS. 

The  fine  tuffs  or  "ash"  beds  occur  plentifully  in  the  Crystal  Falls  district. 
In  many  cases  they  ^^ossess  a  very  well  developed  cleavage,  and  were  very 
puzzling  in  the  field  on  account  of  their  striking  resemblance  to  true  sedi- 
mentary slates.  They  are  of  interest  as  emphasizing  the  resemblance 
between  pre-Cambrian  ejectamenta  and  Tertiary  and  Recent  ones.  In  one 
respect  they  differ  from  the  modern  forms.  The  dust  from  Krakatao  in 
1885  and  from  other  volcanic  explosions  consists  mainly  of  fragments  of 
minerals  and  glass.  The  constituents  of  the  Crystal  Falls  beds  are  usually 
fine  lava  and  glass  fragments  and  less  commonly  minerals. 

The  rock  fragments  are  angular,  vesicular,  and  completely  altered. 
The  glass  fragments  are  likewise  angular,  and  have  the  characteristic  curved 
shapes  from  which  they  are  usually  described  as  sickle-shaped  bodies.  (Fig. 
B,  PI.  XXXII.)     Such  are  formed  when  a  pumice  is  broken  up,  and  each 


PYROCLASTICS  OF  HEMLOUlv  FOHMATION.  143 

represents  a  portion  of  tlic  jilass  Ixmudino-  tlie  vesicles.  Here  and  there  is 
a  fragment  with  a  more  or  less  perfect  vesicle  remaining.  The  few  mineral 
fragnu'Uts  fouml — feldspar — were  angular,  Init  (piite  fresh.  The  rocks  sliow 
no  intermixture  of  i-ounded  tragments,  and  they  are  consequently  regarded 
as  volcanic  dust  dei)osited  through  the  air.  These  ash  beds  show  a  delicate 
banding  of  iiner  and  coarser-grained  fragments.  In  a  single  slide  a  grada- 
tion can  be  traced  from  a  moderately  fine  grained  sand  composed  of  distinct 
volcanic  fragments  into  a  very  fine  grained  mass  composed  of  hornblende 
needles,  biotite,  chlorite,  epidote,  and  sphene,  cemented  by  what  is  prob- 
ably quartz,  perhaps  having  associated  with  it  some  feldspar  whose  charac- 
ters could  not  be  determined. 

Relations  of  tuffs  and  ash  (dust)  beds. — Tlic  pyroclastics  sceui  to  predominate  in  the 
northwestern  part  of  the  district  in  the  neighborhood  of  the  small  town  of 
Amasa.  Special  opportunities  for  observing  the  relations  between  the  tuff 
and  the  ash  beds  are  offered  l)y  the  third  cut  of  the  Chicago,  Milwaukee 
and  St.  Paul  Railway  west  of  Balsam,  Michigan.  Gradation  can  be  traced 
from  coarse  tuffs  to  delicately  banded  fine  tuffs.  The  average  thickness  of  a 
single  ash  bed  probably  does  not  exceed  5  feet.  In  the  same  exposures  the 
tuft"  beds  are  from  50  to  100  feet  thick,  and  even  more. 

VOLCANIC   CONGLOMERATES    (TUFFOUENE    SEDIMENTS,  REVER). 

That  certain  of  the  pyroclastics  have  been  brought  together  and 
rearranged  by  the  agency  of  water  is  made  clear  by  their  characteristic 
structure.  Such  rocks  are  the  volcanic  conglomerates.  In  very  jnany 
respects  they  are  strikingly  like  the  various  eolian  deposits,  tuffs,  etc., 
described  above.  They  agree  with  them  in  color.  The  same  varieties  of 
volcanic  rocks  are  represented  that  are  found  in  the  tufts.  They  are  true 
basalt  conglomei'ates. 

The  pebbles  are  ver}-  commonly  sharply  outlined  by  accumulations 
of  epidote  grains  on  the  periphery.  Some  of  the  fragments  have  a  reddish- 
brown  to  purplish-black  color,  and  stand  out  strongly  from  the  green 
matrix.  Such  pebbles  are  found  to  contain  large  quantities  of  magnetite, 
the  oxide  being  in  beautiful  sharp  crystals  and  in  absolutely  fresh  condi- 
tion, forming  a  sharp  contrast  to  the  altered  condition  of  the  fragments  in 
which  it  occurs.  In  one  case  in  which  the  main  mass  of  the  frag-ments 
now  consists  of  chloi'ite  and  epidote,  magnetite  occurs  in  large  quantity, 


144  THE  CEYSTAL  FALLS  IltON-BEARING  DISTRICT. 

and  in  chains  of  crystals  forming-  dendritic  growths.  The  oxide  is  clearly 
secondary  in  these  altered  rocks.  Since  it  also  occurs  secondarily  in  the 
cement,  it  appears  highly  probable  that  it  is  an  infiltration  product  formed 
during  or  after  the  metasomatic  process. 

In  these  conglomerates  feldspar  fragments  are  far  more  common  than 
they  are  in  the  titffs,  and  they  show  a  well-defined,  round,  waterworn  charac- 
ter. (Fig.  A,  PI.  XXXIII.)  Likewise  masses  of  uralite  are  commonly 
associated  with  the  rounded  feldspar.  The  uralite  is  taken  to  be  altered 
Ijyroxene  fragments,  though  no  proof  of  this  beyond  its  association  could  be 
offered,  as  the  fragments  show  no  characteristic  pyroxene  outlines.  The 
well-rounded  nature  of  the  volcanic  pebbles  makes  it  certain  that  they 
have  been  deposited  through  the  mediation  of  water  and  enables  one  easily 
to  distinguish  the  typical  examples  in  the  field. 

In  size  the  fragments  diflPer  froin  one  another  just  as  they  do  in  the 
case  of  the  eolian  deposits  (fig.  B,  PI.  XXXIII).  Many  of  the  largest 
are  several  feet  in  diameter,  but  more  commonly  they  vary  from  a  couple 
of  feet  in  diameter  to  small  pebbles.  Partly  filling  the  interspaces  and 
aiding  in  cementing  the  larger  fragments,  witli  which  they  are  associated, 
are  very  fine  grained  fragments  derived  from  the  trituration  of  the  water- 
worn  lapilli  and  blocks.  The  coarse  bowlder  conglomerates  grade  through 
finer  conglomerates  into  very  fine  material.  This  fine  material  shows  beau- 
tifully marked  false  bedding.     (Fig.  C,  PI.  XXVII.) 

In  one  of  the  finer-grained  rocks,  in  addition  to  the  usual  sedimentary 
banding,  there  are  bands  which  appear  to  have  been  caused  by  a  further 
sorting  of  the  materials,  some  of  these  bands  being  composed  almost  exclu- 
sively of  uralite.  They  consequently  represent  bauds  which  were  originally 
composed  mainly  of  pyroxene  fragments.  In  this  case,  when  the  fine  ejecta- 
menta  settled  through  the  water  they  were  separated  according  to  size  of 
gi'ain  and  specific  gi'avity,  as  in  ore-dressing  processes. 

Under  the  microscope  other  points  of  difference  in  addition  to  those 
above  mentioned  are  noted  between  the  conglomerates  and  the  tuffs  or 
eolian  deposits.  The  fine-grained  rocks  corresponding  nearly  to  the  vol- 
canic sand,  do  not  consist  of  distinguishable  rock  fragments,  but  of  clearly 
rounded  feldspar  grains  which  have  been  enlarged  by  peripheral  additions 
of  feldspar  substance,  bunches  of  uralite,  some  chlorite,  and  of  sphene 
secondary  after  titanic  iron.     The  photomicrograph.  Fig.  A,  PI.  XXXIII, 


PYKOCLASTICS  OF  HEMLOCK  FORMATION.  145 

illustrates  the  appearauee  of  the  thin  .section  of  such  consolidated  sand, 
which  possesses  in  no  place  the  structure  of  an  igneous  rock,  and  which, 
moreover,  grades  into  a  finely  banded  rock  composed  of  minute  needles  of 
hornblende,  chlorite,  and  grains  of  epidote,  lying"  in  a  clear  minutely  crys- 
talline groundmass  of  quartz  or  of  feldspar,  or  possibly  of  both. 

The  tiner  material  cementing  the  recognizable  fragments  is  the  same  as 
it  is  in  the  tutfs.  The  large  bowlders  and  pebbles  lie  in  a  matrix  of  smaller 
pebbles,  and  these  in  turn  lie  close  together  in  a  paste  which  has  been  com- 
pletely altered,  and  does  not  in  all  cases  show  clastic  characters.  The 
cement  is  composed  of  hornblende,  chlorite,  sericite,  epidote,  feldspar,  and 
quartz,  and  in  one  case  large  porphyritic  rhombs  of  a  ferruginous  carbonate 
are  scattered  through  the  finer-grained  material  of  the  cement. 

In  the  cement  of  the  tuffs  hornblende  is  present  in  large  quantity  and 
feldspar  is  not  so  abundant.  In  the  cement  of  the  conglomerates  feldspar 
seems  to  be  rather  plentiful,  hornblende  is  present  in  a  comparatively  small 
quantity,  and  chlorite  is  more  abundant,  thus  reversing  the  order  of  these 
minerals  in  the  conglomerates. 

SCHISTOSE   PYROCLASTICS. 

At  various  places  in  the  Hemlock  formation  there  occur  clastic  rocks 
w^hich  have  become  schistose.  Two  isolated  exposures  of  pyroclastics  are 
known  whose  characteristics  have  been  so  changed  that,  while  recognizable 
as  elastics,  it  is  impossible  to  say  whether  they  belong  to  the  eolian  or 
the  water-deposited  class.  Upon  weathered  surface  the  rock  is  covered 
with  brownish  ochre,  and  on  fresh  fracture  it  is  dark  green  and  very  schis- 
tose. Neither  in  exposui'es  nor  in  hand  specimens  does  it  give  any  indication 
of  its  origin. 

In  thin  section,  however,  one  may  see  macroscopically  the  fragmental 
characters.  The  fragments  are  elongated  and  rounded.  The  amygdaloidal 
texture  is  also  seen,  showing  the  volcanic  nature  of  the  fragments,  though 
the  majority  of  the  fragments  are  dense.  Under  the  microscope  the  fi-ag- 
mental  nature  of  the  rocks  as  a  whole  and  the  volcanic  character  of  the 
fragments  forming  it  are  still  more  clearly  seen.  In  the  centers  the  frag- 
ments are  seen  to  be  composed  of  chlorite  flakes  in  such  great  quantity  as 
partly  to  conceal  the  character  of  the  clear  white  cement,  which  is  supposed 
to  consist  for  the  most  part  of  quartz,  though  lath-shaped  areas  with  poly- 

MON   XXXVI 10 


146  THE  CRYSTAL  FALLS  IRON-BEARIXG  DISTRICT. 

synthetic  twinning,  showing  the  presence  of  plagioclastic  feldspar,  were 
seen  in  places  on  the  edge  of  the  section.  In  the  chlorite  and  quartz  occur 
large  grains  of  fresh  titaniferous  iron  ore,  altering  on  edge  to  spheue,  and, 
most  striking  of  all,  large  porphyritic,  beautifully  automorphic  calcite  rhombs 
and  muscovite  plates  in  isolated  individuals  as  well  as  in  heaps  of  crystals. 
The  carbonate,  which  predominates,  effervesces  readily  with  cold  HCl,  but 
is  evidently  ferruginous  as  it  is  yellowish  when  altered,  and  from  it  results 
some  of  the  ochre  which  colors  the  weathered  surface  of  the  rock.  In  other 
sections  the  calcite  phenocrysts  are  scalenohedra,  with  very  few  rhombo- 
liedra.  The  terminal  faces  are  not  sharply  defined.  The  sections  resulting 
from  the  scalenohedra  are  long,  lath-shaped,  and  have  pointed  or  irregular 
ends,  ])arallel  extinction,  and  oblique  cleavage. 

In  passing  from  the  centers  toward  the  edges  of  the  fragments,  -we  note 
a  marked  diminution  in  the  amount  of  carbonate,  muscovite,  chlorite,  and 
iron  oxide,  causing  a  consequent  lightening  in  color  of  the  periphery.  This 
gives  the  zonal  structure  noticeable  ujjon  the  macroscopical  examination  of 
the  thin  section.  The  schistose  character  of  the  fragments  is  caused  by  the 
parallelism  of  the  chlorite  flakes. 

The  cement  between  the  fragments  is  composed  of  quartz  in  rather 
coarse  grains,  chlorite  in  larger  flakes  than  in  the  fragments,  and  carbonate 
in  large  porphyritic  crystals,  and  also  in  minute  rhombs  included  in  the 
quartz  grains.  Another  phase  contains  considerable  secondary  plagioclastic 
feldspar  associated  with  the  quartz  in  the  coarser-grained  portion  of  the 
cement.  .  As  in  some  of  the  conglomerates  and  tuffs,  the  fragments  are 
observed  lying  in  juxtapo.sition,  the  only  cement  between  them  being  the 
secondary  interpenetrating  nainerals.  In  some  cases  the  edges  of  the  frag- 
ments have  been  so  welded  that  one  may  pass  from  one  pebble  to  the 
adjoining  one  across  an  intervening  lighter  zone  without  detecting  the 
transition,  unless  changes  in  the  amount  of  chlorite,  iron  oxide,  and  carbonate 
are  noticed. 

Nothing  thus  far  mentioned  would  indicate  the  igneous  origin  of  the 
fragments,  but  that  is  indisputably  proven  by  the  amygdaloidal  texture  of 
the  specimens,  than  which  I  have  seen  none  better,  even  in  the  freshest 
yolcanics.  The  outline  of  the  cavities  is  marked  by  an  accumulation  of 
grains  of  iron  oxide,  and  the  cavities  themselves  are  filled  by  fine-grained 
quartz  having  a  small  amount  of  chlorite  associated  with  it.     (Figs.  A  and  B, 


PYKOCLASTICS  OF  HEMLOCK   FORMATION.  147 

1*1.  XXX.)  'riic'se  specinu'iis  lia\c  the  characters  of  tin-  porpiiyritic  schis- 
tose lavas  described  above,  l)ut  show  clearly  tliat  they  liave  been  derived 
t'roiii  igneous  clastic  rocks. 

Other  .schistose  elastics  occur  in  lar<,fe  quantities  in  sec.  24,  '\\  4(1  X.. 
R.  33  W.  They  are  penetrated  by  a  boss  of"  coarse  jxiikilitic  dolerite 
wiiich  nvdv  have  aided  in  rendering"  them  schistose,  though  their  schistosity 
a"Tees  with  the  general  strike  of  that  of  the  rest  of  tlie  district,  and  is 
probaljl}'  chiefly  due  to  the  general  folding. 

Macroscopically  their  clastic  structure  niav  be  (dearly  seen.  The  peb- 
bles iire  dense  greenish  gi'ay  in  color  and  oval  in  outline.  The  matrix  is  a 
much  darker  green.  The  schistosit}'  of  the  rock  is  marked.  The  ])ebbles 
are  tmifoi-mly  elongated,  and  they  have  the  appearance  of  having  been 
mashed.  The  schistosity  agrees  in  direction  with  the  elongation  of  the 
pebl)les. 

The  microscope  shows  the  pebbles  to  be  basaltic,  with  a  type  of  stiaic- 
ture  intermediate  between  the  navitic  and  iiitersertal  structures,  and  another 
with  approach  to  the  trachytic  structure.  Considerable  brown  mica  is 
present  in  both  kinds,  and  occurs  in  flakes  which  are  probably  secondary, 
though  the  pebbles  sho\\'  few  traces  of  alteration.  The  matrix  consists 
essentially  of  actinolite  in  rather  coarse  needles,  large  grains  of  fresh 
magnetite,  but  ver}'  little  mica,  and  that  such  as  is  seen  in  the  pebbles,, 
all  lying  in  a  cement  of  quartz  and  calcite.  The  passage  between  the 
cement  and  the  pebbles  is  a  more  or  less  gradual  one,  there  being  a  change 
as  we  pass  from  the  center  of  the  pebble,  where  isolated  actinolite  needles 
and  epidote  grains  occur,  toward  the  edge,  where  these  minerals  increase  in 
amount  until  between  them  here  and  there  are  twinned  feldspars.  In  tlie 
matrix  projier  the  quartz  is  the  predominant  white  silicate,  though  here  and 
there  limpid  feldspar  is  also  seen.  The  pebbles  are  gradually  being  eaten 
U}),  so  to  speak,  by  the  actinolite,  and  we  can  imagine  the  final  result  to  be 
an  actinolite-schist  showing  no  clastic  structure,  and  giving  aljsolutely  no 
indication  as  to  the  rock  from  which  it  originated. 

There  is  no  microscopical  evidence  of  mashing  in  the  minerals,  and 
since  this  is  absent  from  the  quartz  in  the  cement,  I  conclude  that  no  original 
clastic  cement  is  now  present,  and  that  the  quartii  is  a  secondary  crystalli- 
zation product  derived  from  infiltrated  material,  and  from  material  obtained 
from  the  adjacent  pebbles.     Whether  the  rock  is  an  eolian  deposit  or  a 


148  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

watei'woru  sediment  can  not  now  he  definitely  detei-mined,  thougli  that  it 
belongs  to  the  first  appears  more  probable. 

Still  another  rock  very  similar  in  general  character  but  differing  in 
detail,  and  showing  a  slightly  different  result,  has  been  examined.  The 
pebbles  are  basalt,  and  in  them  the  secondary  nature  of  the  biotite,  which 
lias  chlorite  ass(iciated  with  it,  is  clearly  shown.  Near  the  centers  of  the 
pebbles  very  little  is  present,  but  it  rapidly  increases  in  amount  toward  the 
l)eriphery,  until  at  the  edge  only  here  and  there  the  feldspars  may  be  seen 
between  the  mica  and  chlorite  flakes.  The  cement  between  the  pebbles 
consists  of  angular  fragments  of  altered  orthoclase  feldspar,  quartz,  and  a 
great  quantity  of  chlorite,  and  some  biotite.  This  cement  is  present  in 
large  (juantity. 

In  both  of  these  last  the  secondary  minerals  are  parallel,  and  produce 
rocks  of  most  decided  schistose  character.  These  schistose  pja-oclastics 
may  be  compared  with  the  rocks  described  by  Williams^  andBayley'  from 
the  related  rocks  in  the  adjoining  Marquette  district. 

THE  BONE  liAKE  CRYSTALUNE  SCHISTS. 

Under  this  name  are  included  certain  crystalline  schists  which  are  best 
developed  in  the  northern  part  of  the  Crystal  Falls  district,  in  the  vicinity 
of  Bone  Lake.  If  one  examined  isolated  specimens  of  certain  of  these 
rocks,  it  would  be  impossible  to  determine  their  origin.  Studied  in  connec- 
tion with  the  alteration  of  the  altered  and  schistose  lavas  and  pyroclastics 
already  described,  the  problem  becomes  greatly  simplified.  These 
schists,  as  will  be  shown  on  the  following  pages,  are  but  extremely  meta- 
morphosed members  of  the  Hemlock  volcanic  formation.  Since  in  the 
limited  area  in  which  the  rocks  occur  the  secondary  characters  are  domi- 
nant, while  the  primary  volcanic  characters  have  nearly  all  disappeared,  a 
brief  separate  description  of  these  rocks  seems  wan-anted,  but  they  are  not 
represented  by  a  separate  symbol  on  the  map. 

DISTRIBUTION. 

The  crystalline  schists  predominate  in  T.  46  N.,  R.  32  W.  Near  the 
western  limit  of  this  township  the  belt  occupied  by  these  rocks  is  about  2 
miles  wide.     As  it  is  followed  to  the  east  past  Bone  Lake,  and  then  to  the 

'Bull.  U.  S.  Geol.  Survey,  No.  62,  cit.,  pp.  185-191. 
•Mon.  U.  S.  Geol.  Survey,  Vol.  XXVIII,  cit.,  pp.  160-169. 


JJONE   LAKE  (JitVSTALLl>'E  SCHISTS.  149 

soiitlR-iist,  it  jiTiulually  iijirniws,  until  iu  sec.  3G,  'V.  4(i  X.,  li.  32  W.,  the 
eusteru  limit  of  the  Jirca  studied  hy  lue,  it  is  only  about  half  a  mile  wide. 
E.xt-ept  iu  the  vicinity  of  Bone  Lake,  where  erosion  has  uncovered  some 
of  the  knol)s,  outcrojis  ar(>  ^^erN'  scarce,  since  the  drift  is  very  lieaNA', 
and  the  drainage  is  poorl\'  develo})ed. 

FIELD    EVIDENCE   OF   CONNECTION   WITH   THE  VOLCANICS. 

If  one  examines  attentive^'  the  Hendock  formation  in  its  typical 
dcvidopnient,  1  )eginning,  say,  in  sec.  27,  T.  45  N.,  R.  33  Vs.,  and  f(dlowiiig 
its  northw;n-d  exteusion  through  sees.  22,  1(!,  and  15  of  the  same  township, 
he  will  observe  instances  of  banding  in  the  tuffs  and  of  schistositv  in  the 
amygdaloidal  lavas  and  pyroclastics.  The  strikes  and  dips  of  the  jjrimarv 
and  secondary  structures  approximately  coincide,  both  having  a  general 
north-south  strike  and  dipping  high  to  the  west.  Tlii-oughout  this  area, 
h()wever,  the  unmistakable  massive  volcanics  are  the  predominant  rocks. 
Continuing  the  examination  farther  nc^rth  into  sec.  34,  T.  46  X.,  R.  33  W., 
rocks  are  found  which  possess  almost  invariablv  a  strongly  marked  schis- 
tositv, but  with  their  volcanic  origin  clearly  shown  by  the  flattened  amyg- 
dules.  This  is  also  true  for  the  exposures  east  of  this  place  on  the  under 
side  of  the  Hemlock  belt,  in  sec.  31,  T.  46  N.,  R.  32  W.  The  strike  of  the 
schistosity  of  the  amygdaloids  varies  from  N.  30°  to  70'^  E.,  and  the  dip  is 
high  to  the  northwest.  Farther  along  this  belt  to  the  northeast,  in  sec.  24, 
T.  46  N.,  R.  33  W.,  schistose  pyroclastics  were  observed  striking  N.  80°  E. 
The  original  characters  of  these  pyroclastics  liave  been  almost  entirely 
obliterated.  The  exposures  next  to  the  east  in  sec.  16,  T.  46  N.,  R.  32  W., 
possess  all  the  characters  of  crystalline  schists.  Somewhat  farther  east, 
however,  associated  with  these  schists  are  isolated  outcrops  in  which  traces 
oi  flow  structure  and  remnants  of  amygdides  were  observed,  and,  in  some, 
traces  of  igneous  textures  were  seen  under  the  microscope.  The  schistosity 
of  these  rocks  strikes  for  the  most  part  south  of  east,  varying  from  N.  65° 
to  80°  W.  and  dipping  to  the  nortJieast.  Following  the  belt  as  it  now  turns 
to  the  southeast,  the  crystalline  schist  characters  prevail,  the  volcanic  char- 
acters being  obliterated.  The  schistosity  at  the  same  time  bends  farther 
around  to  the  southeast,  pointing  toward  the  continuation  of  this  area  of 
volcanics  to  the  southeast,  outside  of  the  area  studied. 

From   field   observations   the    conclusion   seems   iiecessarv  that   these 


150  THE  CRYSTAL  FALLS  IROX-BEARmU  DISTRICT. 

.schists  are  metamorphosed  volcanic  rocks,  and  this  conckision  is  strength- 
eued  l)y  detailed  petrog'raphical  examination. 

PETROGRAPHICAL  CHARACTERS. 

■  The  crystalline  schists  are  tine  to  medium  grained  schistose  rocks 
which  vary  in  C(ilor  from  a  moderately  light  green  for  the  more  chloritic 
phases  to  a  ^ery  dark  green  and  purplish-l^lack  for  those  in  which  the 
hornl)lende,  mica,  and  iron  ores  are  prominent.  The  minerals  of  which 
the  rocks  are  composed,  arranged  in  order  of  importance,  are  hornblende, 
biotite,  feldspar,  chlorite,  epidote  muscovite,  quartz,  magnetite,  hematite, 
ilmenite,  and  rutile.  Under  the  microscojje  the  schistose  structure  is  seen 
to  be  produced  by  the  general  parallelism  of  the  liisilicate  constituents. 
The  por])hyritic  texture  is  seen  in  a  few  specimens,  and  hornblende  forms 
the  phenocrysts. 

Hori:l)lende  occurs  in  iine  needles  and  also  in  coarse  crystals  which 
are  automorpliic  in  the  prismatic  zone,  but  on  which  no  terminations  have 
been  observed.  It  also  occurs  rather  comm(inly  in  sheaf-like  bundles  of 
ragged  crystals.  The  marked  orthopiuacoidal  development  so  common  for 
actinolite  is  quite  noticeable.  The  crystals  show  the  usual  strong  pleoch- 
roism:  c  =  bluish-green,  lj  =  olive  green,  it r=  yellow,  whereby  i:>>lj>a. 
The  hornblende  crystals  frequently  contain  large  fjuantities  of  the  minerals 
of  the  groundmass,  many  of  them  iu  such  quantity  that  there  are  really 
only  skeleton  hornblende  crystals  present.  The  general  character  of  the 
hornblende  in  all  these  rocks  is  that  of  a  secondary  porphyntic  constituent, 
and  seems  to  be  analog'ous  to  such  minerals  as  garnet,  staurolite,  etc.,  which 
are  produced  in  clearly  metamorphic  rocks. 

Brown  biotite  is  rather  common  in  some  of  the  rocks.  Though  usually 
subordinate  to  the  hornblende,  it  is  at  times  the  predominate  bisilicate.  It 
is  light  brown  and  shows  the  usual  characters  of  biotite.  It  is  present  in 
small  irregular  flakes,  and  also  in  larger  individuals  which  show  poor 
pinacoidal  development.  In  one  case  such  a  mica  individual  in  perfectly 
fresh  condition  may  be  seen  with  its  ragged  edges  interlocking  with  the 
fringed  periphery  of  an  altering  feldspar  crystal.  The  liiotite  appears  to 
have  derived  some  of  its  necessary  elements  from  the  feldspar  and  to  be 
eating  int(i  it,  and  consequently  to  be  a  secondary  product. 

Feldspar  is  not  found  as  an  original  mineral  in  any  of  the  crystalline 


BONE  LAKE  CRYSTALLINE  SCHISTS.  151 

schists.  It  occurs  as  a  socond.ary  constituent.  It  is  found,  however,  as  a 
primary  constituent  in  a  few  rocks  which,  as  they  still  possess  remnants  of 
orifi'inal  igneous  textures,  strictly  speaking,  should  not  perhaps  be  included 
with  the  crystalline  schists.  They  represent  more  properly  the  transition 
stao-es  to  the  cr^'staHine  schists,  but  the  process  of  the  alteration  of  the 
feldspar  is  so  well  shown  in  these  that  it  is  considered  expedient  to  mention 
it  at  this  place.  The  original  feldspar  occurs  in  this  transition  phase  in 
the  large  tangled  intergrowths  connnonly  seen  in  andesitic  and  basaltic 
rocks,  as  individual  phenocrysts,  and  as  microlitic  lath-shaped  individuals 
in  the  groundmass.  The  greatest  interest  centers  in  the  phenocrysts,  as  in 
them  the  changes  which  take  place  are  more  clearly  seen.  The  feldspar 
phenocrysts  ai-e  always  cloudy,  due  to  numberless  black  ferruginous  inclu- 
sions. They  also  inclose  the  various  secondary  dark  silicates  composing 
the  gTOundmass,  grains  of  epidote,  ilakes  of  biotite,  and  crystals  of  horn- 
blende. These  are  usually  surrounded  by  very  narrow  clear  zones,  appar- 
ently feldspar.  Near  the  edges  of  such  altered  crystals,  and  especially  in 
the  more  altered  individuals,  these  inclusions  ai-e  more  numerous,  and  are 
accompanied  by  grains  of  quartz  and  new  feldspar  (albite  !).  These  last 
two  have  certainly  been  derived  from  the  alteration  of  the  feldspar,  but 
that  mineral  may  possibl}^  also  have  contributed  something  to  the  produc- 
tion of  the  dark  silicates. 

The  secondary  feldspar,  that  of  the  schists  proper,  is  found  in  grains 
usually  imstriated,  though  in  a  few  cases  striations  were  observed.  This 
feldspar  was  not  determined,  but  is  probably  albite  The  chlorite  is  in  flakes 
scattered  through  the  schists,  showing  the  usual  characters. 

Epidote,  muscovite,  quartz,  and  rutile  appear  as  usual. 

Ilmenite  is  present  in  one  case  as  micaceous  titanic  iron  oxide,  and  is 
then  in  extremely  thin  plates  which  show  a  beautiful  hexagonal  develop- 
ment, though  more  frequently  the  plates  are  rounded.  They  are  transparent 
with  the  characteristic  clove-brown  color.  The  thicker  plates  are  thin 
enough  to  be  transparent  only  along  the  edges. 

The  iron  oxides,  magnetite,  and  hematite  occur  in  some  of  these  rocks 
in  large  quantity.  In  certain  pai'ts  of  the  area  underlain  by  these  schists 
considerable  excavations  have  been  made  in  search  of  iron,  the  presence 
of  which  was  indicated  by  the  magnetic  needle,  and  moderately  large 
bodies  of  ore  have  been  found,  though  in  no  case  in  sufficient  quantity  to 


152  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

admit  of  successful  mining.  Such  ore  bodies  probably  owe  their  presence 
in  great  part  to  processes  active  subsequent  to  the  formation  of  the  schists. 
(See  p.  134.) 

According  to  the  quantity  and  association  of  the  minerals  described 
above  as  occurring-  in  tlie  schists,  the  following-  rocks  mav  result  from 
the  complete  metamorphism  of  tlie  basic  volcanics :  Amphibolites,  chlorite- 
schists,  epidote-schists,  mica-schists,  mica-gneisses,  and  possibly  siliceous 
hematite  and  magnetite  ore.  The  complete  metamorphism  of  dense  basic 
lava  flows  into  crystalline  schists  has  been  described  by  Williams^  for  the 
Menominee  and  Marquette  districts,  and  also  by  Van  Hise  and  Bay  ley  ^  for 
the  Marquette  district.  Williams^  has  also  described  the  production  of 
schists  from  the  igneous  elastics  in  the  Menominee  district  and  similar 
products  have  been  described  from  the  Marquette  di.strict  both  by  Williams* 
and  by  Bayley.^ 

The  above-described  schists  cover  a  considerable  area,  with  only  iso- 
lated exposures  of  rocks  associated  with  them  in  which  volcanic  characters 
are  recognizable.  They  are  confidently  believed  to  represent  extremely 
metamorphosed  volcanics  of  the  same  general  original  character  as  those 
constituting  the  Hemlock  formation  and  belonging  to  the  same  relative 
period  of  extrusion. 

The  same  conclusions  have  been  reached  by  Smyth  for  similar  schists 
along  the  Fence  River  to  the  southeast  of  those  described.  It  is  noticeable 
that  the  most  intense  metamorphism  of  the  volcanics  has  taken  place  in  the 
northern  and  northeastern  ^^fii't  of  the  Crystal  Falls  district,  that  part  in 
which  the  crystalline  schists  have  been  produced,  though  the  explanation 
for  this  can  not  be  offered. 

NORMAL  SEDIMENTARIES   OF  THE   HEMLOCK  FORMATION. 

The  normal  sedimentaries  are  in  small  quantity.  It  has  been  seen 
(p]i.  64,  78)  that  the  Mansfield  slate  is  overlain  by  a  conglomerate  in  which 
volcanic  material  predominates,  but  which  contains  partly  rounded  frag- 
ments of  chert  and  slate  and  round  quartz  grains  derived  from  the  under- 
lying sedimentaries.     But  for  the  interming'liug  of  this  normal  clastic  debris 

I  Bull.  U.  S.  Geol.  Survey  No.  62,  cit.  ^  Op.  cit.,  p.  158. 

•Mod.  U.  S.  Geol.  Survey,  Vol.  XX VIII,  cit.,  pp.  152-159.  °0p.  cit.,  pp.  160-169. 

=  0p.  cit.,  p.  133. 


NOKMAL  SEDIMENTAKIES  OF  HEMLOCK  FORMATION.  153 

with  the  l)vnl(•hl^sti^•s,  the  coni^loinerate  shows  iiothiu;^-  ditfercut  from  the 
volcaiiie  con<>louierate  already  described.  It  is  a  transition  rock  between 
the  tuffs  and  tlie  normal  sedimentaries. 

Similarly,  in  sec.  34,  T.  4.')  N.,  R.  33  W.,  a  gradation  occurs  in  the 
upper  horizon  of  the  Hemlock  formation  from  the  ^■ok•anic  conglomerates 
to  the  true  normal  sediments.  The  sediments  are  slates  about  17.5  feet 
thick,  containing-  lenticular  raas.ses  of  limestone.  These  beds  dip  80°  to  the 
west,  generally  strike  north,  but  vary  in  places  a  few  degrees  to  the  west. 
They  are  underlain  by  conglomerates  containing  well-rounded  volcanic 
pebbles.  This  volcanic  conglomerate  grades  from  the  coarse  conglomerate 
up  into  Avhat  might  be  termed  a  water-deposited  volcanic  sand.  Tlie  peb- 
bles are  all  of  volcanic  material.  Between  the  conglomerates  and  slates  is 
a  small  area  without  outcrop.  Overlying  the  slates  is  a  succession  of  tuffs 
and  lava  flows. 

The  slates  in  color  range  from  light  gray  and  green  to  purplish  red, 
and  the  lenses  of  limestone  vary  from  cream  color  to  purplish  red.  In  thin 
section  the  slates  are  seen  to  be  composed  of  a  felt  of  sericite,  chlorite,  and 
quartz,  with  associated  innumeralile  minute  rutile  crystals,  and  here  and 
there  a  large  spot  of  limpid  quartz.  A  ferruginous  carbonate  is  present  in 
all  of  them  in  porphyritic  rhombs.  Where  chlorite  is  abundant,  the  slates 
are  a  light  green.  Where  iron  oxide  is  abundant  and  the  chlorite  less  plenti- 
ful, the  slates  are  purplish. 

The  lenses  of  limestone  are  rather  pure,  consisting  mainly  of  calcite, 
with  some  few  scattered  areas  of  cherty  silica.  On  the  edges  of  the  lenses 
some  of  the  slate  material  is  found  forming  bands  in  the  carbonate.  These 
intermediate  phases  grade  on  the  one  hand  into  the  pure  carbonate,  and  on 
the  other  hand  into  the  slate  beds.  From  the  crust  of  limonite,  which  may 
be  seen  on  the  weathered  surface  of  the  rock,  the  calcite  is  evidently  rather 
ferruginous.  The  process  of  alteration  is  clearly  seen  under  the  micro- 
scope, where  many  of  the  grains  are  surrounded  by  rims  of  hydrated  oxide 
of  iron  and  hematite. 

ECONOMIC  PRODUCTS. 

BUILDING  AND  ORNAMENTAL  STONES. 

The  rocks  of  the  Hemlock  formation  are  not  likely  to  be  much  used 
for  building  purposes.     The  compact  basalts  possess  in  a  high  degree  the 


154  THE  CRYSTAL  FALLS  lEONBEARING  DISTRICT. 

two  essential  features  of  strength  and  durability.  For  trimming  in  contrast 
with  lighter  stones  they  might  be  found  desirable,  and  it  may  be  suggested 
that  they  are  especially  suitable  for  mosaics  in  which  rich  greens  are 
desired.  They  are  of  too  somber  a  color  to  l)e  used  in  large  quantity  for 
anything  else  than  foundations.  Moreover,  the  difficulty  and  consequent 
expense  of  quarrying  them,  and  their  remoteness  from  cities  of  large  size, 
will  operate  strongly  against  their  use. 

The  pyroclastics  are  natural  mosaics,  and  some  of  them  have  a  very 
pleasing  appearance  (PI.  XIII)  and  are  suitable  for  table  tops,  wains- 
coting, etc. 

ROAD   MATERIALS. 

The  importance  of  good  roads  in  aiding  in  the  material  development 
of  a  region  can  hardly  be  overestimated,  and  in  the  building  of  good  roads, 
especially  in  thinly  inhabited  regions,  the  proximity  of  good  road  material 
is  of  prime  importance. 

Thus  far  the  15  miles  of  good  road  between  Crystal  Falls  and  tlie 
adjacent  mining  villages  have  been  covered  with  the  ferruginous  chert  and 
slates  from  the  dumps  of  the  mines,  and  unroll  themselves  to  the  traveler 
like  red  ribbons  laid  through  the  green  woods. 

No  rock  is  better  suited  for  use  in  building  macadamized  roads  than 
the  liasalt,  and  of  this  the  Hendock  formation  offers  an  inexliaustible 
supply.  The  fine-grained  compact  basalts  are  by  far  the  best  rocks 
obtainable,  and,  other  things  being  equal,  should  of  course  be  chosen 
rather  than  the  scoriaceous  and  consequently  weaker  facies,  but  these 
weaker  kinds  and  also  the  pyroclastics  are  preferable  to  the  cherts  and 
slates  which  have  been  used.  The  cherts  are  very  hard  and  durable,  but 
the  dust  and  sand  froiu  them  possess  but  slight  capacity  for  cementation. 
Consequently  the  roadways  upon  which  quartzite  and  chert  have  been  used 
are  more  likely  to  wash  out  than  are  the  roads  macadamized  with  basalt, 
since  the  dust  in  this  latter  case  serves  as  a  cement  which  binds  the  larger 
fragments  more  firmly  together.  The  road  commissioners  have  thus  far 
used  very  little  basalt,  chiefly  for  the  reasons  that  no  crusher  was  at  their 
disposal,  and  the  chert  and  slates  were  at  hand  ready  for  use. 


CHAPTER   V. 
THE  UPPER  HURONIAN  SERIES. 

The  upper  series  of  tliis  district  is  connected  in  tlie  northeastern  part 
of  the  area  witli  the  Upper  Marquette  series  of  the  Marquette  district 
ah-eady  described  in  the  Fifteenth  Annual  Report  and  Monograph  XXVIII. 
In  these  reports  the  Upper  Marquette  series  is  regarded  as  part  of  the  Upper 
Huronian.  As  has  been  stated,  the  Crystal  Falls  district  is  the  southwestern 
extension  of  the  Marquette  district,  and  consequently  we  should  expect  the 
chief  formations  of  the  two  districts  to  be  continuous,  as  they  are.  Because  of 
the  drift  and  becaiise  of  a  change  in  the  character  of  the  rocks,  in  mapping 
the  western  part  of  the  Crystal  Falls  district  it  has  not  been  practicable  to 
divide  the  Upper  Huronian  into  several  formations,  corresponding  to  those 
in  the  ^Marquette  district.  No  independent  name  will  be  given  to  it,  but 
it  will  simply  be  called  Upper  Huronian,  with  the  understanding  that  it 
corresponds  stratigraphically  to  the  Upper  Marquette  series. 

DISTRIBUTION,  EXPOSURES,  AND  TOPOGRAPHY, 

Beginning  in  the  northeastern  part  of  the  area  discussed  by  me  (see 
PI.  Ill),  this  series  covers  the  southern  parts  of  T.  46  N.,  Rs.  31  and  32, 
where  it  is  only  4  miles  in  width.  It  is  here  a  northwest-southeast  syncline. 
From  this  place  it  stretches  beyond  the  northern  limit  of  the  map.  With 
slight  interruptions  where  intrusives  occur,  it  extends  in  a  broad  area  to  the 
west  and  south  about  the  Hemlock  volcanics  to  a  point  lying  beyond  the 
limit  of  the  map.  On  the  eastern  side  of  the  district  it  abuts  against  and  is 
folded  in  synelines  in  the  Archean  granite. 

Exposures  are  scanty  for  the  greater  part  of  the  area  in  the  Crystal 
Falls  district  underlain  by  the  Upper  Huronian  series.  This  is  due  to  two 
conditions,  first,  to  the  soft  character  of  the  rocks  constituting  the  series,  and, 
second,  to  the  presence  in  places  of  the  Cambnan  sandstone,  and  more 
especially  to  the  deep  covering  of  glacial  drift  which  is  found  spread  over 
the  entire  district.     The  Upper  Huronian  is  composed  in  great  measure  of 

155 


156  THE  CRYSTAL  FALLS  lUON-BEARING  DISTRICT. 

slates,  which  are  interbedded  with  much  smaller  quantities  of  graywackes 
and  chert.  The  slates  are  eroded  much  more  readily  than  the  associated 
harder  beds,  and  therefore,  except  along  valleys,  we  rarely  find  the  soft 
slates  exposed.  The  graywackes  and  cherty  rocks  are  the  ones  ^^'llich  form 
the  striking  topographical  features  of  the  landscape,  the  slates  forming 
softly-rounded  hills.  The  drift  is  also  an  important  factor  in  the  scarcity 
of  outcrops.  In  the  northern  and  western  parts  of  the  districts  especially 
the  drift  is  very  heavy.  In  this  portion  the  youthfulness  of  the  topography 
is  emphasized  by  numerous  swamps,  lakes,  and  generally  imperfect  di-ain- 
age.  In  the  southern  an<l  southwestern  parts  of  the  district,  owing  to  the 
presence  of  larger  streams,  and  consequently  more  advanced  erosion,  the 
drift  has  been  removed  to  a  greater  or  less  extent,  so  that  the  topographical 
forms  approach  much  nearer  to  those  of  an  unglaciated  region.  For 
instance,  the  general  strike  of  the  graywacke  and  cherty  ferruginous  slate 
beds  in  the  southern  portion  of  the  area,  T.  42  N.,  Rs.  32  and  33  W.,  can 
be  closely  followed  by  the  north-south  to  northwest-southeast  ridges  which 
they  form,  the  intervening  valleys  being  in  all  probability  underlain  by  the 
softer  carbonaceous  clay  slates.  Also  in  this  vicinity,  from  the  Chicago 
and  Northwestern  Railway  eastward  to  the  Michigamme  River,  exposures 
of  intrusives  with  some  sedimentaries  stand  out  from  the  sand  plains  as 
rounded  knobs. 

MAGNETIC  LINES. 

A  considerable  amount  of  detail  magnetic  work  has  been  done  in 
the  vicinity  of  the  ore-bearing  areas,  in  the  hope  that  with  the  assistance 
aftV)rded  by  the  magnetic  needle  the  iron  belts  might  be  better  traced  than 
tliev  could  be  by  means  of  the  very  scanty  outcrops.  I  shall  here  describe 
those  lines  of  magnetic  disturbance  which  have  been  traced  for  considerable 
distances.     They  are  indicated  on  the  map,  PI.  Ill,  by  solid  blue  lines. 

Magnetic  line  D. — Tliis  lluc  of  uiaximum  uiagnetic  disturbance  was  traced 
northwest  from  near  the  southeast  corner  of  T.  46  N.,  R.  32  W.,  around 
Bone  Lake,  then  southwest  and  south  tlu-ough  T.  46  N.,  R.  33  W.,  until 
finally  lost  near  the  south  side  of  sec.  34  of  the  same  township.  The 
tracing  of  this  line  was  begun  where  outcrojjs  were  wanting,  and  it  was 
not  possible  to  connect  it  directly  with  any  magnetic  formation  until  sec. 
34,  T.  46  N.,  R.  33  W.,  was  reached.  Here  it  was  connected  with  outcrops 
of  magnetitic  slate  and  gravwacke  which  overlie  the  Hendock  formation, 


MAGNETIC  LINES.  157 

but  with  no  contact  exposed.  Tlirouoliout  its  extent  tlie  lint;  of  disturbanco 
is  separated  from  the  line  of  outcroj)s  of  the  Hemlock  volcanics  by  a  short 
interval.     It  is,  however,  always  distinctly  separated  from  them. 

Magnetic  line  E — Tliis  magnetic  line  passes  directly  tlu'ough  the  open  pits 
at  the  Hemlock.  As  the  line  is  traced  north  from  this  point  it  passes  just 
west  of  an  amyydaloidal  lava  one-half  mile  north  of  the  mine.  From  this 
point  until  it  is  lost  in  sec.  16,  T.  45  N.,  R.  33  W.,  there  is  no  evidence  in 
regard  to  the  nature  of  the  rock  causing  the  attraction.  Tracing  the  line 
south  from  the  Hemlock  mine  it  is  found  to  swing  about  200  paces  east  of 
the  Michigan  mine,  near  the  north  line  of  sec.  9,  T.  45  N.,  R.  33  W. 
A  quarter  of  a  mile  farther  south  it  swings  back  again,  apparently  in  the 
line  of  the  continuation  of  the  iron-bearing  formation,  which  it  follows  for 
one-half  mile  farther,  where  it  is  lo.st.  The  only  place  along  this  line  where 
it  has  been  possible  to  determine  the  I'ock  causing  the  attraction  is  at  the 
Hemlock  mine.  Here  it  was  found  that  it  is  not  the  ore  foi'mation  proper 
which  is  magnetic,  but  that  it  is  the  foot  wall.  This  is  a  magnetitic  slate, 
about  42  feet  m  thickness,  as  shown  by  the  diamond-drill  borings. 

The  above  are  the  only  lines  of  maximum  magnetic  disturbance  of 
this  part  of  the  Crystal  Falls  district  which  it  has  been  found  possible  to 
connect  in  any  way  closely  with  the  iron-bearing  rocks.  A  large  number 
of  lines  of  distui'bance,  however,  were  traced  within  the  limits  of  the 
Hemlock  formation,  but  on  account  of  their  slight  economic  importance 
they  are  not  inserted  on  the  majj.  In  these  cases  the  influence  on  the 
needle  is  evidently  exerted  by  the  magnetite  of  the  lavas  and  pyroclastics, 
and  in  proof  of  this  the  lines  can  very  commonly  be  connected  with 
exposures  of  the  various  volcanic  rocks.  It  is  of  interest  to  note  that  the 
trend  of  the  lines  in  the  volcanics  invariably  agrees  with  that  of  the  tutf 
beds,  and  with  the  general  strike  of  the  formations  of  the  district,  and  the 
reader  is  reminded  of  the  suggestion  already  offered  (p.  1 34)  that  they  may 
be  caused  by  magnetite  accumulated  by  secondary  processes,  especially 
active  in  the  tuff  beds  and  scoriaceous  portions  of  the  lava  flows. 

THICKNESS. 

Since  the  Upper  Huronian  sediments  cover  a  broad  area,  their  thick- 
ness must  be  very  considerable.  Owing,  however,  to  the  scarcity  of 
exposures,  it  is  impossible  to  give  even  an  approximate  estimate. 


158  THE  CEYSTAL  FALLS  IRON-BEAlilXG  DISTlilCT, 

FOLDING. 

The  extreme  northwestern  part  of  the  area  has  not  been  stuclied  in 
such  detail  as  to  enable  the  minor  folds  to  be  determined.  In  general,  the 
series  may  be  said  to  fold  around  the  Lower  Huronian,  foUowino-  the 
general  outline  indicated  by  its  color,  as  shown  on  PI.  Ill,  and  having  a 
steep  dip  away  from  it.  In  sec.  20,  T.  45  N.,  R.  33  W.,  large  outcrops  of 
chert  are  folded  in  a  most  complicated  fashion  and  are  locally  brecciated. 
South  from  this  point  the  evidence  of  subordinate  cross  folds  is  marked. 
As  a  result,  the  line  between  the  Lower  Huronian  and  Upper  Huronian  is 
undulatory.  The  indentations  in  the  Lower  Huronian  represent  minor 
cross  synclines,  and  the  protuberances  represent  minor  cross  anticlines. 

CRYSTAL  FALLS  SYNCLINE. 

Near  Crystal  Falls  is  the  most  important  of  these  synclines.  This 
town  and  a  number  of  small  outlying  mining  villages  are  situated  on  a 
syncline.  The  character  of  this  syncline  is  shown  better  by  the  distri- 
bution of  the  Hemlock  volcanics  than  by  the  sedimentaries,  owing  to  the 
scarcity  of  the  outcrops  of  the  latter  (Pis.  XVII  and  XVIII).  Tlie  broad 
belt  of  northwest-southeast  trending  volcanics,  situated  3  miles  northeast  of 
Crystal  Falls,  bends  in  sees.  11,  12,  and  13,  T.  43  N.,  R.  32  W.,  to  tlie 
south,  and  gradually  changes  to  a  slight  southwest  trend.  In  the  reentrant 
ans'le  of  this  volcanic  formation  is  the  CrA^stal  Falls  syncline,  its  course 
Ijeing  that  of  a  southwestward-opening  U.  The  axial  line  of  this  U  probably 
has  a  westward  pitch,  corresponding-  with  the  general  folding  of  this  part  of 
the  district. 

Near  the  center  of  the  U  and  just  a  little  northwest  of  Crystal  Falls, 
in  sees.  17  and  20,  T.  43  N.,  R.  32  W.,  is  an  area  underlain  by  volcanics, 
which  trends  east  and  west,  and  can  be  followed  westward  into  sec.  1,  T. 
43,  R.  35,  beyond  the  limits  of  the  area  represented  on  the  map.  It  varies 
in  width  from  one-fourth  mile  to  4  miles,  averaging  about  2  miles.  The 
contacts  of  these  volcanics  with  the  overlying  Upper  Huronian  sediments 
are  not  exposed.  Hence  definite  proofs  of  their  interrelations  can  not  be 
given.  The  volcanics  have  been  folded  with  the  sediments,  and  subsequent 
erosion  has  exposed  them  along  the  axis  of  an  anticline. 

The  southei'n  arm  of  tlie  curved  syncline  bends  around  tlie  extreme 
southern  projection  of  the  Hemlock  volcanics  in  sees.  1  and  2,  T.  42  N.,  R. 


PLATE    XIV. 


159 


PLATE    XIV. 

IDEALIZED   STRUCTURAL   MAP  AND   DETAIL   GEOLOGICAL  MAT,  WITH    SECTIONS,  TO  SHOW  THE  DISTRIBU- 
TION'  AND   STRUCTURK   OF  THE    lURONIAN   ROCKS   IN'   THE   VICINITY   OI'  CUYSTAL  KALLS.  .MICHIGAN. 

Idealized  .structural  map  of  the  viciuity  of  Crystal  Falls.  An  attempt  has  beeu  made  to 
illustrate  upon  this  map  the  distributiou  of  the  Hurouian  rocks,  aud  at  the  same  time  our  conception 
of  the  general  features  of  the  structure  of  this  area.  The  drainage  is  merely  introduced  for  the 
purpose  of  orientation.  The  topography  as  here  represented  does  not  agree  with  the  true  topography 
of  the  area.  The  bottom  of  the  geological  basin  now  occupies,  as  the  result  of  erosion,  the  highest 
places  topographically. 

Detail  geological  map,  with  sections,  to  show  the  distribution  and  structure  of  the  Hurouian 
rocks  in  the  immediate  vicinity  of  Crystal  Falls.     This  serves  as  a  key  to  the  accompanying  idealized 
structural  map. 
160 


FOLDlNd   OF   iri'l'KU  HUUONIAN  SEKIES.  161 

31  W.,  and  swing-s  e;ist  imrtli  of  Lakc^  Mary  into  st'c.  32,  'V.  43  N.,  R.  31  W. 
Ht'ir  tt'iTUfiinon.s  slates  aiv  ex])ose(l,  borderinfi'  tlic  ]\Iicliig-annne  River 
at  the  so-called  Gliddeu  exploration.  The  extension  of  these  lowermost 
U))j)er  lluronian  heds  east  from  this  }X)int  soon  passes  nndcr  the  sand  plains 
and  drift  hills  and  is  lost.  The  higher  heds  of  the  series  are,  however, 
exj)osed  in  the  lower  course  of  the  Michig-anmie,  Paint,  and  Bi-ule  rivers, 
which  give  good  sections  across  them.  In  this  portion  of  the  area  discussed 
the  extension  of  even  these  higher  parts  of  the  formation  can  not,  however, 
be  followed  farther  east  than  the  Michigamme  River. 

That  the  Crystal  Falls  synclinal  basin  is  not  simple,  but  has  minor 
rolls,  is  shown  by  the  way  in  which  the  Upper  Huronian  series  indents  the 
Lower  Huronian  at  the  eastern  end.  Also  the  close  and  complicated  folding 
is  shown  by  mining  work,  and  can  be  nicely  seen  in  the  open  pits  of  the 
Columbia  and  Crystal  Falls  mines,  in  the  exposures  in  the  I'ailroad  cut  near 
the  Crystal  Falls  mine,  and  also  along  both  banks  of  the  Paint  River  near 
the  town  of  Crystal  Falls.  PI.  XIV  shows  the  general  character  of  this 
svncline.  The  folding  has  produced  extensive  "reibnngs-breccias."  Near 
Crystal  Falls,  along  the  river  bank,  about  one-fourth  mile  south  of  the  rail- 
road bridge,  may  be  seen  such  a  breccia,  which  has  been  formed  at  the 
junction  of  a  chert  with  the  slates. 

TIME  OF  FOLDING  OF  THE  UPPER  HURONIAN. 

The  latest  folding  to  which  the  rock  of  tlie  Crystal  Falls  district  has 
been  subjected  is  that  which  affected  the  Upper  Huronian  and  likewise 
involved  the  underlying  Archean  and  Lower  Huronian  rocks.  Therefore 
the  determination  of  this  period  of  folding  is  of  especial  interest,  as  mark- 
ing the  close  of  orogenic  movements  in  this  district. 

Overlying  the  Upj^er  Huronian  is  the  Potsdam  Cambrian,  or  Lake 
Superior  sandstone.  The  beds  of  this  formation  are  horizontal,  or  else 
show  a  very  slight  tilting,  following  the  general  inclination  of  the  district, 
which  perhaps  to  a  great  extent  may  be  explained  by  the  initial  dips  of  tlie 
beds.  They  overlie  with  strong  unconformity  the  upturned  and  strongly 
plicated  beds  of  the  Upper  Huronian.  This  unconformity^  marks  a.  lapse  of 
time  represented  in  other  districts  by  the  following  events:  (1)  A  period  of 
u^jheaval  and  denudation  of  the  Upper  Huronian;  (2)  the  subsidence  and 
deposition  upon  the  truncated  Upper  Huronian  sediments  of  the  hetero- 

MON    XXXVI 11 


162  THE  CRYSTAL  FALLS  lEONBBAKING  DISTRICT. 

geiieous  volcanic  and  sedimeiitary  Keweenawan  series;  (3)  the  ii])lieaval 
and  truncation  of  the  Keweenawan,  in  which  movement  of  course  tlie  Upper 
Huronian  in  the  Keweenawan  areas  was  likewise  involved.  Subsidence  of 
the  laud  areas  and  the  transgression  of  the  Cambrian  sea  followed,  with 
deposition  of  the  horizontal  Lake  Superior  sandstone  upon  the  inclined 
Keweenawan  and  Upper  Hui-onian  rocks.  The  Upper  Huronian  of  the 
Crystal  Falls  district  may  have  been  involved  in  one  or  both  of  the  foldings 
which  took  place  prior  and  subsequent  to  the  Keweenawan;  or,  second, 
since  no  Keweenawan  deposits  are  known  in  the  Crystal  Falls  district,  it 
may  be  that  it  suffered  ah  early  period  of  powerful  orogenic  movement, 
which  raised  the  rocks  above  the  sea,  and  was  synchronous  with  the  pre- 
Keweenawan  upheaval.  A  long  period  of  erosion,  accompanied  perhaps 
by  other  less  important  orogenic  movements,  may  have  followed  contem- 
poraneous with  the  activity  of  the  Keweenawan  volcanoes  and  the  f)scilla- 
tory  movements  of  tlie  Keweenawan  region.  The  latter  I  conceive  to  be 
the  more  jjrobable  view.  If  this  is  correct,  the  intense  folding  of  the  Upper 
Huronian  sediments  in  the  Cr3'stal  Falls  district  took  place  immediately 
preceding  the  deposition  of  the  Keweenawan  series  in  other  parts  of  the 
Lake  Superior  region. 

KELATIONS    TO    OTHER    SERIES. 

It  has  been  seen  that  in  the  western  part  of  the  district  the  Hemlock 
volcanics  are  the  highest  member  of  the  Lower  Huronian.  At  the  end  of 
the  volcanic  activity  there  must  have  taken  place  a  very  general  transgres- 
sion of  the  sea,  as  is  eAadenced  by  the  continuous  belt  of  sedimentary  rocks 
which  encircle  the  volcanics.  The  very  marked  change  in  the  character  of 
the  rocks  from  suliaerial  volcanics  to  true  sedimentaries  partly  marks  the 
division  of  the  Upper  Huronian  and  Lower  Huronian  series.  The  deter- 
mining points  in  favor  of  this  subdivision  are  found  in  the  eastern  part  of  the 
district  described  by  Smyth,  and  in  the  Marquette  district  still  farther  north- 
east. In  only  one  jjlace  in  the  western  part  of  the  Crj^stal  Falls  district,  in 
sec.  26,  T.  44  N.,  R.  33  W.,  has  a  contact  between  the  two  series  been  obtained. 
A  drill  hole  here  passed  through  a  mottled  slate  just  before  entering  the 
Lower  Huronian  volcanics.  A  similar  slate  was  obtained  at  Amasa  over- 
lyino-  conglomeratic  volcanic  material,  which  outcrops  at  the  surface,  but 
no  direct  contact  has  been  found.  With  most  careful  examination  I  have 
been  unable  to  determine  whether  the  conglomeratic  rock  is  a  true  volcanic 


RELATIONS  OF  UPI'Elt  UUKONIAN  SEHIES.  103 

tiirt"  (li'|)osit(.'<l   iipou   tlif  laiiil,  or  is  water-deposited  volcanic  material,  and 
thus  possibly  a  hasal  conjilouierate  of  the  U])])er  Ilunmian. 

Insec.  34/r.  4()  X.,  II.  ,')3  W.,  the  Lower  lliironianand  I'pper  Iluronian 
are  found  in  ver\-close  proxiniitv.  Here  the  Upper Huronian  is  a  fei'i'uyinous 
graxwacke  and  is  separated  li\'  oidv  aliout  10  yards  from  the  schistose  vol- 
caiiics.  In  tliis  case  careful  search  failed  to  reveal  the  intermediate  rock. 
In  onlv  one  case  in  addition  to  the  Amasa  instance  has  a  distinct  conglom- 
erate been  found  which  can  be  considered  as  a  possible  basal  conglomerate- 
Tliis  is  in  sec.  9,  T.  42  N.,  R.  31  W.,  along  the  Michigamine  River.  Here 
there  is  a  thick  mass  of  conglomerate  overlain  bv  soutliward-dijjping  schists 
The  conglomerate  contains  pebbles  of  extremely  altered  basic  amygdaloidal 
rocks  and  of  acid  rocks  which  rest  in  a  matrix  of  chlorite-schist.  This 
detrital  rock  is  such  as  might  be  derived  from  the  Hendock  volcanics,  but 
between  it  and  these  volcanics  is  found  a  mass  of  ferruginous  muscovitic  and 
chloritic  schists  at  the  Cllidden  exploration  (sec.  32,  T.  45  N.,  R.  31  W.),  wdiich 
are  very  similar  to  those  occurring  immediately  south  of  the  conglomerate, 
and  like  them  have  apparently  a  southern  dip.  The  true  relations  between 
this  conglomerate  and  the  schists  at  the  (Hidden  workings  are  not  certain. 
The  conglomerate  niay  be  below  them.  In  that  case  the  Gliddeu  schists 
would  correspond  to  those  south  of  the  conglomerate,  the  beds  having 
received  their  present  distribution  from  the  close  folding  to  which  they  have 
been  subjected. 

If  such  be  the  case,  the  schists  at  the  Glidden  are  the  northern  limb  of 
a  syncline,  and  the  conglomerate  and  the  overlying  schists,  with  an  average 
dip  of  70°  S.,  represent  the  southern  limb  of  a  steep  anticline,  whose  crest 
and  northern  limb  have  been  cut  oft'  and  covered  up.  The  considerable 
width  of  the  conglomerate  exposed  may  be  partly  due  to  the  fact  tliat  it 
has  been  doubled  upon  itself. 

That  the  folding  in  this  part  of  the  district  was  probably  fully  sufhcient 
to  produce  such  structural  relations  and  also  the  petrographical  changes 
of  the  conglomerate  matrix  to  a  chloritic  schistose  mass  is  shown  by  the 
changes  which  the  sedimentaries  south  of  them  in  this  part  of  the  district 
have  undergone.  If  this  interpretation  is  correct,  the  space  between  the 
schists  at  the  Glidden  exploration  and  the  Hemlock  volcanics  should  be 
occupied  by  the  equivalents  of  the  conglomerate.  Section  K-L,  PL  VI, 
embodies  this  idea  of  the  structure. 


164  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

RELATIONS   TO  INTRUSIVES. 

The  Upper  Hurouian,  as  well  as  the  Lower  Huronlan,  has  been  pene- 
trated bj^  intrusive  rocks.  The  diflference  in  the  character  of  the  intrusives 
of  the  two  series  is,  however,  interesting-.  As  has  been  seen  (p.  77),  the 
Lower  Huronian  is  cut  by  vast  masses  of  basic  rocks  and  by  rare  dikes  of 
acid  rocks.  In  the  Upper  Huronian  of  the  southern  part  of  the  district  the 
acid  rocks  are  more  abundant  but  still  subordinate  to  the  basic  intrusives. 
North  of  Crystal  Falls  is  a  great  east  and  wf  st  basic  dike.  Similar  rocks  are 
known  in  a  few  small  bosses  near  Crystal  Falls.  Moreover,  on  the  Michi- 
gamnie  River,  in  sees.  31  and  32,  T.  43  N.,  R.  31  W.,  and  in  a  few  places 
to  the  southeast  of  this  area,  the  Upper  Huronian  is  cut  by  the  southern 
continuation  of  the  basic  masses  whose  principal  occurrence  is  in  tlie  Lower 
Huronian  area.  Finally  basic  and  ultrabasic  intrusives  pierce  the  Upper 
Huronian  sediments  in  sees.  15,  28,  and  29,  T.  42  N.,  R.  31  W ,  and  in  a  few 
other  places.  The  acid  rocks  occur  in  isolated  knobs  near  Crystal  Falls, 
in  sec.  28,  T.  43  N.,  R.  32  W.,  and  in  sec.  4,  T.  42  N.,  R.  32  W.  They 
increase  in  (piantity  toward  the  southeast,  and  in  the  vicinity  of  Lake 
Tobin,  in  sees.  21  and  28,  42  T.  N.,  R.  32  W.,  tlie}'  form  a  series  of  small 
hills  rising-  Ixjldly  out  of  the  sand  plains;  and  finally  they  occur  in  larg-e 
quantity  in  sees.  19,  20,  and  29,  T.  42  N.,  R.  31  W.,  between  the  Paint  and 
the  Michigamme  rivers. 

CORRELATION. 

The  Upper  Huronian  sedimentary  rocks  were  first  studied  in  the  field 
by  Brooks,  and  the  conclusion  reached  by  him  that  they  are  late  Huronian.^ 

Rominger  correlates  these  same  rocks  very  correctly  with  the  schists 
exposed  aro^^nd  Lake  Michigamme,  although  he  has  the  erroneous  idea  that 
they  follow  down  the  ^lichigamme  River,  instead  of  making  a  wide  curve 
to  the  west,  as  subsequent  study  of  the  area  has  shown.^  He  considers  the 
rocks  as  forming  the  middle  portion  of  his  Arenaceous  Slate  group.' 

Recent  work  in  the  district  has  shown  the  Upper  Huronian  rocks  to  be 

'  Geolojiy  nf  the  Menominee  region,  by  T.  B.  Brooks:  Geol.  of  Wisconsin,  Vol.  Ill,  1880,  p.  44. 

-"The  mioa-schists  seem  to  continue  southward  along  the  course  of  the  Michigamme  River,  as 
we  find  iu  its  lower  course.  5  or  li  miles  north  from  its  entrance  into  the  Brule  River,  and  from  there 
down  to  the  mouth,  mica-schist  to  lie  the  iirevailing  surface  rock.  Along  the  lower  course  of  the 
adjoining  Paint  River  the  mica-schists  likewise  are  the  only  rocks  seen  in  the  exposures."  (Geol.  of 
Michigan,  Vol.  V,  1895,  p.  81.) 

»0p.  cit.,  p.  79. 


CORRELATION  OF  UPPP^K   HURONIAN  SERIES.  165 

iinqn(>stioniil)ly  tliu  wcsturu  (•iiiitiiiuatioii  of  the  .Michiginiinic-  fonnatinn,  In 
wliicli  tlu'  rocks  correspond  |»etro<i;Taj)liically.  The  Micliifianime  fonnalioii 
has  recentlv  been  carefully  studied  by  Van  Hise,  and  described  l)y  liiiii  in 
detail  in  Monograph  XXVIII  (Chaj).  IV).  A  less  detailed  description  is 
g-iven  in  the  Fifteenth  Annual  Report  (pp.  .'')98-604).  Since  such  great 
])etrograp]ucal  silnilarit^' between  the  Upper  Iluronian  dej)osits  in  the  west- 
ern half  of  the  Crystal  Falls  district  and  the  above  formation  in  the  adjoining 
districts  exists,  and  since  nothing  of  exceptional  interest  has  been  observed 
in  their  stud}-,  the  reader  is  referred  to  the  articles  mentioned  for  details. 
The  following  general  description,  while  based  ujjon  the  study  of  many 
exposures,  specimens,  and  75  sections  of  these  Crystal  Falls  rocks,  may  still 
be  considered  to  some  extent  as  an  abstract  of  the  above  articles  in  which 
the  few  changes  made  necessary  by  the  slightly  different  characters,  have 
been  incorporated. 

PETROGRAPHICAL,   CHARACTERS. 

From  the  above  general  statement  it  is  seen  that  the  Upper  Huronian 
comprises  rocks  both  of  sedimentary  and  of  igneous  origin. 

The  preponderant  deposits  of  the  western  half  of  the  Crystal  Falls 
district  were  muds  and  grits.  With  these  were  subordinate  quantities  of 
carbonates.  In  a  few  places  sheets  of  basic  rocks  were  intruded  between 
the  sedimentary  beds  and  are  now  foiuid  alternating  with  them.  Widely 
distributed  basal  conglomerates,  coarse  quartzitic  conglomerates,  and 
quartzites,  such  as  characterize  the  lowest  horizon  (the  Goodrich  quartzite) 
of  the  Upper  Huronian  of  the  Marquette  district,  are  absent.  Work  already 
completed  outside  of  the  immediate  area  covered  by  this  report  shows  the 
presence  of  a  small  area  of  surface  volcanics  associated  with  the  modified 
Upper  Huronian  sediments.  This  evidence  of  contemporaneous  A'olcanic 
acti^^ty  is  closely  paralleled  by  the  Clarksburg  volcanics  of  the  Upper 
Marquette  of  the  adjoining  district.' 

SEDIMENTARY    ROCKS. 

The  sedimentary  rocks  of  the  Ujjper  Huronian  series  in  the  western 
part  of  the  Crystal  Falls  district  are  graywackes,  feri-uginous  graywackes ; 
micaceous,  carbonaceous,  and  ferruginous  clay  slates  and  their  crystalline 

'Fifteenth  Ann.  Rept.  U.  S. Geol.  Survey,  cit.,  pp.  604-607;  Monograph  U.  S.  Geo].  Survey,  Voi. 
XXVIII,  cit..  pp.  460-486. 


166  THE  CRYSTAL  FALLS  IROlSr-BEARING  DISTRICT. 

derivatives;  and  thinly  laminated  cherty  siderite-slate,  ferruginous  chert, 
and  iron  ores.  With  these  we  find  only  in  two  places  rocks  of  a  well- 
developed  conglomeratic  nature. 

Some  of  the  rocks  have  undergone  great  metamorphism,  and  we  find 
the  graywackes  and  slates  passing  into  chlorite-schists,  mica-schists,  and 
mica-fneisses.  The  ore  deposits  of  the  distinct  are  associated  with  the  least 
altered  sedinientaries. 

The  graywackes  and  slates  are  found  chiefly  in  the  northern  and 
western  parts  of  the  district,  while  the  single  conglomerate,  the  metamor- 
phosed or  micaceous  graywackes  and  slates,  the  mica-schists,  and  the  mica- 
gneisses  are  confined  to  tlie  southern  portion. 

Near  Crystal  Falls  on  both  banks  of  the  river,  between  the  wagon  and 
railroad  bridges,  there  is  exposed  a  conglomeratic  phase  of  graywacke. 
Several  bands  of  these  coarse  conglomeratic  graywackes  are  interlamiuated 
with  bands  of  fine-grained  graywacke  and  chert.  A  well-developed  chert 
reibungsbreccia  is  also  associated  with  these.  I  do  not  consider  this  con- 
glomeratic graywacke  as  representing  anything  more  than  a  purely  local 
and  very  slight  nnconformity.  This  is  evidently  the  same  occurrence  of 
conglomerate  which  has  already  been  mentioned  by  Wadsworth.^ 

The  well-developed  conglomerate  found  in  sec.  9,  T.  42  N.,  R.  31  W., 
along  the  Michigamme  River,  contaios  pebbles  of  both  basic  and  acid 
eruptive  rocks  in  a  chlorite-schist  matrix.  Toward  the  south  the  rock 
grades  up  into  chloritic  graywackes  and  chlorite-schists,  which  possess  the 
ordinary  characters  of  similar  rocks  in  other  portions  of  the  area.  The 
graywackes  and  slates  of  the  district  in  general  differ  from  each  other 
chiefly  in  coarseness  of  grain.  They  are  commonly  interbedded  in  the 
same  exposures.  The  rocks  vary  in  coarseness  from  medium-grained 
graywacke  to  aphanitic  slates,  and  in  color  from  gray  to  green  and  black, 
the  aphanitic  slates  being  usually  the  darkest.  These  fine-grained  rocks 
always  show  well-developed  slaty  cleavage.  Throughout  the  area  the 
rocks  are  very  thoroughly  consolidated,  and  in  places  where  they  have 
been  most  altered  they  are  completely  crystalline  schists. 

The  crystalline  rocks  which  are  believed  to  have  developed  from  the 
elastics  are  found  in  the  southern  portion  of  the  district,  beginning  in 
sec.  16,  T.  42  N.,  R.  31  W.,  and  are  well  exposed  in  the  river  section  at  Nor- 

'  Sketch  of  the  geology  of  the  iron,  gold,  and  copper  districts  of  Michigan,  by  M.  E.  Wads- 
worth:  Ann.  Rept.  State  Board  of  Geol.  Survey  for  1891-92,  1893,  p.  128. 


rETIJOGKAPIIICAL  CHARACTERS  OF  UlTEli  HURONIAN.       167 

way  farry  (portai^v).  Tlic  rocks  iit  this  point  are  an  altcniatiuy  succession 
of  inica-sdiists,  some  of  wliicli  are  very  ([uartzitic,  and  hornblende-gneisses 
(altered  igneous  rocks)  in  thick  beds.  In  these  the  Ix'ddini;'  and  schistosity 
agree,  or  else  ditter  so  slightly  as  not  to  be  noticeable.  These  rocks  out- 
crop in  bold  knobs  for  some  distance  away  from  nnd  on  both  sides  of  the 
Michigannne  River  at  the  carry  and  also  form  the  steep  clitfs  between  which 
the  river  flows.  In  some  places  they  are  more  or  less  ferruginous,  and 
at  one  point  near  the  head  of  the  rapids  exjjloring  for  iron  has  Ijeen  done. 

These  crystalline  schists  .jre  sei)arated  from  the  undoubted  sedimen- 
taries  at  the  north  by  an  interval  of  about  one-half  mile,  and  are  separated 
to  the  south  by  a  smaller  interval  from  the  next  rocks,  which  are  also  of 
sedimentary  origin,  though  of  highly  metamorphosed  cliaracter.  These 
latter  consist  of  micaceous  graywackes  which  grade  over  by  increasing 
metamorphism  int<i  rocks  which  are  indistinguishable  from  mica-schists  and 
mica-gneisses.  There  can  he  no  doubt  as  to  the  clastic  character  of  these 
rocks,  as  oue  may  see  on  the  outcrops  of  the  least  altered  phases  the  normal 
as  well  as  the  false  bedding.  The  bedding  of  these  micaceous  graywackes 
agrees  with  that  of  the  mica-schists  at  the  Norway  carrv,  and  in  them  the 
schistosity  is  usually  nearly  parallel  to  the  bedding,  thougli  at  times  cutting 
it  at  varying  angles.  These  rocks  vary  in  grain  from  fine  to  very  coarse. 
The}^  are  most  all  a  light-gray  color.  In  two  cases  the  presence  of  large 
porphyritic  crystals  of  staurolite  was  observed.  Garnet  was  found  in  but 
a  single  specimen,  and  no  andalusite  was  seen.  The  scarcity  or  absence  of 
such  minerals  is  made  noticeable  by  the  fact  that  they  are  so  abundant  in 
the  adjoining  Marquette  district.  The  contrast  is  the  more  marked  since  the 
Crystal  Falls  rocks  are  cut  by  and  included  in  granite,  and  in  the  Mar- 
quette district  these  intrusives  are  absent.  Splendid  sections  through  the 
metamorphosed  sediments  are  offered  by  the  river  sections  in  sees.  14,  24, 
35,  and  36,  T.  42  N.,  R.  32  W.,  on  Paint  River,  and  at  Peevie^  Falls,  sec. 
32,  T.  42  N.,R.  31  W.,  on  the  Michigamme  River. 

It  should  be  noted  that  thus  far  no  gi-iinerite-schists  which  owe  their 
origin  to  the  metamorphism  of  feiTuginous  sediments  have  been  observed 
in  the  Crystal  Falls  district;  nor  does  the  brilliant  red  jasper — jaspilite — 
which  accompanies  certain  of  the  ores  in  the  Marquette  district,  occur 
associated  in  large  quantity  with  them  in  the  Crystal  Falls  district. 

'  This  name  has  been  given  by  the  lumbermen  to  the  falls  as  they  lose  so  many  peevies  here  in 
breaking  jams. 


168  THE  CRYSTAL  FALLS  lEON-BBAEING  DISTRICT. 

The  iron-bearing-  rocks  of  llie  Upper  Huronian  comprise  cherts, 
siderite-slates,  ferruginous  cherts,  iron  ores,  and  subordinate  quantities  of 
ferruginous  graywackes  and  chiy  slates. 

The  least  altered  of  these  is  a  siderite-slate.  This  is  a  fine-grained  gray 
rock  composed  almost  entirely  of  siderite,  usually  in  rounded  rhombohedral 
crystals,  with  very  little  minutely  crystallized  silica  between  them  in 
places.  Wherever  they  have  been  exposed  to  the  weather  any  length  of 
time,  these  rocks  have  a  deep  reddish-brown  oxidation  crust.  Alteration 
also  follows  along  crevices,  and  thus  the  siderite  is  rapidly  oxidized.  The 
main  ])roducts  derived  from  these  siderites  are  like  those  of  the  more 
important  ore-producing  parts  of  the  Penokee  and  Marquette  districts, 
namely,  hematite  and  limonite.  Little  magnetite  has  been  found.  These 
siderites  are  interbanded  with  the  black  carbonaceous  clay  slates.  In  some 
cases  the  dividing  line  is  sharp.  In  others,  as  the  siderite  lessens  in  quan- 
tity, fragmental  material  increases  until  only  a  few  crystals  of  siderite  are 
found  scattered  through  the  elastics.  Their  association  with  the  carbona- 
ceous fragmentals  would  seem  to  indicate,  as  pointed  out  by  Van  Hise.Hhat 
the  siderite  owes  its  formation  to  tlie  presence  of  organic  material. 

The  ferruginous  cherts  (the  term  is  here  used  as  defined  by  Van  Hise) 
are  banded  chert  and  hematite,  with  some  magnetite,  in  which  the  iron 
oxide  is  derived  from  a  previously  existing  siderite,  and  the  cherty  bands 
are  not  of  fragmental  origin.  This  alteration  from  the  siderite  to  hematite 
may  be  easily  followed  from  the  fresh  siderite  through  that  which  is  slightly 
discolored,  to  the  reddish-brown  earthy  mass,  and  then  to  the  crystalline 
hematite.  Such  alteration  processes  have  been  illustrated  and  clearly 
described  a  number  of  times  by  Van  Hise,  so  that  no  further  mention  will 
be  made  of  them. 

The  ferruginous  graywackes  may  be  described  as  rocks  which  are 
partly  of  fragmental  and  partly  of  chemical  origin.  For  instance,  the  tran- 
sition may  be  traced  from  a  rather  micaceous  magnetitic  graywacke,  in 
which  ordinary  and  false  bedding  may  be  seen,  to  a  rather  schistose  rock, 
in  which  magnetite  is  predominant,  Init  in  which  is  considerable  fragmental 
quartz  and  secondary  muscovite  and  chlorite.  This  rock  represent.s  an 
original  grit  containing  more  or  less  siderite.     Metamorphism  has  changed 

I  Fifteenth  Ann.  Eept.  U.  S.  Geol.  Survey,  oit.,  p.  601;  Mon.  U.  S.  Geol.  Survey,  Vol.  XXVIII,  cit.,  p.  447. 


PETHOGKAPHIOAL  CHARACTERS  OF  UPPER  HURONIAN.       169 

the  sideritc  to  nui^iuftite,  and  pnxliiccil  tVoiii  the   tiue  frag-iuental   mud  tlie 
imiscovite  and  cldorire. 

MICROSCOPICAL    DESCRIPTION    OF    CERTAIN    OF    THE    SEDIMENTARIES. 

In  tlie  foUowin^j;-  pa<^-es  1  sliall  describe  in  a  brief  way  the  graywackes 
anil  shxtes,  the  most  common  rocks  of  tlie  district,  and  the  rocks  which 
have  been  ])rodnced  from  tliem  by  metamor])hism. 

The  g-raywackes  and  slates  consist  chiefly  of  readil}-  distinguishable 
fragmental  quartz  and  feldspar  grains,  which  are  embedded  in  a  matrix 
consisting  of  fine-grained  quartz,  feldsj^ar  (?),  biotite,  muscovite,  chlorite, 
some  siderite,  epidote,  small  quantities  of  magnetite,  hematite,  and  iron 
pyrites,  and  a  dark  clayey  mass.  This  mass  appears  to  contain  a  consider- 
able amount  of  black  carbonaceous  material  and  reddish-brown  ferruginous 
matter  in  finely  disseminated  specks.  The  greater  the  quantity  and  finer 
the  character  of  this  matrix  the  more  difiicult  it  becomes  to  determine  its 
constituents  with  any  degree  of  certainty.  In  the  slates  the  matrix  plays 
the  chief  role,  while  in  the  graywackes  the  large  fragmental  grains  form 
the  predominant  material.  By  a  diminution  in  quantity  of  the  matrix  and 
fragmental  feldspar  grains,  the  coarser-grained  elastics  approach  very  closely 
to  true  quartzites,  but  in  no  case  was  a  pure  quartzite  found. 

nie  constituents  which  can  be  recognized  without  difficulty  as  original 
ones  are  the  larger  grains  of  feldspar  and  quartz.  These  show  pressure 
phenomena  of  all  grades,  from  slight,  wavy  extinction  to  complete  granula- 
tion. Many  of  the  large  fragmental  quartzes  are  mashed  into  oval-shaped 
areas  or  are  broken  into  numbers  of  frag-ments.  The  large  feldspars  are 
broken,  and  are  altering  to  quartz  and  secondary  clear  feldspar  with  a 
simultaneous  production  of  epidote  and  mica.  In  their  least  altered  condi- 
tion the  original  feldspars  are  cloudy,  and  hence  may  be  readily  distin- 
guished from  the  limpid  secondary  grains. 

The  small  mineral  particles  of  the  matrix,  including  the  mica,  do  not 
show  undulatory  extinction  like  the  large  fragmental  quartzes  and  feldspars. 
These  micaceous  minerals  are  in  automorphic  plates,  and  wrap  around  the 
quartz  grains,  and  in  some  cases  likewise  project  into  them.  These  con- 
stituents of  the  matrix  are  all  believed  to  be  secondary  minerals  derived 
from  the  original  clayey  matrix,  and  from  the  alteration  of  the  feldspar 
fragments,  with  the  possible  addition  of  infiltrated  material.     At  places  all 


170  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

of  thes^e  minerals  occur  together,  liut  more  commonly  one  finds  various 
combinations  of  certain  of  them.  When  muscovite  is  present  in  large 
quantity,  it  is  usually  not  accompanied  by  biotite  or  chlorite,  the  iron  and 
magnesium  necessary  for  the  production  of  liiotite  and  chlorite  evidently 
not  having  been  present.  These  last  two,  however,  are  always  associated. 
As  the  mica  increases,  the  schistosity  of  the  rock  increases  in  a,  corre- 
sjionding  naanner,  and  the  rocks  liecome  those  which  may  be  spoken  of  as 
micaceous  graywackes. 

These  micaceous  graywackes  represent  a  somewhat  ni(ire  advanced 
stage  of  metamorphism  of  the  rocks  than  the  graywackes  just  described, 
and  the  extremely  altered  varieties  of  these  are  very  close  to  the  mica- 
schists  and  mica-gneisses,  according  to  the  respective  amounts  of  secondary 
feldspar  jJi'esent.  No  distincti(in,  however,  can  be  made  in  the  iield  between 
some  of  the  less  metamorphosed  graywackes  and  these  micaceous  ones. 
The  chief  difference  appears  to  be  in  the  fact  that  in  the  micaceous  gray- 
wackes the  larger  feldspars  are  almost  completelj^  altered  and  the  finer 
matrix  completely  recrystallized  into  readily  distinguishable  mineral  parti- 
cles. In  these  more  metamorphosed  rocks  the  parallel  intergrowth  of 
secondarA'  muscovite  and  biotite  is  nicely  shown,  a  thin  leaf  of  biotite  being- 
included  l^etween  two  lamella-  of  nmscovite.  A  considerable  quantity  of 
epidote  is  scattered  in  large  grains  tlu-ough  the  micaceous  graywackes, 
besides  occumng  in  aggregates  of  small  grains.  Some  crystals  of  apatite 
and  tourmaline  were  observed.  Rutile  is  found  in  some  quantity,  and  with 
it  is  also  spheue,  both  of  them  possibly  resulting  from  the  alteration  of 
titanium-bearing  iron  ores  in  the  original  graywackes.  The  iron  present 
in  the  original  graywackes  as  siderite  and  the  minute  specks  of  oxide  have 
been  collected  into  large  crystals  of  magnetite  and  also  into  aggregates  of 
smaller,  well-defined  magnetite  crystals. 

The  alteration  of  the  feldspar  and  the  production  from  it  of  quartz, 
secondary  feldspar,  ejiidote,  and  mica  is  well  shown  in  one  case.  In  this 
the  nucleus  of  original  feldspar,  in  the  center,  contains  minute  grains  of 
epidote  and  flakes  of  muscovite,  besides  reddish,  presumably  ferruginous, 
specks.  These  with  a  low  power  cause  the  feldspar  to  appear  cloudy. 
Surrounding  this  core  is  a  mottled  zone  in  which  secondary  felds^Dar  and 
quartz  occur.  In  one  place  in  this  zone  a  flake  of  biotite  is  observed.  Some 
epidote  also  occurs  in  it,  but  no  muscovite. 


PKTUOGUAl'HU'AL  OIIAKACTEIIS  OF  UPPER  HUROISIAN.       171 

Tlic  iiltcM-iitidU  of  tlui  t'eldispar  usually  l)c-;;ius  at  the  periphery,  and 
jii-aduallv  advances  toward  the  center.  It  thus  l)reaks  tlie  ori<i-inal  oraiii  up 
into  irreji-ular  areas  and  strinji-ers  of  feldspar,  many  of  which  arc  attaclicd  to 
the  unaltered  center.  IVtween  these  n-sidual  areas  of  feldsp;n-  th(M-e  are 
irrcuuhu-  yrains  of  .secondary  linn)id  felilspar  and  ((uartz.  Tlie  fartlier  the 
alteration  is  advanced  the  less  of  the  irregular  center  may  l)e  seen,  and  in 
the  final  sta<;-e  the  fehlspar  core  disappears. 

A\'hile  the  alteration  nearly  always  heifins  at  the  periphery,  one  case 
was  noticed  where  it  ap[)arently  be<i'an  at  various  places  in  the  g-rain,  the 
result  l)eing  the  production  of  a  secondary  micropoikilitic  structure.  This 
original  feldspar  is  cloudy,  with  the  usual  alteration  products,  ])ut  scattered 
through  this  are  a  number  ()f  more  or  less  roundish  s])Ots  of  quartz,  the 
majority  of  which  extinguish  simultaneoitsly,  and  have  a  different  position 
of  extinction  from  the  including  feldspar.  Considering  these  two  elements 
alone,  the  structure  is  near  the  micropegmatitic;  but  there  are  other  areas 
which  extinguish  in  different  positions  from  both  the  quartz  and  the  original 
feldspar.  These  are  of  decidedly  more  angular  shape  than  the  secondary 
quartzes,  and  appear  to  be  secondary  acid  feldspar.  The  small  size  of  these 
secondary  minerals  prevents  the  use  of  any  })hysical  tests  other  than  the 
differences  in  refraction.  The  nnmded  quartz  appears  in  many  respects 
very  much  like  the  corrosion  quartz  of  the  French  petrograjijhers.  The 
majority  of  the  secondary  feldspars  are  imstriated,  but  a  few  show  striations. 
No  satisfactory  sections  upon  which  to  make  measurements  were  found. 
Biotite  and  nuiscovite  flakes  are  included  in  the  quartz,  and  smaller  auto- 
morphic  plates  of  biotite  niay  be  seen  lying  partly  within  the  altered  feld- 
spar grain,  as  though  growing  partly  at  its  expense. 

These  highly  metamorphosed  micaceous  rocks  included  under  the  gen- 
eral term  "micaceous  graywackes"  have  the  interlocking  groundmass  struc- 
ture of  the  schists,  but  some  of  the  larger  grains  sliow  clastic  forms.  No 
sharp  line  can  be  drawn  between  these  metamorphosed  sediments  on  the 
one  hand  and  the  mica-schists  and  mica-gneisses  on  the  other. 

In  the  mica-schists  and  mica-gneisses  all  of  the  original  mineral  grains 
have  been  completely  crushed  and  recrystallized,  and  we  can  find  no  micro- 
scopical criteria  which  enable  us  to  class  them  with  the  sedimentary  rocks. 
Dynamic  action  in  the  district  had  sufficient  power  and  duration  to  comjjlete 
locall}'  the  metamorphism  of  the  original  sedimentaries  and  produce  per- 


172 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 


fectly  crystalline  schists,  as  described  by  Van  Hise  in  the  Penokee  and 
Marquette  districts.^  No  rocks  corresponding  in  content  of  carbon  to  the 
carbonaceous  slates  which  occur  among  the  rocks  around  Crystal  Falls  and 
south  of  that  town  have  been  found  among  these  crystalline  schists.  These 
crystalline  schists  are  throughout  moderateh'  fine  grained,  and  consist  of 
quartz,  feldspar,  and  mica,  with  associated  epidote,  rutile,  tourmaline,  and 
iron  oxides,  and  in  a  few  exceptional  cases  crystals  of  staurolite  and  garnet. 
In  some  of  the  rocks  quartz  and  mica  are  preponderant  and  feldspar  is  prac- 
tically wanting,  and  we  have  mica-schists.  In  others  all  three  essential  min- 
erals are  present,  and  we  have  mica-gneisses.  The  presence  of  the  feldspar, 
and  to  some  extent  the  proportion  of  the  mica  and  other  minerals,  depend 
on  the  character  of  the  original  sediments.  Conclusive  evidence  of  the 
sedimentar)'  origin  of  these  schists  is  furnished  by  their  occurrence  in  the 
field,  where  are  found  all  gradations  between  them  and  rocks  of  unques- 
tionably sedimentary  character. 

In  PI.  XV  there  is  reproduced  the  part  of  Brooks's  PI.  IX  in  Vol.  Ill 
of  the  Geological  Survey  of  Wisconsin  which  comes  within  the  Crystal 
Falls  district  and  includes  a  part  of  the  area  luiderlain  by  the  Huroniau 
sediments.  There  is  here  given  his  macroscopical  description^  of  the  rocks 
collected  in  his  study  of  the  area. 

Brooks^s  macroscopical  description  of  rocks  collected  in  the  Crystal  Falls  district. 


No.  of 
bed. 


ThickDeas 
in  feet. 


Mica-scMst,    alternating   with   gneiss,   generally   flaggy    or 

schistose  (2452) '       1,250 

About  IJ  miles  west  autl  a  little  north  is  an  outcrop  of 
mica.-sehist  containing  staurolite  (2160).  The  same  rock 
crosses  the  Paint  1  mile  farther  west  and  north,  where  it 
also  contains  staurolite.  This  is  important  and  interest- 
ing, since  this  miueral  characterizes  the  same  bed  of  mica- 
schist  (XIX)  in  the  Marquette  region,  where  it  is  associated 
with  andalusite.  Neither  mineral  has  been  observed  else- 
where in  the  series. 


I  Mon.  U.  S.  Geol.  Survey,  Vol.  XIX,  cit.,  pp.  332-343;  Mon.  U.  S.  Geol.  Survey,  Vol.  XXVIII,  cit., 
pp.  449-450. 

^Geology  of  the  Menominee  iron  region,  by  T.  B.  Brooks:  Geol.  of  Wis.,  Vol.  Ill,  1880,  Pt.  VII, 

p.  496. 


PETKOGRAPEIICAL  CHAKAOTEKS  OF  UPPKK  HUKONIAN. 

Brooks's  macroscopical  description  of  rocks,  etc. — Continued. 


173 


Xo.  of 
bed. 


t. 


Hornblende-Kchist  or  yneiss,  black,  slaty,  liamled  with  Kneissic 
layers  (245:^ ) 

(iniias  iind  luica-nrhial 

Covered 

Stauiolilir  iiiica-schisl  ou  south,  overlaid  with  niica-arliial  :inil 
gneiss  ( 2455) 

Covered 

V.   \   (! iieiss 

\v.   !  Mint-schist 

Micaceous  ijuartz-schist  or  (jnciss  (2381,  2457) 

Covered 

Gneiss 


Thickness 
ill  fe»t. 


100 

mo 

130 

400 
150 
100 
100 
250 
500 
150 


He  then  iiieutiniis  the  occurrence  farther  np  the  Miehigainnie  Kiver  of 
exposures  of  "a  heavy  l)e(l  of  uneiss,  dipping  north  at  a  high  angle,"  and 
also  speaks  of  "outcrops  along  the  river  of  hornhlendic  and  other  rocks,  often 
granitic  in  appearance."  He  writes  also  of  "chloritic  and  hornblende  schists 
exposed  for  a  thickness  of  over  1,000  feet"  at  the  Norway  portage,  farther 
up  the  river.  "They  dip  south  at  a  high  angle  under  tlie  granite  horn- 
blende Ijelt  just  described,  and  are  proljably  the  equivalents  of  some 
portion  of  the  Long  Portage  series  on  the  opposite  side  of  this  synclinal, 
although  these  differ  from  the  prevailing  rocks  of  that  series  in  being 
decidedly  more  chloritic  and  hornblendic' 

It  is  seen  from  the  above  quotations  that  the  general  characters  of  the 
rocks  were  recognized  by  Brooks.  His  schists  are  for  the  most  part  the 
metamorphosed  sediments,  and  the  hornblendic  and  granitic  rocks  are  the 
various  basic  and  acid  rocks  Avhich  intrude  them. 

Specimens  from  these  interesting  beds  were  collected  by  the  Michigan 
and  Wisconsin  surveys,  and  were  described  by  Julien,  Wichmann,  and 
Wright  in  lithological  reports  appended  to  the  reports  of  those  survevs." 

'  Geology  of  Jleuouiinee  iron  region,  by  T.  B.  Brooks:  Geol.  of  AVis.,  Vol.  Ill,  1880,  Pt.  VII,  i).497. 

-  Microscopical  observations  on  the  iron-bearing  rocks  from  the  region  soutli  of  Lake  Superior, 
by  Arthur  Wichmann:  Chapter  V  of  Brooks's  Geology  ofthe  Menominee  iron  region,  Geol.  Survey  of 
Wis.,  Vol.  Ill,  1880,  pp.  600-G5(3. 

Lithology,  by  A.  A.  Julien:  Geol.  of  Mich.,  Vol.  II,  1873,  pp.  1-197.     Appendix  A. 

Geology  of  the  Menominee  iron  region,  by  C.  E.  Wright:  Geol.  Survey  of  Wis.,  1S80,  part  8, 
pp.  (;90-717. 


174  THE  CRYSTAL  FALLS  IROD-BEAEING  DISTRICT. 

Specimens  of  these  mica-scliists  and  chlorite-schists  were  regarded  by 
Wichniann  as  nonfragmental.'  Tliis  is  little  to  be  wondered  at,  since  lie 
had  never  studied  their  field  relations.  In  the  field  these  rocks  can,  however, 
be  traced  into  rocks  of"  unquestionably  fragmental  origin.  The  same  speci- 
mens were  described  as  "micaceous  quartz-schists"  by  AYright."  Wright 
also  mentions  a  staurolitiferous  mica-schist  in  the  Michigannne  River,  and 
also  in  the  Paint  River.  The  second  occurrence  is  2  J  miles  northwest  from 
the  first  and  in  the  direction  of  its  strike.  Julien  describes  metamorphic 
rocks  from  Long  Portage  as  "fine-grained  grayish  black  gneisses."'  From 
these  descriptions  it  is  seen  tliat  the  least  altered  sedimentaries  on  the  one 
hand  and  the  crystalline  schists  on  the  other  were  recognized  by  these 
earliest  students  of  the  metamorphic  rocks.  However,  the  fact  to  which 
I  would  especially  call  attention,  that  the  crystalline  schists  are  derived 
from  the  clastic  rocks  b}'  metamorphism,  was  evidently  not  understood. 

The  contact  action  produced  by  igneous  intrusions  into  these  series 
of  sedimentaries  will  be  discussed  in  connection  with  the  intrusive  rocks 
(p.  194  et  seq.). 

IGNEOUS   ROCKS. 

The  igneous  rocks  which  are  found  to  have  penetrated  the  Uppei- 
Huronian  after  the  important  folding  of  tlie  rocks  took  i)lace  are  not 
included  here,  but  may  be  found  described  under  the  heading  "Intrusives" 
(p.  187).  In  this  place  it  is  desired  to  call  attention  to  certain  hornblende- 
gneisses  wliicli  occur  near  J^orwa)'  carry,  on  the  Michigannne  River,  and 
also  extend  in  large  outcrops  west  of  the  river  for  about  2  miles  and  east 
for  about  a  mile.  These  are  interlaminated  in  thick  masses  with  the  mica- 
schists.  Thev  are  perfectly  crystalline  hornblende-gneisses.  They  consist 
of  connnon  hornblende,  quartz,  feldspar,  and  some  iron  oxide.  The  horn- 
blende is  present  in  large  (juantity,  the  parallel  })lates  of  that  mineral 
giving  the  rock  its  schistosity.  None  of  the  minerals  are  automorphic,  but 
all  occur  in  interlocking  grains.  Without  going  into  a  detail  description 
of  these  rocks,  it  will  suffice  i)erhaps  to  state  that  they  are  similar  in  all 
respects  to  hornblende-gneisses  which  in  other  parts  of  the  Lake  Superior 

'Op.  cit.,pp.  635,646. 
-Op.  cit.,  p.  693. 
■  Op.  cit.,  p.  130. 

'Bull.TJ.  S.  Geol.  Survey  No.  62,  by  <i.  II.  Williiims,  1890;  Mou.  U.  S.  Gcol.  Siirvoy,  Vol.  XXVIII, 
jip.  152-1.59,  203,  208. 


ORE  DEP08ITrf  OF  UPPER  IIUKONIAN.  175 

reo'ion  have  Ix'Oii  traced  into  ij^-iicous  rocks/  ^''hcsi' jiiicisscs  are  believed  to 
be  i<;neous  rocks,  either  intrusives  whicli  were  injected  ])arallel  to  the  ])ed- 
ding-  ot"  the  U])|)er  Huronian  sediments  prior  to  tlic  folding,  or  contempo- 
raneous \(ilcanics.  'riie\'  have  l)een  uietaniorpliosed  and  rendered  schistose 
bv  the  same  forces  which  metamorjjhosed  the  sediments.  This  explains  the 
perfect  agreement  of  their  schistosity  with  that  of  the  adjacent  sediments. 

ORE  DEPOSITS. 

HISTORY   OF   OPENING   OF   THE   DISTRICT. 

For  a  number  of  years  after  the  opening  of  the  mines  of  the  Menomi- 
nee range,  prospectors  worked  in  Aarions  places,  among  others  in  the  vicinity 
of  Crystal  Falls,  seeking  to  follow  the  iron  range  west  of  the  Menominee 
River.  As  a  resnlt  of  this  endeavor,  the  deposits  at  Florence,  Wisconsin, 
and  then  those  farther  north  and  west  at  CJrystal  Falls,  Michigan,  were  in 
turn  located.  It  was  not  until  1881  that  sufficient  exploratory  work  had 
been  done  at  Crystal  Falls  to  warrant  a  belief  in  the  future  of  this  iron- 
bearing  area.  In  April,  1882,  the  Chicago  and  Northwestern  Railway  com- 
pleted its  branch  to  C'rystal  Falls,  and  the  shipment  of  ore  began.  The 
Amasa  deposits  were  not  exploited  to  any  great  extent  until  the  year  1888, 
when  the  Chicago  and  Northwestern  Railway  built  a  branch  from  Crystal 
Falls  to  Amasa.  The  Chicago,  Milwaukee  and  St.  Paul  Railway,  in  1893, 
completed  a  line  from  Channing  to  Sidnaw,  which  runs  through  Amasa. 

DISTRIBUTION. 

The  iron-bearing  rocks  trend  nortlnvest  and  southeast  from  Crystal 
Falls.  East  of  Crystal  Falls  some  of  the  ore  deposits  are  found  in  prox- 
imity to  the  Henilock  volcanics,  and  follow  along  a  line  located  a  short 
distance  from  them.  Otlier  deposits  are  those  at  Amasa,  about  12  miles 
northwest  of  Crystal  Falls.  These  are  near  the  contact  between  the  Upper 
Huronian  and  Lower  Huronian,  and  above  the  Hemlock  volcanics,  like 
the  deposits  east  of  Crvstal  Falls.  Four  miles  north  of  Amasa  are  the 
explorations  in  sec.  20,  T.  45  N.,  R.  33  W.,  in  which  the  iron-bearing  beds 
are  exposed.  Another  exposure  of  the  iron-bearing  formation  is  in  sec.  34, 
T.  46  N.,  R.  33  W.,  about  4  miles  still  farther  north. 

These  are  the  northernmost  known  exposures  of  the  iron-bearing  rocks 
of  the  Upper  Huronian  in  the  Crystal  Falls  district.  However,  dial- 
compass  and  dip-needle  work  has  located  a  line  of  magnetic  attraction  for 


176  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

about  12  miles  to  the  north  aud  east.  By  meaus  of  this  line  of  magnetic 
attraction,  with  the  assistance  aflforded  by  occasional  outcrops  of  Lower 
Huronian,  Hemlock  volcanics,  the  possible  continuation  of  the  iron-beai'ing 
belt  was  approximately  located. 

I  shall  take  up  the  four  localities  mentioned  in  which  an  iron-bearing 
formation  has  been  found,  and  discuss  them  in  some  detail,  beginning  at  the 
northern  and  least  important  and  passing  to  the  southern  aud  most  important 
part  of  the  district. 

WESTEKN    HALF    OF    SEC.  3-t,  T.  40  N.,  E.  33  W. 

In  the  western  half  of  sec.  34,  T.  46  N.,  R.  33  W.,  there  are  outcrops 
of  a  magnetic  graywacke  which  grades  into  a  rock  that  might  properly  be 
called  a  magnetite-schist  but  for  the  tact  that  its  partial  fragmental  natui-e  is 
still  apparent.  The  rock  contains  a  varying  quantity  of  magnetite,  always 
enough  to  exercise  great  influence  over  the  magnetic  needle.  However,  in 
no  case  have  true  ore  deposits  been  found  in  it,  although  the  vicinity  has 
been  extensively  test  pitted.  The  strike  is  in  general  north  and  south,  Avith 
a  high  dip  to  the  west,  thus  agreeing  in  general  character  with  the  trend  of 
the  Hemlock  volcanics.  The  highest  outcrop  of  the  volcanics  is  a  schistose 
amygdaloid.  After  an  interval  of  no  exposure  of  about  30  feet,  graywacke 
appears,  and  this  grades  up  intt)  the  magnetitic  beds. 

SEC.  iiO,  T.  45  N.,  R.  33  w. 

To  the  south,  in  sec.  20,  T.  45  N.,  R.  33  W.,  are  outcrops  of  ferruginous 
chert,  which  in  places  contains  "bands  and  shots"  of  ore,  the  thicker  bands 
being  an  inch  and  a  half  across.  These  outcrops  have  tempted  prospectors 
to  do  considerable  exploring  by  means  of  both  test  pits  and  diamond-drill 
holes.  The  results  have  been  negative.  The  general  map,  PI.  HI,  shows 
that  the  Upper  Huronian  at  this  place  indents  the  Lower  Huronian  series, 
indicating,  as  has  already  been  said,  the  presence  of  a  westward-pitching 
svncline.  The  presence  of  th"s  syncline  is  further  shown  by  the  strike 
obtained  on  the  outcrops  of  chert  found  at  this  locality.  For  the  most 
part,  a  nearly  north-south  strike  i)revails.  The  greater  part  of  the  northern 
ledges  give  an  east-west  strike,  with  a  variation  of  lint  a  few  degrees  to  the 
north  of  east.  The  southernmost  outcrops  show  a  strike  which  varies  from 
X.  27°  E.  to  N.  34°  W.     The  dip  is  in  all  cases  high,  ranging  from  80°  to 


U.S  GEOLOGICAL  SURVEY 


MONOGRAPH    XXXVI  PL, XVI 


R    33  W 


R   33  W 


DETAIL   GEOLOGICAL,    MAP 

VICINIT\^    OF  AMASA,  MICHIGAN 


JULIUS  aiENaCO  LITM.NY 


SCALE  :  2  INCHKS  -  1  MILE 


CONTOUR  INTERVAL    20  FEET 
0  Outcrops  wthoul  obser\-ed  strike  or  dip 
T  Outcrops  vdth  dptermined  strike  and  dip 
X  Outcrops  with  slatinoas  or  Nrhisto.sity 
.  Tost  pits  bottomed  in  rook 

ALGONKIAN 


LOWER     HIIRONIAN 


HEMLOCK  FORMATION 
Yoltamcs  Normal  Sediments 


'^'hl 


UPPER  HURONIAN 
UNDIVIDED 

I        ^        I 


ORE  DEPOSITS  OF  UPPER  HURON  IAN. 


177 


DRILL  HEMLOCK 


87°.  Tlu'  si'vero  dotoniiatiou  is  olcarly  shown  l)ytlu'  plication  of  the  beds, 
and  by  faults  whose  extent  can  not  be  determined,  but  which  are  accom- 
panied by  rather  extensive  reiljuugsbreccias.  The  breccias  are  cemented 
b\-  iron  oxide. 

THE   AMASA   AREA. 

The  Aniasa  deposits  must  of  necessity  be  very  briefly  described,  as  I 
have  l)een  unable  to  obtain  much  information  concerning  the  relations  of 
the  rocks  as  shown  in  the  closed  mine.  In  the  early  days  of  the  mine  it 
was  thought  by  the 
mine  captain  that  the 
volcanics  fonned  the 
foot  wall  of  the  ore, 
and  on  liis  authority 
Van  Hise  says,  "The 
ore  of  the  Hemlock 
mine  rests  upon  a 
stratum  consisting  of 
surface  volcanic  mate- 
rial "  ^  ^"^'  ^^' — '^^***^^®  section  illustrating  results  of  diamond-drill  work. 

Probably  this  is  a  mistake,  for  the  section  from  west  to  east  (fig.  11), 
i.  e.,  from  the  higher  to  the  lower  beds,  obtained  in  two  drill  holes,  is  as 
follows : 

Feet. 

Gray  sericitic  ulate,  discolored  liy  iron 115 

Chert  and  jasper 59 

Pyritiferous  black  slate  aud  quartzito 180 

Ore  formation .* 3q^ 

Magnetitie  slate 40 

Mottled  slates,  red  and  green,  containing  iron.     Drilling  ceased  after  passing  through 70 

The  thickness  of  the  beds  given  is  the  true  one  and  not  the  thickness 
passed  through  by  the  drill,  which  cut  through  the  beds  at  an  angle.  These 
beds  projected  to  the  surface  are  found  to  be  immediately  underlain  by 
greenstone,  in  some  places  massive,  in  others  tufaceous.  Moreover,  an 
identical  section  is  shown  by  a  drill  hole  4  miles  southeast  of  Amasa,  in  sec. 
26,  T.  44  N.,  R.  33  W.  It  was  carried  deeper,  however,  and  after  passing 
through  the  mottled  slate  was  bottomed  in  greenstone.     From  these  drill 


LOWER 


huroniaN 


The  iron  ores  of  the  Marquette  district  of  Michigan,  by  C.  R.  Van  Hise;  Am.  Jour.  Sci.,  vol. 
43,  1892,  p.  130. 

MON   XXXVI 12 


178  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

holes  it  appears  eertaiu  that  the  ore  formation  is  immediately  underlain  and 
overlain  by  black  slates.  The  foot  and  hanging  slates  are  much  alike,  the 
hanoino-,  however,  being  verj^  pyritiferous,  and  the  foot  containing  much 
more  iron  than  the  hanging.  This  iron  is  in  the  form  of  hematite  and  mag- 
netite. Below  the  black  magnetic  slate  is  the  ferruginous  mottled  slate, 
which  apparently  lies  next  to  the  Lower  Huronian  Hemlock  volcanics. 
The  so-called  ore  formation  consists  of  banded  chert  and  jasper,  with  which 
the  hematite  bodies  are  associated. 

The  results  obtained  from  these  holes  show  the  lenticular  character  of 
tne  ore  bodies  and  the  difficulty  in  finding  them.  One  of  the  holes  passed 
through  the  ore  formation,  l)ut  missed  the  ore  body,  which  subsequent 
undero-round  work  showed  it  would  have  struck  had  it  cut  the  formation  50 
feet  farther  north. 

The  distribution  of  the  Huronian  rocks  in  the  vicinity  of  Amasa  is 
shown  on  the  map,  PI.  XVI. 

THE  CRYSTAL  FALLS  AREA. 

The  most  of  the  observations  upon  the  ore  bodies  and  their  relations  to 
surrounding  beds  have  naturally  been  made  in  the  vicinity  of  the  town  of 
Crvstal  Falls,  where,  owing  to  the  extensive  development  of  the  mines,  the 
underground  conditions  could  best  be  studied.  The  conclusions  reached, 
however,  are  confidently  believed  to  hold  good  for  the  entire  Upper  Huro- 
nian of  the  district. 

In  the  description  of  the  folding  of  the  Upper  Huronian  it  was  stated 
that  the  Crystal  Falls  area  is  in  a  syuclinorium  forking  as  the  result  of  a 
subordinate  central  anticline  so  as  to  produce  a  U  opening  to  the  south  of 
west.  It  is  in  this  basin  that  the  important  mines  of  the  Crystal  Falls  dis- 
trict are  situated.  One  row  of  mines — the  Hollister,  Ai-meuia,  Lee  Peck, 
and  Hope— as  shown  by  the  map  (PL  XVII),  lies  to  the  west  and  north- 
west of  the  main  mass  of  Hemlock  volcanics  between  Crystal  Falls  and 
Mansfield.  A  second  and  more  important  set  of  mines  follows  an  east-west 
line  south  of  the  subordinate  area  of  volcanics,  which  lie  just  north  of  Crys- 
tal Falls  in  the  midst  of  the  Upper  Huronian  sediments.  The  second  set  of 
mines,  including  the  Crystal  Falls,  Great  Western,  Lincoln,  Paint  River, 
Lamont,  Youugstown,  and  Claire,  lies  near  the  axis  of  the  syncline — that 
is,  along  the  line  of  major  folding,  and  consequently  greatest  mashing.    The 


17 


U.S. GEOLOGICAL  SURVEY 


DKTAII,  GE0L0(;K;AI,  M.U'  OF  THK  MCIMT^OF  (m'STAL  I'AIJ.S  AM)  MAN'SFIKI.D.  SHKF.T  I 


I-ONTUL-Ii  1.NIEUVAL.20  Hi  1. 1 

ned  wnlic  Juitl  <li|i  -f  Outcrvpe  wiili  alatuicss  or  sctiiSU)Bl(>- 


LOTVER  IjURONlAN" 


MAN9FI&l.n  SLATE  HKMLOCK    FORMATION 

I    Aim  1  rAJK:^^! 


lliPP55_5S^oNiAN" 


INTRUSIVE 


DIOHITE 


18 


I 


T^ 


US-GEOLOOrCAL  SURVEY 


MONOGPAPH    XXXVI  PL   XVIIl 


DKTAII.  GI:01.0GR:AK  MAP  Ol-'  THK  MClXm  OK  CRVSTAL  FM.LS  AND  NLVN'SMKLO.  SlIEP:'!'  IT 


CONTOUW   lNTKH\-.'U..ZO  I'L^KT 
cltijja  "11"  uotDiTiunBi]  BinKi-  unaiUp  'V  OUUit-pn  vvilli  a 

■  Tcsi  T""*  ''olioiiicil  111  rork 
AlXJONKTAN 


LOWKR  irt-nONLW  UPPER  HLTtONlAW 

HEMl^CK    VOnUXnOtt  L'KPn.TDCU 


nOLERITE 


DIORITK 


OKE  DEPOSITS  OF  UPPEK  UURONIA>;. 


179 


CV'luinlnu,  Duuu,  Miistodou,  juid  others  to  the  west  (PL  XVIII),  sire  prol)- 
ablv  the  westeru  conthiuatiou  of  tliis  hue  of  luiiies,  and  follow  the  trend  of 
the  main  synclinal  axis  of  the  district.  The  position  of  these  mines  with 
reference  to  the  main  structural  features  of  the  district  can  be  seen  on  the 
relief  map  and  the  sketch  map  corresponding  to  it,  PI.  XIV. 

The  section  made  through  the  closely  folded  Upper  Huronian  lieds  by 


w 


Fill.  12.— Sketcli  illustraliiig  coiitortioD  of  Ujiper  Huroiiiaii  strata. 

the  Paint  River  affords  the  best  opportunity  in  the  district  for  studying  the 
rocks,  but  the  rocks  are  so  crumpled  that  even  here  the  succession  was  not 
made  out  with  certainty. 

The  sketch  fig.  12,  by  W.  N.  Merriam,^  shows  the  folding  of  the  slate  and 
chert  strata  as  seen  in  the  railroad  cut  between  the  Paint  River  and  the  Lin- 
coln mine.  The  strike  of  the  rocks  is  about  N.  80°  E.  The  sketch  is  taken 
looking  almost  along  the  strike 

of  the  beds.     In  fig.  13  a  sec-  ,..-•''"  \. 

ond  sketch  is  given,  also  hj 
W.  N.  Merriam,  which  illus- 
trates the  rapid  change  in  strike 
in  these  beds,  due  to  the  con- 
tortion of  the  strata.  This 
change  is  seen  near  the  east 
end  of  the  wagon  bridge  just 
across    the   Paint  River  from 

OrVStal     Falls         At    this    nOint      ^'^'  ^^' — S^^tch  showing  change  of  strike  of  Upper  Huronian  beds 
•^  '  i  ilue  to  the  folds. 

the  beds  bend  from  a  strike  ot 

S.  40°  E.  to  W.  10°  S.     The  change  takes  place  by  means  of  three  very 

sharp  bends. 

The  following  are  the  observations  made  by  Rominger  upon  these  expo- 
sures near  Crystal  Falls: 

Among  the  recently  discovered  productive  fields  for  irou  mining,  the  vicinity  of 
Crystal  Falls  has  become  famous  for  its  wealth  in  ore.    The  formation  enclosing  the  ore 


'  Manuscript  notes. 


180  THE  CRYSTAL  FALLS  IRON-BEAKING  DISTRICT. 

deposits,  has  there  a  great  thickness,  but  its  determination  by  actual  measurement  is 
,  impossible,  on  account  of  the  much  folded  condition  of  the  strata,  and  for  want  of  con- 
nected exposures  transverse  to  the  stratification.  Estimating  its  thickness  to  several 
thousand  feet  is  surely  not  far  beyond  the  truth.  This  folded  condition  of  the  strata 
is  in  many  instances  an  obstacle  in  the  decision,  whether  in  a  given  locality  we  have 
under  observation  a  descending  or  an  ascending  succession  of  beds. 

If  we  follow  the  railroad  from  Crystal  Falls  village  upward  along  the  bed  of 
Paint  River,  we  find,  in  the  first  cut  the  road  makes  into  rock  beds,  a  series  of  hard, 
black  slates,  transversely  intersected  in  almost  vertical  positions,  and,  according  to 
their  cleavage  planes,  dipping  in  southwest  direction.  This  cross  cut  is  210  steps  long; 
thence,  for  the  distance  of  100  steps,  no  rock  ledges  are  touched  by  the  roadbed,  but 
on  the  left  side  of  the  road  similar  slate  rocks  are  denuded,  which  apparently  represent 
a  continuation  of  the  former  succession.  From  here  for  eighty  steps  a  cut  is  made 
through  similar  slate  rocks,  but  interlaminated  with  numerous  quartzite  seams; 
further  on,  the  intersection  of  slates  in  alternation  with  quartz  seams  continues  for 
quite  a  while,  but  these  slate  rocks  are  more  graphitic  than  the  former  and  readily 
disintegrate,  on  exposure,  into  splintery  fragments,  as  they  contain  a  large  proportion 
of  iron  pyrites  and  rusty  ferruginous  seams  causing  the  decay.  By  this  time  we  have 
reached  close  to  the  river  below  its  falls,  and  find,  laid  open  in  its  embankments  formed 
by  the  bluffs  thirty  feet  high,  a  further  conformable  series  of  graphite-schists,  300  feet 
wide.  Beneath  the  graphite-schists,  close  to  the  water  level  at  the  foot  of  the  falls, 
succeeds  an  ore  belt  six  feet  wide  at  the  surface,  but  widening  to  fifteen  feet,  followed 
into  the  hillside.' 

Below  the  ore  belt  follows  an  immensely  large  succession  of  thinly  laminated 
banded  ferruginous  quartz-schists  of  dark,  rusty  color,  which  beds,  in  steeply  erected 
position  crossing  the  river  bed  diagonally,  give  a  cause  to  falls  eight  or  ten  feet  in 
height.  The  exposed  succession  of  beds  amounts  at  the  falls  to  a  thickness  of  over 
800  feet.  Intermixture  of  pyritous  shaly  seams  with  the  quartzite  beds,  induces  their 
rapid  disintegration  on  exposure,  into  shelly  fragments  covered  with  an  iridescent 
varnish  like  coating  of  oxide-hydrate.  These  beds  are,  in  the  embankments  on  the 
opposite  river  side,  remarkably  corrugated,  describing  in  their  flections  perfect  coils.^ 

CHARACTER  OF  THE  ORE. 

The  ore  obtained  from  the  Crystal  Falls  district  is  chiefly  soft  red 
hematite,  though  iu  places  it  is  hydrated  aud  graded  as  brown  hematite 
(limonite).  The  ore  is  very  porous  and  shows  many  crystal-lined  cavities. 
At  places  a  hard  steel  hematite  ore  is  found,  which  runs  as  high  as  70  per 
cent  metallic  iron.  This  ore  occurs  in  very  small  quantities  associated  with 
the  soft  ores,  and  appears  for  the  most  pai-t  to  have  formed  iu  geodal 
cavities.  When  the  cavities  are  still  partly  open,  the  Ore  has  botryoidal  and 
stalactitic  forms.     The  ores  are  very  similar  to  the  ores  of  the  Michigamme 

'Iron  and  copper  regions  of  the  Upper  and  Lower  Peninsulas  of  Michigan,  by  C.  Rominger: 
Geol.  of  Mich.,  Vol.  V,  1895,  Pt.  1,  p.  74. 
-Ibid.,  p.  75. 


ORE  DEPOSITS  OF  UPPEK  IIURONIAN. 


181 


slatos  of  the  Upper  ^I;ir(|iu'tte  series,  Ijut  (litter  very  coiisidei'ablv  from  tliose 
of  the  Lower  ]\Iai-quette  series,  in  whieh  the  hard  hematites  and  magnetites 
arc  important  ores,  and  from  the  ores  of  the  Menominee  district,  wliieh  pro- 
duces Uirg-e  quantitie(5  of  soft  Ijhie  hematite,  some  inartite,  and  also  some 
specular  ore. 

The  following  figures  show  the  average  composition  of  the  ores  for  the 
district.  They  were  taken  from  analyses  furnished  b}-  the  management  of 
the  various  mines  and  from  the  reports  of  the  State  commissioner  of  mineral 
statistics  of  Michigan. 

The  metallic  iron  of  the  ores  ranges  from  54  to  63  per  cent,  the  aver- 
age being  about  59  per  cent.  Phosphorus  in  exceptional  cases  is  as  low 
as  0.05  per  cent,  though  usually  ranging  from  0.1  to  0.7  per  cent,  most  com- 
monly approaching  the  higher  figure.  Silica  averages  about  3  per  cent. 
These  analyses  show  the  ore  to  be  ratlier  low  grade.'  It  is  due  to  this  that 
this  district  has  been  so  sensitive  to  the  prices  of  iron  ores.  A  low  market 
price  makes  the  cost  of  production  exceed  the  selling  value,  and  under 
these  conditions  work  necessarily  stojis. 

Some  of  the  ores  in  the  Crystal  Falls  district  contain  a  ver}'  high 
percentage  of  AI2O3,  CaO,  and  also  of  manganese.  It  is  reported  that 
some  very  good  deposits  of  manganese  have  been  found,  one  unauthenti- 
cated  statement  being-  to  the  effect  that  an  analysis  of  the  ore  runs  as  follows: 
Metallic  iron,  17.46;  manganese,  29.81;  phosphorus,  0.064;  silica,  0.009(?). 

'  Brooks  states  that  "  the  ores  are  unlike  those  in  the  more  easterly  part  of  the  Menominee  region 
in  being  richer  in  iron,  freer  from  silica,  and  in  containing  more  water."  (Analysis  68,  p.  302.)  Geol- 
ogy of  Michigan,  Vol.  I,  part  1,  p.  182. 

Since  the  above  was  written  the  volume  on  Mineral  Resources  of  the  United  States,  1896,  Part 
V,  of  the  Eighteenth  Annual  Report  of  the  United  States  Geological  Survey,  has  appeared,  and  the 
following  analyses  of  ores  from  the  Crystal  Falls  district  are  taken  from  Mr.  John  Birkiubiue's  article 
on  iron  ores  in  that  report. 

The  analyses  were  prepared  for  the  ore  association  at  Cleveland.  Ohio,  and  show  the  average 
cargo  analyses  of  iron  ore  as  shippeil  from  the  various  mines.  The  analyses  were  made  from  ores 
dried  at  212^,  the  amount  of  natural  moisture  being  added. 


Iron. 

Silica. 

Phos- 
phorus. 

Man- 
ganese. 

Alu- 
mina. 

Lime. 

Mag- 
nesia. 

Orgjinic 
Sul-        and 
l)hur.  ivolatile 
'  matter. 

Mois- 
ture. 

Crystal  Falla  ..,. 
Dunn ^. 

58.55 
58.61 
60.20 
60.86 
61.00 

4.25 
3.88 
4.71 
3.86 
4.50 

.721 
.573 
.309 
.240 
.350 

.20 
.58 
.39 
.45 
.30 

1.16 
1.88 
2.81 
1.67 
2.75 

2.64 
1.80 
1.87 
2.14 
.50 

.77 

.83 

1.32 

1.73 

.30 

.  0U8  t      2.  92 
.033  1      5.44 

.010    

.010    

.075    

7.20 
8.70 
6.62 
6.50 
9.00 

Lincolu 

Mastodon 

182 


THE  CRYSTAL  FALLS  IROK-BEARING  DISTRICT. 


One  of  the  best  results  gives  as  high  as  61.5  per  cent  metallic  Mn. 
The  ores  thus  range  from  a  manganiferous  iron  ore  to  a  manganese  ore. 
The  further  statement  is  made  that  the  bed  lies  very  close  to  the  surface, 
and  is  from  6  inches  to  3  feet  in  thickness.  From  4  to  6  feet  of  bog  iron 
is  found  underlying  tlie  bed  of  manganese. 

RELATIONS  TO  ADJACENT  ROCKS. 

The  ore  is  associated  with  white  or  reddish  chert,  which  in  places  is 
jaspery.  The  cherty  iron  formation  passes  into  ore  by  a  decrease  of  the 
silica.  An  intermediate  phase  is  chert  with  "bands  and  shots"  of  ore.  In 
places  the  chert  is  more  or  less  brecciated,  and  the  ore  often  has  a  similar 
character.  Commonly  the  ore  is  completely  surrounded  by  the  chert  beds, 
or  chert  and  ore,  forming  the  so-called  mixed  and  lean  ore.  In  such  cases 
theA"  form  both  the  foot  and  hanging  walls  of  the  ore 
bodA".  But  the  ore-bearing  chert  formation  is  always 
associated  with  black  carbonaceous  slates,  which  consti- 
tute the  base  on  which  the  ore-bearing  formation  rests. 
In  the  Youngstown  mine  3  feet  of  so-called  "graphite" 
was  jjassed  through  before  the  usual  carbonaceous  slates 
were  reached.^  The  hanging  wall  is  also  carbonaceous 
slate.  At  places  thin  quartzitic  beds  which  approach  a  true  quartzite  are 
associated  with  the  slate. 

The  ores  occur  in  the  cherts  in  pockets  and  lenticular  masses,  which 
always  agree  in  greater  dimensions  with  the  strike  of  the  beds  with  which 
thev  are  associated.  The  lenticular  character  is  well  shown  in  the  Dunn, 
Columbia,  and  Great  Western  mines.  In  the  Dunn  mine  the  bodies  over- 
lap. In  the  Great  Western  mine  in  1887  seven  different  ore  bodies  in  an 
east-west  line,  separated  by  areas  of  barren  rock,  mostly  slate,  were  being 
mined.  In  following  these  isolated  ore  bodies  to  the  east,  at  various  places 
thev  are  found  to  turn  around  a  horse  of  rock.  Their  occurrence  is  illus- 
trated by  the  horizontal  section,  tig.  14.  Evidently  the  ore  bodies  accumu- 
lated in  westward-pitching  synclinal  troughs,  in  which  the  hanging  wall 
appears  to  the  miners  as  a  horse  of  rock. 

The  ore  bodies  in  general  pitch  to  the  west  at  varying  angles  corre- 


FiG.  14.— Sketch  to  illus- 
trate the  occurrence  of  ore 
bodies. 


'This  information  was  furnished  by  Mr.  C.  T.  Roberts,  of  Crystal  Falls, 
obtain  a  specimen  of  the  graphite  for  examination. 


It  was  not  possible  to 


ORE  DEPOSITS  OF  UPPER  HURONIAN.  183 

.spoiiding-  ro  the  pitches  of  the  axes  of  tlie  syiielhies  in  which  thev  occur. 
Tlie  pitdu's  of  these  folds  in  turn  correspond  to  the  westward  pitch  of  tlie 
Crvstal  Falls  syncliuoriuin,  of  which  the  secondary  synclines  containing 
rlie  ore  l>odies  are  a  part.  A  typical  example  of  the  occurrence  is  shown  in 
the  Armenia  mine  ore  body,  which  is  found,  according-  to  Van  Hise,  "at  the 
bottom  and  on  the  sides  of  a  synclinal  trough,  pitching  at  an  angle  of  about 
^,-o"i      rpi^^^  trend  of  the  axis  is  to  the  south  and  west. 

The  dip  of  the  ore  bodies  is  always  steep,  and  generally  to  the  south, 
but  varies  in  places  to  a  few  degrees  north. 

ORIGIN. 

The  fact  tliat  tlie  important  mines  in  tlie  district  are  located  in  a 
synclinal  basin  and  that  they  all  possess  an  impervious  footwall  of  black 
slate  gives  very  clearly  the  reason  for  their  existence  and  indicates  their 
mode  of  origin.     They  are  concentrates  in  synclinal  troughs. 

In  the  ]\Iarquette  and  Penokee-Gogebic  districts  the  ore  Ijodies  are 
frequently  found  associated  with  dikes  of  dolerite  (diabase),  which  have 
been  altered  to  "diorite "-schists,  and  so-called  soapstone  or  paint  rock.^ 
Only  one  such  association  is  known  for  the  Crystal  Falls  district.  Wads- 
worth  mentions  having  seen  a  dike  in  the  Paint  River  mine.^ 

In  the  field  notes  of  the  Lake  Superior  survey  for  1<S91,  I  find  the 
statement  made  that  "the  strata  of  the  ore  formation,  which  here  strikes 
nearly  east  and  west,  is  cut  by  an  eruptive  dike  which  runs  about  north- 
west and  southeast.  This  dike  hades  to  the  west,  and  forms  with  the 
hanging  slates  of  the  ore  formation  a  trough  pitching  to  the  west  at  a  very 
steep  angle.  In  this  trough  is  situated  the  ore  body  upon  which  the  Paint 
River  and  the  Monitor  '^  mines  are  working."  This  ore  body  is  stated  to  be 
about  100  feet  wide,  300  feet  long,  and  of  unknown  depth.  When  I  was 
in  the  district,  the  mines  were  closed,  or  only  shipping  from  stock  piles,  so 
that  I  had  no  opportunity  of  verifying  this  observation.  From  this  state- 
ment it  appears  that  in  this  particular  case  the  ore  is  due  to  the  presence  of 


Iron  ores  of  the  Marquette  district  of  Michigan,  l>y  C.  R.  V.an  Hise;  Am.  .Tonr.  .Sci.,  3d  series, 
Vol.  XLIII,  18fl2,  pp.  130. 

-Mernaui  also  mentions  in  his  notes  a  dolerite  dike  found  cutting  the  ferruginous  rocks  at  the 
Glidden  exploration.     In  this  case  it  does  not  appear  that  an  ore  body  was  foruu-d. 

"  Sketch  of  the  geology  of  the  iron,  gold,  and  copper  districts  of  Michigan,  by  M.  E.  Wads- 
worth  :  Kept.  State  Board  of  Geol.  Survey  for  1891-92, 1893,  p.  108. 

■•Now  known  as  Lamont  mine. 


184  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

this  dike,  as  It  occurs  in  a  pitching  trough,  formed  by  its  junction  with  the 
impervious  slate.  These  same  relations  are  well  known  to  be  the  cause  of 
similar  occurrences  in  the  Lake  Superior  districts  above  mentioned. 

The  original  rock  from  which  the  ores  were  formed  was  cherty  iron 
carbonate,  which  in  many  places  is  found  associated  with  the  iron-bearing 
formation.  The  cherty  carljonate  shows  the  various  stages  of  alteration 
from  the  compact  cherty  siderite  to  the  banded  ore  and  chert  rocks  which 
form  the  nuclei  for  the  addition  of  the  iron  obtained  from  the  higher  exten- 
sions of  the  beds.  Percolating  waters  have  been  the  agents  in  this  process 
of  replacement  and  concentration.  Consequently  where  the  rocks  have  been 
most  shattered,  we  find  the  water  was  especially  active.  Hence  it  is,  also, 
that  we  find  the  deposits  in  this  closely  folded  part  of  the  Upper  Huronian. 

As  to  the  origin  of  the  cherty  carbonate  itself,  we  know  nothing 
definite.  Its  association  with  the  carbonaceous  slates  would  indicate  the 
ao-ency  of  organic  matter  in  its  prodiiction,  possibly  in  some  such  manner 
as  is  rather  generally  accepted  for  the  formation  of  the  Carboniferous 
carbonate  ores.  The  Upper  Huronian  ores,  as  well  as  the  Lower  Huronian, 
are  supposed  to  have  been  formed  in  this  same  way,  and  from  the  same 
kind  of  rock.  Under  the  discussion  of  the  Lower  Huronian  ores  (p.  70) 
these  points  were  discussed  more  in  detail,  and  references  given  to  the 
literature,  and  the  reader  is  referred  to  that  discussion  for  further  details. 

SIZE  OF  THE  ORE  BODIES. 

No  definite  general  statement  can  be  made  as  to  the  size  of  the  ore 
bodies,  as  this  varies  considerably.  None  of  the  bodies  which  are  being 
worked,  so  far  as  I  can  learn,  are  less  than  30  feet  wide.  In  one  of  the 
old  mines  crosscuts  disclosed  a  width  of  nearly  200  feet.  This  same  ore 
body  is  reported  to  be  at  least  one-foin-th  of  a  mile  long. 

METHODS    OF    MINING. 

The  first  develoinnent  of  the  iron  ores  of  this  district  was  by  the 
stripping  and  open-cut  method,  very  few  resorting  at  once  to  under-ground 
work.  Nearly  two-thirds  of  the  product  of  certain  of  the  mines  has  been 
from  open-pit  work.  When  the  open  pits  become  too  deep  to  be  readily 
worked  as  such,  shafts  are  sunk  and  the  exploiting  of  the  ore  body  is 
carried  on  under  ground,  at  times  both  open-pit  and  under-ground  work 
being  carried  on  simultaneously.     The  Mastodon  presented  the  unusual 


ORE  DEPOSITS  OF  UPPER  HURONIAN.  185 

siflit  of  ;m  o\)(.'U  pit  c'Xten(liii<4-  down  200  feet,  part  of  tlic  wdrkinf^'s  still 
boiii"'  roofed  over  by  an  enormous  arch  of  ro(dc.  All  work  in  tlie  district 
is  at  present  under  gTound.'  As  a  rule,  the  nndcr-yroun<l  work  has  not,  thus 
far,  been  carried  to  very  great  depth,  the  two  deepest  mines  beinf^^-  the 
Great  Western  and  the  Dunn,  which  are  down,  respectively,  700  and  720 
feet.  The  cithers  are  down  to  depths  varying-  from  100  to  450  feet,  the 
lowest  figures  being,  as  a  rule,  for  the  youngest  mines,  the  more  important 
ones  having  nearly  reached  the  400-foot  level  or  passed  beyond,  it. 

In  the  early  mining  days  of  the  district  an  extensive  system  of 
timbering  was  resorted  to,  but  gradually,  as  the  cost  of  timber  increased 
and  this  item  became  burdensome,  careful  attention  was  paid  to  this  point, 
and,  where  practicable,  the  system  of  caving  or  the  sj'stem  of  tilling  was 
introduced  (Mastodon).  At  the  present  time  most,  if  not  all,  of  the 
important  producers  are  mined  with  open  stopes,  pillaj's  being  left  only 
when  necessary.  From  this  it  would  appear  that  the  rocks  had  not  been 
much  broken,  but  it  should  be  borne  in  mind  that  the  ore  deposits  them- 
selves are  later  than  the  folding,  and  in  the  process  of  their  formation  many 
of  the  cracks  in  the  surrounding  beds  could  have  been  filled  with  ore  or 
other  material  and  the  rocks  thus  quite  rigidl}'  united  again.  That 
cementation  has  taken  place  is  evident  from  an  examination  of  almost  any 
hand  specimen  or  slide,  where  one  may  see  veins  of  infiltrated  quartz 
traversing  them  at  various  angles.  The  extremely  wet  character  of  the 
Great  Western  would  seem  to  indicate  that  localh'  the  rocks  may  still  be 
very  much  fissured. 

PROSPECTING. 

Owing  to  the  impossibility,  with  our  present  knowledge,  of  mapping 
the  various  beds  of  the  Upper  Htironian,  it  is  not  possible  to  g'ive  any 
directions  with  reference  to  the  exact  lines  which  should  be  followed  in 
searching  for  ore.  However,  since  the  areas  which  are  underlain  by 
igneous  rocks  have  been  delimited,  there  is  no  longer  an}"  excuse  for  wast- 
ing time  and  money  in  prosjjecting  in  these  unpromising  portions  of  the 
district.  W^here  indications  point  to  considerable  rock  movements,  and 
where  the  sideritic  rocks  are  found  associated  with  impervious  slates, 
explorations  are  warranted. 

'  This  waa  written  in  1896.  .Since  tlien  a  large  part  of  tlie  Mansfield  ore  body  has  been  stripped, 
and  this  \u:\y  be  worked  at  present  by  open-cut  methods. 


186  TOE  CRYSTAL  FALLS  IRON-BEAKING  DISTRICT. 

PRODUCTION  OF  ORE  FROM  THE  CRYSTAL  FALLS  AREA. 

In  the  following  table  the  iirst  column  contains  the  names  of  the  mines 
or  combinations  of  mines  of  the  Crystal  Falls  area.  These  are  arranged 
alphabetically  for  ease  of  reference,  and  not  according  to  date  of  opening 
or  amount  of  ore  produced,  as  is  so  usually  the  case.  In  one  case,  that  of 
the  Claire  mine,  the  mine  was  operated  by  the  company  operating  the 
Youngstown,  and  its  output  accredited  to  that  mine  until  1891,  when  the 
two  mines  were  separated.  Following  the  name  under  which  a  mine  is 
known  at  present,  there  is  given  in  parentheses  in  chronological  order  the 
name  or  names  by  which  the  mines  were  formerly  known.  The  second 
column  gives  the  location  of  the  mine.  Following  these  data  there  are 
arranged  in  columns  the  yearly  shipments  from  the  time  of  the  first  open- 
ing to  the  closing  of  the  mines,  or  to  January  1,  1899.  In  many  cases  the 
ore  body  had  been  definitely  located  and  considerable  work  done  and  ore 
accumulated  upon  stock  piles  several  years  before  the  first  shipments  were 
made,  but  it  would  be  impossible  to  give  the'  exact  date  of  the  opening  of 
the  mine  unless  we  considered  the  year  of  first  shipment  as  such.  In  a 
column  following  the  yearly  shipment  the  total  shipment  for  each  mine  is 
given.  This  is  followed  by  a  column  giving  the  year  in  which  the  maxi- 
mum shipment  was  made,  and  by  another  giving  the  amount  of  this  ship- 
ment. In  the  horizontal  column  at  the  foot  of  the  page  may  be  found  the 
total  shipments  for  each  year  and  the  total  product  of  the  district  since  its 
first  exploitation.  The  figures  for  the  district  have  been  obtained  either  by 
correspondence  with  the  mining  companies  or  from  the  annual  reports  of 
the  commissioner  of  mineral  statistics  of  Michigan.  Acknowledgments  are 
due  to  certain  of  the  companies  which  have,  through  their  managers,  been 
ver-v  obliging  in  furnishing  infonnation  concerning  the  mines  they  operated. 

From  a  comparison  of  the  total  shipment  of  the  area  for  1898  with  the 
total  shipment  of  the  Menominee  range  2,522,265  long  tons  and  of  the  entire 
Lake  Superior  region  14,024,673  long  tons  for  the  year,  it  will  be  seen  that 
the  Crystal  Falls  area  furnished  13  per  cent  of  the  total  iron-ore  shipment 
of  the  range  and  2^  per  cent  of  the  region.^ 

'  Total  shipments  for  1898  were  obtained  through  Mr.  .John  Birkinbiue  from  the  Iron  Trade  Review. 


IRON-ORE  SHIPMENTS  OF  THE  WESTERN  HALF  OF  THE  CRYSTAL  FALLS  DISTRICT,  GIVEN  IN  TOSS  OF  2,240  POUNDS. 


Name  of  mine. 

Location. 

Operated  by — 

1S6-2 

_ 

issa 

1$S4 

ISSo 

18S6 

1VS7 

18)«» 

1S89 

1800 

1S91 

1S98 

1893 

1894 

1805 

1890 

1897 

I8D8 

Total 

shipmpnl  of 

indiviihial 

miDes  and  uf 

district  10 

date. 

T.ar 
ot  niitx- 
ilniiiu 
Bliip. 
ment. 

Ani„,iiit 
orshi),- 
nieiit. 

-Van.e  „r  ,„ine. 

E.J  SE.  J  sec.  23.  T.  ja  N.,  R.  33  W , 

47,775 

26,649 

2,045 

76.460 

121. 963 

484. 190 

1.341 

266. 331 

33.246 

1,025,679 

375,742 

375, 023 

4,098 

17,818  . 

140.871 

2,844 

45. 174 

8,204 

306, 845 

425.290 

1,702 

222,371 

158,899 

1889 
1692 
1896 
1882 
1898 
1886 
1801 
1892 
1^97 
1890 
1892 
1892 
1892 
1892 
1889 
1893 
1890 
1895 
1890 
1890 

47,775 
57,351 
87,202 

•1,341 
128, 233 
17,684 
162,721 
87,487 
96,032 

2,020 
15,543 
42,819 

2,844 
26,020 

4,006 
69,558 
66, 559 

1,071 
62.654 
41,460 

ClairC 

>'E.4SE.Jsec.l9.T.43N.,K.32^V 

NW.|NW.^SfC.31.T.43N..E.32W 

Lot3.scc.20.T.43K.,  B.32W 

Claire  Mining  Co 

55, 000 
70,770 

57,351 
57.682 

9,012 
22, 426 



Annenia<Siiiitii,. 

15, 940 
1,341 

4.334 

.77. 

U,282 

2.337 

10,936 

11.365 

60, 133 

10, 300 

70,867 

87,202 

24.623 

14.199 

Crvstal  Falls 

ColtUBbia  (1.  Uuion ;  2-  .Sbeldon  i  iScliaf.^r; 

Crystal  Fiills 

SE.iNE.jBBC.21,T.43N.,E.32W 

XE.jS'W.4  8ec.24,T.42N.,K.33'W 

Corrigau.McKinnev  &  Co 

3.975 

14,387 

44,526 

95.210 

123,233 

Delphic 

3,410 

508 

9.813 

17,081 

1,8)1 

118.001 
21,861 

151,828 
38,451 

156, 653 
71,719 

162,721 
62,464 
35. 531 

1,057 

133, 945 
87,487 
6.5,459 
1.021 
15,  S43 
42, 819 
2,844 
26,020 

58, 261 

661 

11,323 

90,886 

47.081 

31,062 

49,381 

Great  WesterD  (ImD  Star) 

NE.iSW.Jaeo.21,T.43N.,B.32'n' 

NW.JSW.i8ec.4,T.44N.,R.33-\V 

Iron  Star  Co 

5S7 

22, 825 

20,722 

25,725 

23,2(0 

DuoD. 

Hemlock 

1,046 

05.767 

96,032 

69,865 

Holliater 

NW.5ST;r.jBec.l3,T.43N..E.32W.'. 

1                   i 

2,020 

HoUister. 

Hoped-  Blaney;  2,  AVauneta) | 

NE.JSE.l8ec.27,T.43N.,R.32W 

1 

2,275 
13,777 

Laniont  (Monitor) ' 

Lot(J,NW.iSE.jBec.20.T.43N..E,32W 

W.jNE.iaec.2C.T.43N..R.32W...  



2.090 

21, 620 

31.139 

26, 226 

2,600 



Lament  (Monitor). 

Lee  Peck. 

Lincoln  (Fairbanlcs) 

Manhattan  (Soutli  Mastoclon). 

Mans&tld  (Caleilonial.t 

Msalodon. 

Lee  Peck  ..-. 



■n'.iSW.Jsec.21,T.43N..R.32-W 

Lincoln  Mining  Co 

8,131 

453 

1        ■ 

1,813 

8,767 

i 1 

Manbattan  (Sonlh  Mastodon) 

KE.J  SE.  1  sec.  13. T. 42 N., E. 33  W 

2,722 

4.000 

1,476 
18.303 
66,559 

.     ..  . 

1 

Mansfield  (Caledonia)* 

KW.iNW.i8eo.20.T.43N..E.31W 

SE.JNE.Jsec.l3.T.42?r.,E.33-W 

De  Soto  Mining  Co 

;:;.:;:::::::::: 

49,836 
45.370 

69,259 
9,150 

09,558 

23, 485 

505 



39,612 

60,877 

Miistoilon ' 

3,477 

18, 577 

18, 020 

11,73? 

jl,»0 

49,  lis 

51,293 

63.086 

23,781 
1,071 

Michigan 

NE.jN\V.j8ec.9,T.44K.,E.33-\V 

XE.iSE.i8ec.20,T.43N..E.32-n- 

Michigan  Exploring  Co 

210 

Paint  Ki ver 

Paint  River  Iron  Co 

Illinois  Steel  Co 

6.515 
6.188 

5,073 
15,292 

11,652 
8,  .143 

2.373 

13, 033 

25,638 

10.210 
34,  418 

12,506 
12.700 

32,700 
7,471 

02,654 
44.460 

45. 435 
3,705 

18,390 

Paint  River. 
Youngstown, 

Yoiingstown 

J.nv.}  Sn'.isec.20.  T.43N..  E.32W 

13 

601 

1 

Total 

42. 1S9 

70,  SCI 

06, 019 

22,953 

138,902 

140, 621 

2J2, 799 

378, 322 

.^40, 040 

559, 828 

580,070 

220, 040 

12,000 

134,096 

274. 576 

286. 610 

392, 555 

4,036,448 

MON  XXXVI — fact' ]).  ISi; 


■  Previous  to  IfiDl  Clairo  and  Yonngstown  products  were  quoted  together  and  credited  to  Youngstown,  though  they  were  about  eiiually  divided, 
t  LowiT  Iluionian.    Only  pruduclive  EeSBi-mer  mine  in  Crystal  Falls  area. 


1 


■ 


CHAPTER    YI. 
THE  INTKUSIVES. 

Under  tliis  general  head  there  is  here  included  an  extremely  varied 
assortment  of  rocks  exhibiting  in  common  intrusive  relations  to  sedimentary 
and  igneous  rocks.  This  division  is  here  used  merely  because  it  simplifies 
the  classification  of  the  rocks  of  the  district,  and  the  term  "intrusives"  is  not 
to  be  interpreted  as  synonymous  with  the  "dike  rocks"  (ganggesteine)  of 
some  authors,  a  petrographical  division  which,  in  the  opinion  of  the  writer, 
is  not  warranted. 

These  intrusive  rocks  differ  very  materially  in  field  occurrence,  petro- 
gi-aphically,  and  in  point  of  age  from  the  igneous  rocks  thus  far  described. 
In  age  much  younger  than  the  volcanics,  they  still  bear  a  close  resemblance 
to  some  of  them;  indeed,  some  forms  are  identical  in  character.  Massive 
granular  rocks  are  the  common  forms.  Porphyritic  varieties  are  very 
subordinate. 

The  rocks  are  all  considered  as  intnisives  into  either  the  Lower  or  the 
Upper  Huronian.  In  most  cases  the  intrusive  relations  may  be  said  to  be 
rather  inferred  than  demonstrated,  for  the  direct  contacts  have  been  observed 
in  very  few  cases.  However,  where  isolated  sets  of  knobs  of  eruptive  rocks 
are  found  in  areas,  the  greater  portion  of  which  are  underlain  hj  sedimen- 
taries,  the  natural  inference  is  that  they  penetrate  these  sedimentaries. 
Where  isolated  sets  of  knobs  are  composed  of  the  same  kind  of  rock  or 
show  variations  of  the  same  type,  they  may  be  presumed  to  be  connected. 
For  the  most  part  the  dikes  and  bosses  are  too  small  to  admit  of  indication 
upon  the  accompanying  map.  Wherever  their  size  has  warranted  it,  they 
have  been  represented,  as  in  the  case  of  the  acid  intrusives  between  the  Paint 
and  Michigamme  rivers,  and  of  the  basic  intrusives  north  of  Crystal  Falls. 

187 


188  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

ORDER  OF  TREATMENT. 

For  the  sake  of  eon^•eniellce  and  to  avoid  repetition,  the  age  of  the 
intrusives  a.s  a  whole  i.s  first  determined,  and  then  follows  a  brief  descrip- 
tion of  the  effect  of  the  folding  of  the  district  on  the  distribution  of  the 
eruptives. 

The  rocks  described  ha^-e  been  di^aded  into  those  of  acid,  basic,  and 
ultrabasic  comjjosition,  and  the  usual  order  of  discussion  from  the  acid  to 
the  basic  will  be  followed.  In  Section  I  rocks  are  described  which  are 
geologically  disconnected.  In  Section  II  is  a  description  of  a  series  of 
rocks  which  constitute  a  geological  unit,  and  are  especially  interesting  from 
a  petrogenetic  standpoint.  Here  also  the  order  of  discussion  is  from  the 
acid  to  the  basic.  The  geographical  distribution  of  the  rock  of  each  divi- 
sion is  given,  only  those  outcrops  being  accui'ately  located  which  are  of 
very  large  size  or  which  for  other  reasons  are  of  special  interest  from  a 
stratigraphical  oi-  a  petrological  standpoint. 

After  the  petrographical  description  of  each  of  the  rocks  a  statement 
will  be  made  of  its  lield  relation  to  the  adjacent  rocks,  if  its  relations  have 
been  discovered.  Tliis  will  be  followed  by  a  brief  account,  in  those  rare 
cases  in  which  a  contact  has  been  observed,  of  the  metainorphic  action 
which  it  has  caused  in  the  sediments  through  which  it  was  forced. 

AGE   OF  THE   INTRUSIVES. 

The  intrusives  have  forced  their  way  through  the  Lower  and  Upper 
Huronian  sedimentaries,  but  have  never  been  found  to  penetrate  the  hori- 
zontal Lake  Siiperior  (Cambrian)  sandstone.  These  facts  alone  are  con- 
clusive proof  that  their  period  of  intrusion  falls  in  the  time  which  elapsed 
between  the  deposition  of  the  Upper  Huronian  and  that  of  the  Cambrian. 

Ill  tlie  discussion  of  the  time  of  the  folding  of  the  Upper  Huronian  the 
conclusion  was  reached  that  this  folding  preceded  the  deposition  of  the 
Keweenawan  series.  If  the  intrusives  to  be  described  had  existed  at  the  time 
of  folding,  they  must  certainly  have  suffered  from  the  orogenic  movements. 
Examination  of  the  exposures  of  the  intrusives  has  not  shown  schistose 
masses,  nor  has  detailed  microscopical  study  disclosed  tlie  eataclastic  textures 
which  accompany  powerful  dynamic  movements,  except  in  one  case,  which 
is  described  on  p.  1 94,  and  is  presumed  to  be  due  to  purely  local  movements. 
Such  being  the  facts,  the  conclusion  follows  that  the  intrusives  were  iiitro- 


AGE  OF  INTKUSIVES.  189 

cluced  subsequent  to  the  folding  of  the  Upper  Huroniun,  (.)r  are  of  Kewee- 
nawau  or  post-Keweenawan  age. 

A  closer  approximation  to  the  age  of  tlic  intrusives  is  not  possible, 
unless  we  rely  upon  petrographical  similarity.  The  dolerites  of  the  Crystal 
Falls  district  are  similar  to  those  forming  the  flows  and  dikes  of  the  Kewee- 
nawan  on  Keweenaw  Point.  They  are  also  similar  to  the  basic  intrusives 
of  the  Marquette  district,  with  which  this  is  jjractically  a  geological  unit, 
and  likewise  they  agree  petrographically  with  the  dolerite  dikes  of  the 
Penokee-Gogebic  district.  In  both  districts  the  late  intrusives  have  been 
considered  to  be  of  Keweenawan  age.^  Rominger,"  in  his  report  for  1881 
and  1884,  calls  attention  in  a  general  statement  to  the  possible  connection 
of  the  doleritic  dikes  penetrating  the  Michigan  Huronian  with  the  flows 
and  dikes  of  the  "Copper-bearing  formation"  (Keweenawan). 

While  congelation  by  means  of  petrographical  similarity  would  not 
hold  for  widely  separated  areas,  it  seems  to  be  well  worth  considering  for 
areas  which  are  so  closely  connected  as  are  the  iron  districts  of  the  U})per 
Peninsula  of  Michigan. 

Judging  from  the  evidence  thus  presented,  the  dolerites  of  the  Crystal 
Falls  area  are  probably  contemporaneous  with  the  intrusions  of  the  Penokee- 
Gogebic  and  Marquette  districts  and  with  the  volcanics  of  Keweenawan 
time. 

EBI.ATIONS    OF    FOXiDHSTG    A3^D   THE    DISTRIBUTIOK   OF   THE 

INTRUSIVES. 

In  the  preceding-  chapter,  in  the  sections  on  folding  of  the  Upper 
Huronian,  p.  158,  it  was  shown  that  the  main  folds  of  the  district  follow  an 
approximately  northwest-southeast  course,  and  that  upon  these  were  super- 
imposed minor  folds  approximately  at  right  angles  to  these.  The  lines  of 
weakness  parallel  to  the  axes  of  the  main  folds  have  been  taken  advantage 
of  by  certain  of  the  intrusives,  especially  the  dolerites. 

A  glance  at  the  general  map  (PI.  Ill)  shows  that  the  dolerite  dikes 
which  have  been  traced  for  considerable  distances — that  is,  ai'e  more  than 
great  knobs  uncovered  by  erosion — have  a  northwest-southeast  trend,  in 
agreement  with  the  general  direction  of  the  major  folding  of  the  district 

1  Mod.  U.  S.  Geol.  Survey,  Vol.  XIX,  p.  349;  Vol.  XXVIII,  p.  218. 
=  Geol.  of  Mich.,  Vol.  V,  cit.,p.  6. 


190  THE  CRYSTAL  FALLS  lEON-BEARIXG  DISTRICT. 

The  only  apparent  exception  is  that  part  of  the  great  mass  in  T.  43  N.,  R. 
31  W.,  which  extends  north  and  south  along  the  Michigamme  River;  but 
this  is  reall}'  not  an  exception,  since  the  folds  of  the  Mansfield  slates  here 
run  in  the  same  direction. 

SECTION    I.— UNRELATED    INTRUSIVES. 
CLASSIFICATION. 

Under  the  above  heading  are  included  intrusive  rocks  which  occur  in 
such  isolated  outcro^js  that  no  definite  relations  can  be  shown  to  exist 
between  them  and  other  igneous  rocks  of  similar  or  related  characters. 

The  intru.sives  described  under  this  heading  comprise  rocks  of  acid, 
basic,  and  ultrabasic  composition,  represented  respectively  by  the  granites, 
the  dolerites  and  basalts,  and  the  ^jicrite-porphyries. 

ACID   INTRUSIVES. 

The  acid  intrusive  rocks  may  be  divided  into  ordinary  biotite-granite 
(granitite)  with  a  micropegmatitic  variety,  and  muscovite-biotite-granite 
and  rhyolite-porphyry. 

GEOGRAPHICAL    DISTRIBUTION    AND    EXPOSURES    OF    GRANITES. 

The  biotite-granite  proper  does  not  occur  in  the  district  in  large  quan- 
tity. It  is  found  in  dikes  penetrating  the  Upper  Huronian  rocks  in  sees.  15 
and  22,  T.  42  N.,  R.  31  W.,  and  in  dikes,  cutting  diorite  intrusives  in  the 
Upper  Huronian,  in  sec.  22,  T.  42  N.,  R.  31  W. 

The  micropegmatitic  variety  of  the  biotite-granite  is  confined  to  an 
area  underlain  by  the  Lower  Huronian  rocks.  It  occurs  at  N.  750,  W.  740, 
sec.  17,  T.  43  N.,  R.  31  W.,  and  N.  1750,  W.  1580,  sec.  29,  T.  43  N.,  R.  31 
W.,  in  small  quantities  in  isolated  dikes,  cutting  the  dolerites  which  penetrate 
the  Lower  Huronian  series. 

Owing  to  their  small  size,  none  of  the  above-mentioned  exposures  are 
represented  on  the  maps. 

The  muscovite-biotite-granite  forms  large,  bold,  isolated  knobs  in  sees. 
19,  20,  29,  and  30,  T.  42  N.,  R.  31  W.,  between  the  Paint  and  Michigamme 
rivers.  These  knobs  are  so  closely  related  petrographically  that  they  are 
presumed  to  represent  a  large  boss,  and  they  are  therefore  represented  as  a 
unit  on  the  geological  map  of  the  district,  PI.  HI.  Other  smaller  areas  of 
granite  occur  and  are  shown  on  the  detail  map,  PI.  XVIII. 


ACID  INTltUSlVES.  191 

BIOTITE-GRANITE. 

The  biotite-granites  vary  iu  colur  tVoiu  li<>'lit  reddisli-browii  to  dark- 
^av  and  greenisli  rocks,  ami  iu  j^-rain  from  tine  to  coarse.  The  structure 
ordinai'ih'  is  tliat  of  a  normal  granite.  In  some  of  them  the  micropey- 
niatitic  intergrowth  of  quartz  and  feldspar  may  be  observed  in  small  quan- 
tity. In  others  this  forms  the  characteristic  part  of  the  rock,  and  these 
rocks  may  be  properly  termed  micropegmatitic  granites.  In  most  of  the 
sections  the  usual  constituents  in  ordinary  proportions  occur. 

In  all  the  rocks  the  main  mass  of  the  quartz  forms  irregular  grains, 
molded  on  the  other  constituents.  Sometimes  round  areas  of  quartz  are 
included  in  the  feldspars.  The  quartz  contains  very  commonly,  and  usually 
in  great  quantities,  liquid  inclusions  with  dancing  as  well  as  stationary 
bubbles. 

The  feldspar  is  nearly  always  of  two  kinds — orthoclase  and  plagio- 
clase.  Microcline  was  also  observed,  but  in  neglectable  quantity.  These 
feldspars  in  the  great  majority  of  slides  show  fairly  good  rectangular  out- 
lines, and  in  some  cases  these  are  strikingly  well  developed.  In  some  slides 
the  plagioclase  is  observed  in  rectangular  crystals  and  the  oithoclase  is 
found  in  large  irregular  plates  which  are  jnolded  on  the  plagioclase,  show- 
ing conclusively  their  relative  age.  The  plagioclase  is  finely  twinned 
according  to  the  albite  law,  and  also  in  some  slides  exhibits  pericline  twin- 
ning. A  case  was  observed  in  which  two  crystals,  one  showing  albite 
twinning,  the  other  both  albite  and  pericline  twinning,  were  grown  together 
so  as  to  correspond  to  the  Carlsbad  twins  of  orthoclase.  The  jjlagioclase 
gives  low  extinction  angles,  which  show  it  to  be  rather  acid.  The  amount 
of  plagioclase  in  some  of  the  sections — for  example,  in  those  of  the  numer- 
ous small  dikes  cutting  the  schists  near  Norway  portage  in  sec.  15,  T.  42  N., 
R.  31  W.,  and  the  one  cutting  the  gabbro  at  the  SE.  corner  of  sec.  22, 
T.  42  N.,  R.  31  W. — is  very  large,  denoting  an  increase  in  soda  and  lime 
and  indicating  a  relationship  to  the  diorites.  The  geological  relations  are 
not  such,  however,  as  to  enable  this  connection  to  be  shown  in  default  of 
chemical  analyses. 

The  orthoclase  is  for  the  most  part  untwinned,  or  else  shows  simple 
Carlsbad  twinning.  In  some  sections  the  feldspars  are  quite  fresh,  but  in 
others    they  are  seen  to  be  opaque,  porcelain-hke,  and  iu  still  others  the 


192  THE  CKYSTAL  FALLS  IRON-BEARING  DISTRICT. 

original  feldspar  material  is  almost  entirely  replaced  by  a  mass  of  musco- 
vite,  with  some  little  epidote-zoisite  and  biotite.  The  muscovite  in  these 
secondary  aggregates  gives  excellent  though  small  rectangular  sections, 
showing  its  fine  cleavage  very  distinctly.  Well-determinable  kaolin  flakes 
were  not  found. 

The  mica  is  well  represented  Ijy  both  Ijiotite  and  muscovite.  Both 
occur  in  very  well  developed  crystals,  the  muscovite  showing  the  most  per- 
fect development.  The  biotite  has  j^artly  altered  to  chlorite,  with  a  simul- 
taneous production  of  rutile,  sagenite,  and  sphene.  Between  the  chlorite 
laminae  one  frequently  sees  lenticular  areas  of  secondary  calcite.  Quite 
commonly  the  sagenite  is  found  included  in  these  areas. 

In  onlv  one  specimen  was  hornblende  oljserved.  This  was  from  a 
granite  dike  whicli  cut  the  dolerite.  The  hornblende  is  of  a  noncompact 
variety,  which  upon  the  edges  is  finely  fibrous.  It  corresponds  exactly  to 
that  which  is  found  in  the  adjacent  dolerite. 

The  contact  between  the  granite  and  dolerite  appears  irregular,  as 
though  the  dolerite  had  been  to  some  extent  broken.  As  the  contact  is 
approached  from  the  granite  side  the  hornblende  increases  in  quantity.  It 
is  thought  probable  that  the  hornblende  in  the  granite  along  the  contact  is 
secondary  after  pyroxene,  and  that  this  pyi'oxene  was  obtained  by  the 
inclusion  of  fragments  of  the  dolerite. 

Accessory  minerals  are  not  present  in  large  quantity. 

Iron  oxide  is  not  present  in  great  quantity,  and  when  seen  it  is  usually 
titaniferous,  as  rutile  is  found  as  an  alteration  product.  In  one  case  the 
hexagonal  plates  show  the  presence  of  ilmenite.  Apatite  is  rare,  as  a  rule, 
though  occurring  in  some  sections  in  considerable  quantity.  Zircon  is 
scarce,  as  are  also  sphene  and  rutile.  Epidote  is  rather  common.  In  some 
cases  it  is  seen  in  biotite  surrounded  by  a  pleochroic  halo,  and  in  such  cases 
it  is  probably  original.  The  secondary  minerals,  muscovite,  biotite,  chlorite, 
epidote,  sphene,  rutile,  and  calcite,  show  their  usual  characters.  Calcite  is 
abundant,  and  is  more  or  less  ferruginous.  It  is  found  in  rhombohedra  and 
also  in  irregular  masses.     In  all  cases  its  secondary  origin  is  clear. 

MICROPEGMATITES. 

The  micropegmatitic  varieties  of  the  biotite-granite  show  the  same 
variations  in  color,  from  reddish  to  gray  and  greenish,  and  in  grain  from 


ACID  INTKUaiVES.  193 

tine  to  mc'iliuiu,  as  do  tlu'  biotite-iirauitcs  j)r()[)er.  In  o'euenil  tlu;y  niuy  be 
described  as  biotite-granites  in  wliicli  the  niicropegniatitie  intergrowtii  of 
quartz;  ami  feldspar  instead  of  being  sul)ordinate  preponderates.  Many  of 
the  well-crystallized  feldspars  are  surrounded  by  a  border  of  micropegraa- 
tite,  which  varies  from  a  narrow  strip  to  a  very  wide  border,  usually  in 
inverse  ratio  to  the  size  of  the  feldspar  nucleus.  The  feldspar  in  the  inter- 
growth  is  continuous  with  that  of  the  nucleus.  A  coarsely  radial  arrange- 
ment of  the  niicropegniatitie  intergrowtii  was  frequently  observed.  Where 
the  feldspars  and  quartz  are  predominantly  poi-jihyritic,  and  micropegmatite 
forms  the  groundmass,  the  rock  grades  over  into  the  rhyolite-porphyries  with 
micropegmatitic  groundmass — the  inappropriately  named  granophyres  of 
Eoseiibusch. 

The  biotite  of  the  micropegmatitic  granites  has  partly  altered  to 
chlorite  and  sageiiite.  In  some  of  these  rocks  the  biotite  is  collected  into 
large  aggregates  of  imperfect  individuals,  which  surround  larg-e  pieces  of 
iron  ore.  In  some  instances  it  is  included  in  the  plagioclase.  The  biotite 
flakes  in  the  feldspar  are  sometimes  so  numerous  as  to  conceal  almost  com- 
])letely  the  feldspar  substance.  In  one  instance  the  feldspar  of  such  a 
micropegmatitic  intergrowtii  is  completely  replaced  by  biotite.  These  sec- 
ondary biotite  flakes  surrounding  the  remaining  more  or  less  rounded  quartz 
areas  of  the  micropegmatite  produce  a  rock  which  is  strikingly  like  a  mica- 
schist  in  places,  although  it  is  of -unquestionably  eruptive  character. 

Tourmaline  is  a  rare  accessory  in  these  granites,  a  small  smoke-brown 
crystal  having'  been  observed  in  one  section.  Titaniferous  iron  ore  alterinff 
to  leucoxene  or  sphene  and  rutile  is  found,  as  are  also  the  common  accessory 
minerals — apatite,  rutile,  and  zircon.  They  contain  also  the  same  secondary 
minerals  as  the  normal  biotite-g'ranite. 

MUSCOVITE-BIOTITE-GRANITE. 

These  are  medium-grained  rocks,  and,  owing  to  the  fact  that  the  mus- 
covite  is  more  abundant  than  the  biotite,  have  a  light-gray  color. 

The  muscovite  is  noticeably  automorphic  with  respect  to  the  biotite, 
though  the  biotite  is  also  in  well-developed  automorphic  plates.  Plagio- 
clase is  present  in  these  granites  in  very  large  quantity.  It  shows  an  excel- 
lent zonal  development,  with  diminishing  angle  of  extinction — that  is, 
increasing  acidity — from  the  center  outward.     The  center  of  the  individuals 

MON  XXXVI 13 


194  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

is  nearly  always  extensively  altered,  while  the  outer  zones  are  compara- 
tively fresh.  A  maximum  extinction  angle  of  15°  against  the  twinning 
planes  in  the  zone  perpendicular  to  010  was  observed  on  an  unaltered  zone 
surrounding  an  altered  core.  This  would  indicate  the  feldspar  to  be  per- 
haps as  basic  as  labradorite  at  the  center.  The  other  essential  minerals, 
quartz  and  orthoclase,  occur  in  usual  quantity  and  show  nothing  of  espe- 
cial interest.  Sphene  and  apatite  are  the  only  accessory  minerals  present. 
An  apatite  crystal  was  observed  which  was  included  in  quartz,  and  con- 
tained the  brown  apparently  vitreous  core  so  frequently  seen  in  the  apatites 
of  basic  rocks.  Where  included  in  biotite,  it  is  surrounded  by  a  pleo- 
chroic  halo. 

No  analyses  were  obtained  of  these  granites,  but  from  the  quantity 
and  character  oi  the  feldspar  as  noted  above,  these  rocks  are  thought  to  be 
closely  related  to  dioritic  rocks.  Indeed,  it  is  a  (question  if  they  should 
not  be  classed  as  quartz-diorites. 

RELATIONS  OF  GRANITES  TO  OTHER  INTRUSIVES. 

In  two  cases  alreadv  mentioned  granite  dikes  cut  the  diorites.  Granite 
also  cuts  the  gabbro,  and  dikes  of  granite  were  observed  penetrating  the 
dolerites,  thus  indicating  that  tlie  granites  are  younger  than  tliese  igneous 
rocks. 

DYNAMIC    ACTION    IN    GRANITES. 

An  examination  of  the  granites  with  particular  reference  to  pressure 
phenomena  shows  that  the}'  exhibit  a  great  difference  in  this  respect.  Some 
show  scarcely  any  traces  of  pressure,  while  others  quite  closely  associated 
may  have  been  affected  thereby  to  such  an  extent  that  a  more  or  less 
strongly  wavy  extinction  of  their  mineral  constituents  is  general.  However, 
but  a  single  instance  of  a  supposed  granite  possessing  an  excellent  cataclastic 
structure  and  imperfect  schistosity  was  observed,  and  this  rock  was  so 
extremely  altered  as  to  render  dcnibtful  a  determination  of  its  original 
character. 

CONTACTS  OF  GRANITES  AND   SEDIMENTARIES. 

The  largest  intrusive  granite  mass  is  found  l>etween  the  Paint  and 
Michigamme  rivers,  in  sees.  19,  20,  29,  and  30,  in  T.  42  N.,  R.  31  W. 
The  granite  is  a  inusco\'ite-biotite-granite.  The  sedimentaries  are  mica- 
ceous graywackes,  which  have  been  described  on  p.  170.     Let  it  suffice 


ACID  INTliUSIVES.  195 

here  to  repeat  that  thev  li;ivc  Ix-cii  luiicli  mashed  and  recrystalhzcd,  Imt 
that  some  of  them  still  show  their  fraymeiital  nriuiii.  New  hiotite,  musco- 
vite,  feldspar,  iiiid  cjuartz  have  developed.  Where  most  altered,  they  are 
mica-schists  and  mica-gueisses.  These  chang-es  are  presumed  to  Ije  due,  for 
the  most  part,  to  the  orogeuic  forces  which  were  active  prior  to  the  iutrusiou 
of  the  fi'ranite  (p.  170).  If  su(di  be  the  case,  the  g-ranite  beg-an  its  metamor- 
phic  action  upon  a  rock  already  greatly  changed  from  its  original  couditiou. 

EVIDENCE    OF    INTRUSION. 

The  intrusive  character  of  this  large  mass  of  granite  is  indicated  by 
its  stratigraphical  position,  and  is  further  confirmed  by  the  mechanical 
effects  ])roduced  by  the  intrusion  of  the  granite  upon  the  sedimentaries,  by 
the  contact  effects  produced  in  the  sediments,  and  by  the  presence  in  the 
sedimentaries  of  granite  dikes  forming  offshoots  from  the  main  mass. 

The  mechanical  effects  are  well  shown  by  the  inclusion  of  sedimentary 
fragments  and  by  the  dislocation  and  folding  of  the  beds.  Inclusions  are 
rather  common  and  are  usually  of  considerable  size. 

The  dislocation  and  folding  are  lieautifully  shown  at  N.  1970,  W.  570 
paces,  sec.  30,  T.  42  N.,  R.  31  W.  In  general  the  layers  in  the  graywacke, 
which  are  alternately  rich  and  poor  in  mica,  strike  N.  15°  to  30°  W.,  but 
where  the  intrusives  are,  these  layers  are  found  to  strike  almost  due  north 
and  south.  At  the  aliove  location  the  beds  are  folded  into  small,  closely- 
compressed  anticlines  and  synclhies,  which  pluiige  to  the  east  at  an  angle 
of  about  80°.  At  this  place  the  micaceous  graywacke  is  broken  into  small 
pieces,  which  are  thoroixghly  injected  and  cemented  by  the  granite,  thus 
forming  a  typical  eruptive  breccia.  The  granite  cement  is  microgranitic, 
with  comparatively  little  quartz  and  a  small  amount  of  chloritized  mica. 
The  fragments  of  micaceous  graywacke  in  the  breccia  appear  to  be  rather 
more  feldspathic  than  usual,  but  otherwise  seem  not  to  have  been  nuich 
affected. 

Owing  to  the  altered  condition  of  the  sediments  i)rior  to  the  granite 
intrusion,  and  to  the  alternation  of  sediments  of  somewhat  varying  char- 
acter, we  can  not  expect  to  find  such  clearly  outlined  concentric  zones 
suri'oundinp-  the  aranite  as  in  cases  where  the  sediments  are  uniform.  In 
one  case  a  contact  was  observed  between  the  granites  and  apparently  the 
main  mass  of  the  sediments.     Along  this  line  of  contact  biotite  and  white 


196  THE  CRYSTAL  FALLS  IKON-BEARING  DISTRICT. 

mica  have  developed  in  great  abundance.  Mica  is  well  known  as  one  of 
the  minerals  produced  in  granite  contacts,  and  it  evidently  here  owes  its 
abundance  to  the  presence  of  the  granite. 

At  a  considerable  distance  from  the  nearest  intrusive  outcrop  (2  miles) 
a  mica-schist  was  observed  which  was  characterized  by  numerous  small 
but  prominent  nodules  that  stood  out  upon  its  weathered  surface.  The 
rock  contains  a  considerable  quantity  of  an  apparently  original  chlorite 
in  large  automorphic  plates.  The  nodules  were  produced  by  large  individ- 
uals of  staurolite.  The  staurolite  has  almost  completely  altered,  remnants 
only  of  the  original  indi^^duals  remaining.  These  remaining  grains  show 
a  very  poor  cleavage,  and  extinguish  parallel  to  it.  These  include  blebs  of 
quartz  and  particles  of  iron  oxide.  Thev  have  the  usual  pleochroism  for 
staurolite,  varying  from  golden  yellow  for  c  to  yellowish  white  for  a  and  6. 
The  alteration  products  in  which  the  grains  lie  are  fine  scaly  aggregates 
of  minute  leaves  of  muscovite,  with  here  and  there  larger  plates  of  the 
same  mineral.  A  few  grains  of  quartz  and  a  small  amount  of  iron  oxide, 
possilily  jiartly  original,  are  found  in  the  mass.  This  observation  of  the 
alteration  of  the  staurolite  to  muscovite  confirms  the  observations  of  Thiir- 
ach^  and  Pichler.'  A  similar  staurolitiferous  mica-scliist,  occurring  in  the 
same  locality,  was  described  by  C.  E.  Wright  for  the  Wisconsin  survey.^ 
On  these  particular  specimens  the  characteristic  twins  of  staurolite  may  be 
observed  macroscopically  as  well  as  in  thin  section.  Wright  has  also 
described  a  garnetiferous  mica-schist  from  this  area  of  metamorphic  schists.* 
Both  of  these  schists  contain  prisms  of  bluish  tourmaline  in  considerable 
quantity. 

It  appears  highly  probable  that  these  staurolitiferous  and  garnetiferous 
schists  owe  their  origin  to  the  intnision  of  the  igneous  rocks,  though  no 
well-marked  contact  zones  could  be  outlined. 

The  exomorijhic  contact  effect  of  the  granite  is  more  noticeable  where 
a  large  body  of  the  granite  contains  a  sedimentary  intrusion  than  elsewhere. 

The  determination  of  the  sedimentary  origin  of  the  fragments  included 
in  the  granite  is  based  primarily  ujjon  the  probability  that  in  its  passage 

I  Thiirach,  GrotU's  Zeitschr.,  Vol.  II,  p.  423. 

'  A.  Pichler,  Beitriige  zur  Mineralogie  Tirols,  Nenes  Jahrb.,  1871,  p.  54. 

^Geology  of  the  Menominee  iron  region,  by  C.  E.  Wright:  Geol.  of  Wisconsin,  Vol.  Ill,  1878,  Part 
8,  11.695. 

*  Log.  cit.,  p.  695. 


ACID  INTUUSIVES.  197 

tlirou"-!!  the  seiliiii('ntiU-\'  rocks  whirli  now  surniuiKl   it  tliu  granite  iiichuU-d 
fraj'-meiits  of  tliciu. 

In  addition  to  this  a  well-defined  banding-  is  still  ])resent  in  these  frag- 
ments. Thougli  by  no  means  conclusive  evidence,  this  is  considered  as  an 
indication  of  their  having  been  originally  deposited  through  the  mediation 
of  water.  The  sediments  have  been  comi)letely  recrystallized  into  fine- 
grained mica-gneisses. 

The  sedimentary  fragments  included  in  the  granite  now  sliow  the 
foliowiug  characters.  They  are  composed  of  layers  of  two  kinds.  The 
one  kind  of  layer  is  very  fine  grained,  of  gray  color,  and  consists  predomi- 
nantly of  biotite  in  fairly  good  automorjihic  plates,  muscovite  in  small 
quantity — but  in  automorphic  jilates — even  with  respect  to  the  biotite, 
feldspar,  quartz,  and  iron  oxide.  The  feldspar  is  in  small  equi dimensional 
grains.  Only  one  finely  striated  feldspar  was  observed,  the  greater  part 
possibly  being  orthoclase.  It  shows  in  places  a  well-developed  zonal 
structure,  the  zones  conforming  to  the  outlines  of  the  grains.  The  zonal 
structure  probably  depends  upon  varying  quantities  of  the  soda  and  potash 
molecule.     Quartz  occurs  in  grains  in  very  small  quantity. 

The  second  kind  of  layer  which  is  seen  in  the  fragments  is  nuicli  coarser 
grained  than  is  the  first,  just  described,  and  is  very  much  darker.  It  is 
composed  mainly  of  muscovite  and  biotite,  in  about  equal  quantities,  feld- 
spar, quartz,  magnetite,  and  ilmenite,  with  some  tourmahne.  Both  feldspar 
and  quartz  occur  in  grains,  the  former  in  small  quantity.  The  biotite  is  in 
small  plates  less  well  developed  than  the  muscovite,  though  mostl}-  auto- 
morphic. It  is  partly  bleached  and  has  associated  with  it  here  and  there 
some  secondar}^  epidote.  The  muscovite  is  in  very  large  automorphic 
plates,  some  of  them  twinned  according  to  Tschermak's  law,  and  includes 
flakes  of  biotite  and  grains  of  quartz  and  feldspar.  This  gneiss  contains 
also  a  large  number  of  crystals  of  tourmaline,  showing  strong  dichroism 
from  light  pinkish  to  dark  grayish  blue.  Iron  oxide  is  present,  both  as 
magnetite  and  as  ilmenite.  The  quadratic  individuals  of  the  one  and  the 
hexagonal  plates  of  the  other  at  times  are  very  well  developed.  No  signs 
of  pressure  whatsoever  are  seen. 

The  muscovites  evidently  represent  the  last  product  of  crystallization, 
as  shown  by  their  including  all  the  minerals  which  had  been  previously 
formed.     (Fig.  A,   PI.   XXXIV.)     These  muscovites  are  probably  to  be 


198  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

looked  upon  as  the  product  of  mineralizers,  dependent  upon  the  presence- 
of  the  hot  g-ranite  injection,  to  whose  action  may  also  be  referred  the 
presence  of  the  tourmaline. 

The  line  of  contact  between  the  granite  and  the  sedimentary  frag- 
ments, though  somewhat  irregular,  is  macroscopically  very  well  defined  by 
the  difference  in  size  of  the  mineral  constituents  of  the  two  rocks.  Here 
and  tliere  in  the  fragments  there  are  seen  thin  masses  of  the  granite  which 
were  injected  along  the  planes  of  sedimentation,  and  also  traverse  cracks 
penetrating  the  sediments.  On  the  granite  side  of  such  a  contact  there  is  a 
nan-KW  zone  very  noticeably  richer  in  large  biotite  flakes  tliau  the  granite 
is  ordinarily.  No  diff'erence  can  be  noticed  on  the  sedimentary  side  of  the 
contact.  The  microscopical  examination  of  the  contact  emphasized  the 
endogenous  character  of  the  metamorphic  action.  There  is  more  biotite 
present  than  in  the  normal  granite. 

The  feldspar  in  the  normal  granite  has  a  decided  tendency  toward 
autoniorphic  development.  Where  the  feldspars  of  the  granite  touch  the 
metamorphosed  sediments  they  are  partly  rounded,  and  the  mica  plates 
developed  in  the  sediments  have  in  general  a  parallel  structure  around  that 
side  of  the  feldspar  which  is  turned  toward  the  fragments. 

At  another  point  the  quartz  of  the  granite,  where  it  comes  in  contact 
with  the  sediments,  has  developed  as  an  automorphic  individual,  and  looks 
as  though  it  were  pressed  into  the  fragment.  The  quartz  crystal  contains 
near  its  edge  grains  of  feldspar  and  flakes  of  mica,  thus  making  an  imper- 
fect narrow  poikilitic  zone.  Beyond  this  zone  there  is  the  sedimentary  rock 
jjroper,  and  there  the  mica  plates  lie  parallel  to  the  contours  of  the  quartz. 
An  illustration  of  such  a  contact  is  shown  in  figs.  A  and  B,  PL  XXXV,  as 
seen  in  ordinary  light  and  also  between  crossed  nicols. 

It  appears  that  the  formation  of  the  quartz  and  feldspar  noted  above 

caused  the  arrangement  of  the  constituents  of  the  gneiss  parallel  to  their 

contours.     It  would  thus  seem  probable  that  in  this  particular  case  the 

recrvstallizatiou  of  the  original  graywacke  into  the  gneiss  which  we  now 

find  in  its  place  followed  and  was,  chiefly  the  result  of  the  intrusion  of  the 

m-anite. 

°  BASIC   INTRUSIVES. 

The  basic  intrusives  are  represented  by  metadolerites  and  metabasalts. 
The  dolerites  are  the  most  important,  and  will  be  treated  in  detail.     Rocks- 


BASIC  INTRUSIVES.  199 

very  siniihir  to  tlie  l);iisalts  have  alread}'  been  described  at  length  under  the 
Hendock  volcanics,  and  since  they  are  found  in  loniparatively  few  dikes 
they  will  be  passed  over  with  very  Ijrief  mention. 

METADOLERITE.i 
GEOGRAPHICAL   DISTRIBUTION. 

The  dolerites  of  the  Crystal  Falls  district  for  tlie  most  part  form  liigh 
ridges  extending  in  a  northwest-southeast  direction.  Their  principal  occur- 
rence is  in  the  area  immediately  north,  northeast,  and  east  of  Crystal  Falls. 
Beginning  in  sees.  32  and  33,  T.  43  N.,  R.  31  W.,  there  extends  a  great 
intrusive  mass,  varying  from  a  mile  to  a  mile  and  a  half  in  width,  due  north 
to  sec.  6,  T.  43  N.,  R.  31  W.  There  it  bends  to  the  northwest,  and  ends  in 
sec.  3,'  T.  43  N.,  R.  3"2  W.  The  northwestern  extension  of  this  mass  is  much 
narrower,  never  exceeding  a  half  mile,  and  at  many  places  it  is  only  a  few 
hundi'ed  yards  in  width.  In  the  northern  part  of  T.  42  N.,  R.  31  W.,  are  a 
number  of  knobs  which  are  evidently  connected  below  with  these  large 
masses,  although  the  exposures  are  discontinuous. 

A  large  dike  begins  in  sec.  1,  T.  42  N.,  R.  32  W.,  and  extends  for  about 
5  miles  to  the  northwest  into  sec.  19,  T.  44  N.,  R.  32  W.  This  averages 
about  one-eighth  of  a  mile  in  width,  though  in  places  it  is  three-fourths  of 
a  mile  Avide.  Another  dike  begins  in  sec.  28,  T.  44  N.,  R.  32  W.,  and  runs 
for  3^  miles  to  the  northwest  into  sec.  18,  T.  44  N.,  R.  32  W.,  and,  like  the 
above,  is  narrow,  being  only  about  one-eighth  of  a  mile  in  average  width. 
A  narrow  dike  less  than  one-eighth  of  a  mile  in  width  extends  in  a  high 
ridge  from  in  sec.  16,  T.  43  N.,  R.  32  W.,  to  the  northwest  for  3  miles  and 
ends  in  sec.  7  of  the  same  township  and  range.  Numerous  isolated  knobs 
occur  in  T.  44  N.,  R.  32  W.  A  small  boss  is  in  sec.  24,  T.  46  N.,  R.  33  W., 
and  another  at  1,600  paces  N.,  1,000  W.,  sec.  19,  T.  47  N.,  R.  33  W. 

PETROGRAPHICAL   CHARAf^TERS. 

iiacroscopicai. — Tlic  doleritcs  vary  in  color  from  greenish  to  dark  olive- 
green  and  almost  black.  The  weathered  surface  is  usually  of  a  very  light 
color,  rather  a  light  gray,  with  frequently  a  reddish  tinge.     The  texture  is 

'luse  the  name  " ilolerite "  here  merely  to  iudic;ite  the  macrostiuctural  dift'erence  between  the 
rocks  inclnded  under  it  and  the  fine-grained  and  aphanitio  rocks  of  the  same  composition  included 
under  the  basalts.  It  is  also  extended  to  include  jja^fo  as  well  as  neo  cruptives.  As  nearly  all,  if  not 
all,  the  paleodolerites  have  undergone  great  alteration,  the  prefix  meta — indicating  alteration  without 
reference  to  any  specific  kind — is  very  generally  applicable. 


200  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

medium  to  coarse.  Probably  the  most  striking  textural  characteristic  is  the 
pecuhar  mottled  appearance  described  as  "luster-mottling"  by  Pumpelly,' 
to  which  the  name  poikilitic"  has  of  late  years  been  more  generally  applied. 
This  texture  is  almost  always  brought  out  macroscopically  on  the  weathered 
surfaces  by  the  difference  in  the  weathering  of  the  feldspar  and  the  augite 
or  uralite,  these  lieing  the  prominent  mineral  constituents  of  the  rock. 
This  poikilitic  texture  is  most  common  in  the  coarsest  of  the  metadolerites. 
In  such  rocks  the  augite  or  uralite  areas  are  of  large  size,  quite  commonly 
2  centimeters  in  diameter,  and  show  their  mottled  character  very  plainly  to 
the  naked  eye. 

In  the  medium-grained  dolerites  the  ordinary  ophitic  texture  is  pre- 
dominant, though  in  these  rocks  poikilitic  areas  may  be  seen.  In  fact,  these 
poikilitic  areas  really  possess  an  ophitic  texture,  according  to  the  definition 
of  that  texture  by  A.  Michel  Levy,  for  the  feldspars  are  developed  as  laths 
and  the  pyroxene  is  the  mesostasis  in  which  the  feldspars  lie.^  It  is  thus 
clear  that,  restricting  the  statement  to  these  dolerites,  the  ophitic  texture  is 
at  times  included  in  the  poikilitic,  and  that  under  such  circumstances  the  two 
can  not  be  considered  as  totally  different  and  independent  textures,  but  are, 
on  the  contrary,  practically  identical. 

In  one  dike  of  dolerite  the  influence  which  the  conditions  of  consoli- 
dation exert  upon  the  texture  is  well  shown.  This  dike  is  only  8  feet  wide, 
but  the  center  is  developed  as  a  dolerite,  while  along  the  edges  where  cool- 
ing was  more  rapid,  the  rock  is  a  porphyritic  basalt.  The  porphyritic 
texture  is  caused  by  the  development  of  pyroxene  and  feldspar  phenocrysts, 
which  lie  in  a  dense  basaltic  groundmass. 

Microscopical. — Tlic  OHgiual  miucrals  of  which  the  rocks  were  composed 
were  feldspar,  quartz,  pyroxene,  olivine,  biotite  (?),  apatite,  and  titano- 
magnetite.  The  minerals  which  are  now  present  in  the  rocks  are  for  the 
most  part  secondary.  They  are  hornblende,  muscovite,  epidote-zoisite, 
chlorite,  biotite,  sphene,  leucoxene,  calcite,  albite,  quartz,  and  iron  pyrite. 
Of  these,  hornblende  is  by  far  the  most  prominent  constituent.  A  study  of 
the  isolated  specimens  of  these  rocks  might  result  in  their  determination  as 

'  Metasomatic  develoitmeiit  of  the  coppei-beariug  rocks  of  Lake  Superior,  by  Raphael  Pumpelly: 
Proc.  Am.  Acad.  Arts  Set.,  Vol.  XIII,  1878,  p.  260. 

'On  the  use  of  the  terms  "poikilitic"  aud  "micropoikilitic"  in  petrography,  by  G.  H.  Williams- 
Jour.  Geol.,  Vol.  1, 1892,  pp.  176-179. 

=  Structures  et  classiljcatiou  des  roches  eruptives,  by  A.  Michel  Levy,  Paris,  1890,  }>.  30. 


BASIC  1NTKUS1VE8.  201 

uralitic  dolerites,  cpidioriti^s,  or  even  dioritus,  as  it  is  impossible,  without  a 
stHiiieuce  of  cliaujj^es,  to  deteriniue  wlietlier  the  hornhk'iide  is  original  or 
secondary.  Rare  rocks  contain  quartz  in  sutfieient  (juantity  to  warrant 
their  designation  as  quartz-dolerites.  However,  they  differ  in  no  essential 
respects  from  the  other  dolerites.  The  quartz  is  in  micropegmatitic  inter- 
growth  with  feldspar,  filling  the  angular  interspaces  of  the  rock.  These 
intero-rowths  were  evidently  the  last  elements  to  crystallize. 

The  feldspar  occurs  in  large  automorphic  lath-shaped  crystals,  which 
in  most  cases  show  poly  synthetic  twinning.  In  a  few  cases  unstriated  crys- 
tals were  obseiwed.  Owing  to  the  alteration  of  the  feldspar,  which  has  in 
most  cases  almost  completely  destroyed  the  striations,  it  has  been  impossible 
to  make  many  accurate  measurements.  Measurements  ou  the  zone  perpen- 
dicular to  010  gave  equal  e.xtinetion  angles  against  twinning  planes  of  37° 
as  maximum,  showing  the  feldspar  to  be  bytownite.  The  chief  alteration 
products  of  the  feldspar  are  epidote  and  zoisite.  With  these  are  usually 
associated  more  or  less  muscovite,  some  chlorite,  and,  more  rarely,  scales  of 
biotite.  Accompanying  these  alteration  products  one  very  fi-equently  finds 
hmpid  spots  of  secondary  albite  or  quartz.  Some  few  of  the  feldspars  are 
smoke-colored,  and  as  the  coloring  appeared  homogeneous  even  under  the 
highest  powers,  it  Avould  seem  to  be  due  to  some  pigment  in  the  mineral 
and  not  to  minute  inclusions. 

Pyroxene  is  very  rare,  having  been  observed  in  only  a  few  sections  and 
in  the  majority  of  these  is  present  merely  as  small  remnants  surrounded  by 
its  secondary  product,  uralite.  The  pyroxene  possesses  the  usual  char- 
acters of  common  augite.  The  augite  is  quite  free  from  inclusions.  Along 
the  edge  its  alteration  to  the  light-green  hornblende,  uralite,  can  be  readily 
followed,  and  in  one  case  an  octagonal  basal  section  of  augite  was  observed 
which  was  completely  occupied  by  uralite  fibers. 

The  former  presence  of  olivine  is  based  upon  verj^  slight  proof,  viz, 
the  existence  in  some  of  the  pyroxene  and  uralite  crystals  of  areas  which 
are  oval  or  round  in  shape  and  are  occupied  by  pilite.  The  presence  of  this 
pilite  in  the  altered  augite  might  possibly  be  explained  as  an  alteration 
product  of  the  augite  itself,  but  it  is  difficult  to  explain  why  pilite  should 
develop  in  one  part  of  the  augite  and  secondary  coarse  hornblende  in  the 
other  part.  Moreover,  the  general  characters  of  the  rocks  are  such  as  to 
lead  one  to  expect  to  find  olivine  present  in  some  of  them. 


202  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

Biotite  occurs  in  large  irregular  masses  whicli  are  considered  to  be 
primary,  as  well  as  in  the  scales  which  occur  within  secondary  products  of 
the  rock  and  are  considered  to  be  secondary.  It  is  scattered  throughout  the 
rocks  in  irregular  pieces,  usually  associated  with  iron  oxide.  Where  fairly 
fresh,  it  is  brown  and  shows  its  ordinary  character.  By  weathering  it 
becomes  green,  having  still  a  high  double  refraction.  By  further  weath- 
ering it  passes  into  a  nearly  colorless  mass  that  has  the  faintest  tinge  of 
green  and  scarcely  polarizes  light.  Such  masses  are  crossed  by  lines  of 
hair-like  crystals,  some  of  which  intersect  one  another  at  angles  of  60 
degrees,  extinguish  parallel  to  their  long  directions,  and  show  high  single  and 
double  refraction.  These  are  taken  to  be  rutile.  Other  crystals,  somewhat 
coarser,  also  lie  irregularly  in  the  biotite  masses.  They  show  the  same  inter- 
sections as  the  rutile.  They  are  very  faintly  greenish,  have  a  high  single  and 
double  refraction,  are  positive  in  the  long  direction,  and  have  a  maximum 
extinction  angle  of  46  degrees.  These  characters  were  not  sufficient  to 
determine  the  mineral  by,  and  no  other  characteristics  could  be  observed. 

Ilmenite  and  titano-magnetite  are  in  irregular  grains.  These  minerals 
are  more  or  less  altered  to  leucoxene  or  to  sphene.  Very  frequently  the 
alteration  product  incloses  bands  of  the  iron  oxide,  which  intersect  one 
another  at  angles  of  60  degrees,  pointing  toward  the  hexagonal  character 
of  the  ore.  In  one  case  a  beautiful  example  of  the  alteration  of  such  an 
ilmenite  to  rutile  was  observed.  It  is  exactly  similar  to  that  described  by 
Williams  in  the  case  of  the  greenstones  of  the  Menominee  district.^  By 
low  power  the  mass  has  a  semimetallic  luster,  and,  as  it  seems  to.  be  almost 
solid,  has  very  much  the  general  appearance  of  an  ore,  but  by  higher  power 
it  is  resolved  into  a  mass  of  small  golden-brown  crystals.  These  frequently 
intersect  one  another  at  angles  approximating  60  degrees  (120°),  similar  to 
the  fine  needles  in  sagenite. 

The  hornblende  is  mainly  in  large  xenomorphic  plates  inclosing  the 
automorphic  feldspars.  This  is  the  variety  of  hornblende  known  as  uralite 
and  is  all  presumed  to  be  of  a  secondary  nature.  In  no  case  does  it  pos- 
sess the  compact  nature  of  original  hornblende,  but  is  always  more  or  less 
fibrous,  its  fibrous  nature  being  best  seen  along  the  edges,  and  less  clearly 
shown,  though  still  observable,  where  the  sections  are  thicker.     It  varies 


1 A  letter  to  Neiies  Jahrbiich,Vol.  II,  1887,  p.  263.     The  greenstone-schist  areas  of  the  Menominee' 
and  Marquette  resious  of  Michigan.     Bull.  U.  S.  Geol.  Survey,  No.  62, 1890,  p.  99. 


BASIC  INTRUSIVES.  205 

iVoin  scarc't'lv  colored  needles  to  those  wliieli  are  stroujilv  pleoeliroic.  The 
pleocliroism  varies  from  yellowish  for  a  to  yellowish  oi-  dlixe  •'•reen  for  ItJ, 
and  in  many  cases  to  a  dark  hluish-green  for  c.  In  a  few  cases  mucli  of 
the  hornblende  has  frequently  a  darker  shade  in  the  center  than  at  the 
border,  althon<rh  of  the  same  color. 

A  somewhat  different  variety  of  hornblende  is  observed  occupying' 
round  to  oval  areas  in  the  dolentes.  This  is  in  tang-led  aii-ffresrates  of 
needles,  with  which  some  chlorite  is  associated.  This  hornblende  is  auto- 
niorphic  in  prismatic  zone,  ragg-ed  at  the  ends.  These  aggregates  seem  to 
be  ver}'  coarse  pilitic  pseudomor])lis  after  olivine.  The  areas  occupied  Ijy 
these  aggregates  are  similar  in  appearance  to  the  pseudoamygdules  described 
bv  Pumpelly  as  occurring  in  the  Keweenawan  lavas. 

The  hornblende  is  largely  altered  to  masses  of  chlorite  and  epidote, 
usitally  with  the  production  of  some  calcite,  and  to  this  is  due  the  present 
extremely  cliloritic  and  epidotic  characters  of  niany  of  the  badl}"  altered 
specimens. 

The  secondary  minerals,  chlorite  and  epidote-zciisite,  possess  their  usual 
cliaracters.  The  chlorite  is  present  in  very  large  quantity,  and  next  to  it 
the  epidote-zoisite  is  most  common.  These  two  minerals  make  up  a  large 
proportion  of  the  rock.  In  one  ease  porph}-ritic  scalenohedra  of  calcite 
were  found  in  a  medium-grained  dolerite,  the  occurrence  in  every  way 
being  similar  to  that  described  in  the  volcanics  of  the  same  region. 

None  of  the  original  minerals  of  these  intrusive  greenstones  give  evi- 
dence of  having  been  severely  mashed;  consecpiently  we  inay  safely  conclude 
that  they  have  not  participated  in  the  orogenic  movements  in  pre-Keweena- 
wan  time  which  have  universally  aifected  the  older  rocks  of  the  Crystal  Falls 
district. 

RELATIONS    TO    AD.JACENT    ROCKS. 
Relations  to  Lower  Huronian  Mansfield  slates. Tile    relatioU    of    tile     dolcriteS     tO     the 

Mansiiekl  slate  is  quite  clearlj'  shown  along  the  line  of  contact  between 
them.  This  extends  from  sec.  7  S.  to  sec.  32,  T.  43  N.,  R.  31  W.,  on  the 
east  side  of  the  Michigamme  River,  near  ilanstield.  The  presence  of 
numerous  large  inclusions  of  the  slate  in  the  dolerite  and  the  occurrence  of 
contact  rocks  in  the  slate  plainly  show  that  the  dolerites  are  younger  than 
the  slate.  Another  piece  of  evidence  pointing  to  this  same  relation  was 
found  in  sec.  28,  T.  44  N.,  R.  32  W.     Here  was  found  an  angular  inclusion 


204  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

of  sedimentary  rock  in  a  dolerite.  The  rock  now  possesses  the  characters 
of  a  spilosite,  and  was  evident!}-  brought  up  from  below  by  tliese  iutru- 
sives.  No  slate  is  exposed  near  this  point,  but  it  is  presumed  to  underlie 
this  area,  althoug-li  below  the  exposed  volcanics. 

Relations  to  Lower  Huronian  Hemlock  volcanics. The    dolcote    HdgeS    wllicll    OCCUr    lu 

the  area  underlain  b}'  the  Hemlock  volcanics  are  surrounded  on  all  sides 
by  rocks  of  related  petrographical  character.  The  number  of  localities 
at  which  the  relations  between  the  two  may  be  observed  are  very  few, 
biit  their  relations  where  seen  are  clear.  For  instance,  in  sees.  18  and  30, 
T.  44  N.,  R.  32  W.,  the  coarse  intrusives  break  through  the  volcanics.  In 
sefc.  ?7,  T.  46  N.,  R.  33  W.,  a  boss  of  the  dolerite  occurs  in  the  midst  of  schis- 
tose volcanic  tuffs.  The  volcanics  surrounding  the  intrusives  exhibit  symp- 
toms of  more  or  less  violent  dynamometamorphic  action,  whereas  the  doler- 
ites  in  no  case  show  any  evidence,  microscopically  or  macroscopically,  of  hav- 
ino-  undergone  the  metamorphism  from  which  the  volcanics  have  suffered. 
The  dolerites  are  thus  clearly  younger  than  the  effusives  of  the  district. 

Relations  to  Upper  Huronian. — Ouly  a  fcw  Isolated  doleHtc  outcrops  liave  been 
found  in  the  area  imderlain  by  the  Upper  Huronian.  The  most  conspicu- 
ous outcrops  are  the  large  dike  in  sees.  7,  8,  and  9,  T.  43  N.,  R.  32  W.,  and 
the  knobs  in  Ts.  42  N.  and  43  N.,  R.  31  W.  These  last  are  practically 
continuous  with  the  great  dike  which  penetrates  the  Lower  Huronian  along 
the  Michigamme  River  immediately  to  the  north.  One  isolated  knob  has 
also  been  found  in  the  extreme  northwestern  part  of  the  district  in  sec.  19, 
T.  47  N.,  R.  33  "W.  Although  in  none  of  these  areas  have  the  dolerites  been 
found  in  contact  with  the  Upper  Huronian,  as  it  has  been  shown  by  the 
stratigraph}'  that  these  areas  are  underlain  I))-  the  Ujjper  Huronian  sediments, 
the  statement  seems  warranted  that  the  dolerites  are  intrusive  through  them. 

Relations  to  other  intrusives. — In  ouc  placc  a  dolerite  Is  lutruded  by  a  dolerite 
of  later  age,  and  it  is  highly  probable  that  there  are  many  more  simila,r 
cases  never  observed.  The  dolerites  are  cut  by  small  granite  dikes  at  sev- 
eral places  east  of  Mansfield. 

CONTACT  METAMOKPHISM  OF  MANSFIELD  SLATES  BY  THE  DOLERITE.' 

The  contacts  between  the  dolerites  and  the  sedimentaries  are  very 
rarely  observable.     For  the  most  part  where  the  sedimentaries  are  altered 

'  A  contribution  to  tlie  study  of  contact  metamorphism,  by  J.  Morgan  Clements :  Am.  Jour.  Sci,, 
4tli  series,  Vol.  VII,  1899,  py.  81-91. 


CONTACT  AIETAMOKl'HISM   liV   INTIIUSIVKS.  205 

1)V  cinitiU't  iictiou  tllu^■  arc  surrouiulcd  on  all  sides  i)V  the  dolerites,  hciuii' 
ill  tact  iiiclusious,  but  without  tlic  iiiiiiicdiate  contacts  ex])osed.  Such 
inclusions  arc  rather  numerous  on  the  east  side  of  the  Micliigannne  River, 
iVoni  sec.  29  N.  to  sec.  S,  T.  43  N.,  R.  31  W.,  near  the  boundary  line 
')ct\vccn  the  Mansfield  slates  and  the  dolerites. 

The  Mansfield  slates  are  uniformly  rather  fine  grained,  and  the  contact 
products  are  also  fine-grained  rocks,  which  still  show  in  some  cases  the  fine 
l)anding  of  the  original  slates.  They  are  very  dense  "hornstone"-like 
rocks,  have  a  splintery  and  at  times  almost  conchoidal  fracture,  and  vary  in 
color  from  light  to  very  dark  gray  and  greenish  The  weathered  surfaces 
in  almost  all  cases  are  covered  by  a  thin  white  to  light-yellowish  crust. 
This  weathering  brings  out  very  clearly  the  banded  and  spotted,  character 
of  some  of  the  rocks. 

The  mineralogical  components  are  quartz,  feldspar,  biotite,  chk)rite, 
white  mica,  actinolite,  rutile,  epidote,  spliene,  and  iron  oxide.  Quartz  is  in 
very  minute  grains.  Much  of  the  feldspar  shows  fine  striations,  but  owing 
to  the  minute  size  of  the  grains  their  exact  characters  are  not  determinable 
with  the  microscope,  although  from  the  very  high  percentage  of  soda  shown 
by  analysis  to  be  present  in  the  rocks  the  conclusion  is  drawn  that  they 
are  grains  of  albite. 

Biotite  is  present  in  small  quantity  in  some  of  the  contact  products. 
This  production  of  secondary  biotite  has  been  noted  as  rare  for  "diabase" 
contacts.^  Plates  of  chlorite  and  white  mica  and  needles  of  actinolite,  the 
latter  of  much  larger  size  than  the  individuals  of  the  other  minerals  men- 
tioned, lie  scattered  through  the  fine-grained  mass  of  feldspar  and  quartz. 
Usually  they  are  gathered  together  in  bunches  and  sheaf-like  or  radial  aggre- 
gates, but  they  occur  at  places  in  isolated  individuals.  Scattered  through 
the  slates  is  unmistakable  rutile  in  coarse  crystals  with  pyramidal  ends.  In 
only  one  case  are  the  crystals  very  fine,  and  in  that  case  they  approach 
closely  the  appearance  of  needles  in  the  clay  slates  (Thonschiefernadeln). 
These  needles  are  commonly  aggregated  into  tangled  and  roughly  radial 
growths.     The  needles  show  very  pretty  knee-  and  heart-shaped  twins. 

Various  combinations  of  the  minerals  occur,  and  the  structures  which 
accompany  the  combinations  likewise  vary.  As  a  result  of  these  variations 
there  are  found  the  different  types  of  contact  products  described  as  spilosites, 

I  Mikroskopische  Physiographie,  by  H.  Rosenbusch,  Stuttgart,  1896,  Vol.  ii,  p.  244. 


206  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

desmiisites,  and  adinoles.^     The  general  characters  of  the  minerals  being-  the 
same,  I  shall  describe  the  structure  which  characterizes  the  rocks. 

SPII.OSITES. 

The  ordinary  spilosites  are  distinctly  mottled  in  hand  sjjecimens  and 
show  clearly  to  the  naked  eye  in  thin  section  tlie  oval  spots  which  char- 
acterize them.  These  oval  areas  are  commonly  4  millimeters  long-,  and  in 
rare  cases  even  longer.  They  are  ti-eqiiently  connected,  forming  chains. 
The  spots  are  appreciably  darker  than  the  mass  in  which  they  lie,  and  are 
composed  of  chlorite,  quartz,  feldspar,  and  rutile,  with  a  small  amount  of 
muscovite,  the  chlorite  being  the  chief  mineral.  The  sun-ounding  mass 
consists  essentially  of  nuiscovite,  quartz,  and  feldspar,  with  rutile  crystals 
and  Hakes  of  hematite,  and  witii  a  very  slight  amount  of  chlorite.  The 
different  proportions  of  chlorite  and  nmscovite  seem  to  cause  the  difference 
between  the  spots  and  the  groundnia.ss  (fig.  A,  PI.  XXXVII).  In  some  of  the 
spilosites  Ave  find  a  few  flakes  of  biotite  and  needles  of  actinolite;  however, 
these  are  always  very  subordinate  in  quantity  to  the  chlorite. 

In  the  ordinary'  form  described  these  spots  consist  essentially  of  bisili- 
cates.  Others  also  have  been  noted  in  which  these  spots  are  white  and  lie 
in  the  fine-grained  dark  mass  composing  the  greater  part  of  the  slides. 
Thus  far  it  seems  onlv  one  instance  of  the  occurrence  of  sucli  a  variety  of 
the  spilosites  has  been  described.  This  is  by  Van  Werveke,  to  whose 
description  reference  is  made  by  ZirkeP  and  Rosenbusch.^  These  white 
spots  are  composed  essentially  of  albite  feldspar,  with  onh"  a  minor  amount 
of  chlorite  and  epidote.  The  feldspar  grains  are  much  larger  than  those 
which  take  part  in  the  constitution  of  the  mass  surrounding  the  spots. 
This  mass  is  made  uj)  of  quartz  and  feldspar,  chlorite,  epid(ite,  and  some 
sphene,  with  sheaves  of  actinolite  scattered  through  it,  and  in  one  section 
clumps  of  biotite  were  observed  mixed  with  the  chlorite,  though  in  very 
subordinate  quantity  (figs.  A  and  B,  PI.  XXXVl). 

'  Uber  lien  spilosit  aud  desmosit  Zincken's,  by  Losseu ;  Zcitsclir.  Dentsch.  fJeol.  GeselL,  Vol. 
XXIV,  1872,  p.  701. 

Durcli  iliabaa  veriimlertu  Scliiel'er  im  Gcbiet  iler  Saai'  auil  Mosel,  l),v  Vmu  Werveke:  Leonard's 
Jahrb.,  Vol.  II,  1884,  p.225. 

An  interesting  contact  rock,  witb  note  on  contaet  luetaniorphism,  by  W.  JI.  Hntchiugs :  Geol. 
Mag.,  Vol.11,  pp.  122,  163. 

Numerous  other  references  may  be  found  in  t'bemisolie  Geologie,  by  Roth,  Bd.  Ill,  and  in  petro- 
graphical  works  of  Zirkel  and  Eosenbusch. 

-Zirkel,  Pet.,  Vol.  II,  p.  719. 

' Rosenbusch,  Vol.  II,  3d  ed.,  p.  1177. 


OOXTALT  METAMOHPIIISM  BY  INTKUSIVES. 


20' 


Dirt'e-riuji'  sliij;litly  t'niiu  the  oiiliiiary  spilositc  i.s  oiu'  in  wliicli  the  spots 
<ire  of  iiiicrosi-opical  size,  and  consist  of  r<i<i'<4e(l  Ininches  of  chlorite  and  Jigyre- 
_g'ates  of  sphene  and  epidote,  witli  sonic  Hakes  of  l)iotite,  which  lie  in  a  quartz- 
feldspar  mass.  The  photomicrograph,  tig.  11  of  1^1.  XXXVII,  illustrates  the 
appearance  of  the  rock.  As  these  spots  increase  in  number  the)'  approach  each 
other  and  unite,  forming  streamei's,  which  in  their  turn  unite  and  form  Ijands 
(photomicrograph,  tig.  A,  PI.  XXXVIII,).  The  spilosites  or  spotted  contact 
products  thus  ])ass  over  into  the  desinosites  or  banded  contact  products. 

Analyses  of  spilosites. — Aiialyscs  of  two  of  tlie  spilosltcs  (Nos.  I  and  II)  were 
made  by  Dr.  H.  N.  Stokes  in  the  laboratory  of  the  United  States  Geoloff- 
ical  Survey,  and  are  here  appended.  With  them  there  is  given  an  analysis 
(No.  Ill)  l)y  E.  Kayser  of  a  spilosite  from  the  Harz  Mountains 

Analyses  of  sjMositen. 


J  (32827). 

11(32861). 

III. 

SiOi 

57.  77 

.92 

19.35 

None. 

1.29 

3.37 

Trace. 

1.71 

None. 

Trace. 

4.  3.-) 

8.22 

.  22 

None. 

None. 

.04 

.18 

2..S4 

None. 

None. 

None. 

52.51 

1.70 

19. 00 

None. 

3.31 

7.19 

Trace. 

1.  55 

Trace. 

Trace. 

3.29 

6.72 

.70 

Trace. 

None. 

.15 

.34 

3.26 

Noue. 

None. 

Trace. 

55.  56 

TiO  ■ 

Al.,0) 

18.15 

Cr  .O3 

F&iOs 

5.08 

7.04 

.51 

1.40 

FeO 

MnO 

CaO 

BaO 

SrO 

MgO 

3.17 
4.20 
2.25 

NajO 

KG 

Ll-O 

CO2 

.10 

P.O.. 

H.OatllO^ 

1      2. 79 

H-O  at  110^+ 

S  and  so , 

CI 

F 

Total  

99.76 

99.72 

100. 25 

DESMOSITES. 


Under  the  desnK)sites  are  included  contact  products  composed  of  the 
;same  mineral  constituents  as  the  spilosites,  but  which  show  a  distinctly 
banded  structure.  As  shown  in  the  discussion  of  the  spilosites,  the  two 
must  be  very  closely  related  and  grade  into  each  other. 


E.  Kaj-ser,  from  Roth's  Cheni.  Geol.,  Vol.  Ill,  1890,  p.  143,  No.  IV. 


208 


THE  CRYSTAL  FALLS  IRON-BEARIiSfG  DISTRICT. 


ADIXOI.ES. 


Chlorite  has  been  the  chief  dark  luineral  in  the  contact  products  thus 
far  mentioned,  with  actinolite  as  an  accessory.  In  the  adinoles  actinolite  is 
the  characteristic  constituent.  The  mineral  constituents  in  the  adinoles  are, 
as  a  rule,  more  uniformly  distributed  than  is  the  case  with  the  spilosites; 
however,  the  spots  are  composed  essentially  of  actinolite.  The  actinolite  is 
in  sheaf-like  g-rowths.  These  actinolite  sheaves  lie  in  an  exceedingly  fine 
grained  mass  of  quartz  and  albite,  with  some  flakes  of  ehloi-ite  and  grains 
of  epidote.  The  groundmass  is  formed  of  such  minute  mineral  constituents 
that  no  conclusive  test  could  be  obtained  for  the  determination  of  the 
limpid  grains,  and  their  nature  has  been  concluded  from  the  analyses.  The 
rock  is  rendered  rather  dark  by  minute  black  specks  disseminated  through 
it.  In  places  these  are  collected  in  irregular  or  lenticular  heaps.  They 
seem  to  be  carbonaceous  matter. 

Analyses  of  adinoles. — Thc  followiug  is  au  aualysls  (No.  I)  by  Mr.  George 
Steiger,  of  the  United  States  Geological  Survey,  of  one  of  the  typical 
adinoles  from  this  district.  With  this  there  are  given  for  comparison  two 
adinole  analyses  (Nos.  II  and  III)  by  E.  Kayser.' 

Analyses  of  adinoles. 


I  (324li5). 


III. 


SiO.2 

TiO. 

AI2O3 

Fe,03 

FeO 

MnO 

CaO 

BaO 

MgO 

K;0 

Na;0 

H.O  at  100^  —  . 
H.,0  at  100'-'  +  . 

P.O5 

CO2 

FeSj 

C 


74.16 

.37 

11.85 

.82 

1.66 
.06 

2.10 
None. 

2.10 
.15 

6.57 
.05 
.52 
.08 
.09 


75.25 


.18 


11.80 

Trace. 

1.76 


72.63 
15.81 


32 


.74 
1.02 


1.57 

.61 

7.54 

.81 


1.21 

.75 

8.33 

.61 


.49 


Total 


100.76 


100.15 


101. 10 


'  E.  Kayser,  Zirkel,  Vol.  II,  p.  721,  Analyses  III  and  VI. 


CONTACT  METAMORPniSM   BY   INTRUSIVES.  209 

There  is  still  a  kind  ot'  contact  rock  in  \\  liidi  iictiuoiitc  is  the  chief  dark 
iiiincral,  and  in  which  the  actinolite,  thougii  in  clnnips,  is  mainly  collected 
in  l)ands.  This  convsjidnds  to  the  desmosites  in  structvire,  though  ditiering- 
from  them  in  mineralogical  composition. 

These  chlorite  and  actinolite  contact  rocks  may  be  expected  to  grade 
into  each  other,  and  such  a  gradation  is  sliown  in  one  specimen,  in  which 
actinolite  and  chlorite  are  present  in  about  equal  quantity.  The  actinolite 
occurs  in  crystals  and  sheaves,  forming  the  spots,  whereas  the  main  mass  of 
the  slide  surrounding  the  spots  is  formed  of  chlorite  as  the  raetasilicate  asso- 
ciated with  feldspar,  quartz,  and  some  epidote. 

It  w^ould  be  of  great  interest  to  determine  which  of  the  contact  prod- 
ucts, the  desmosites,  the  spilosites,  or  the  adinoles,  represent  the  greatest 
amount  of  metamorphism,  as  shown  by  the  relations  to  the  intruding  mass. 
Unfortunately,  the  records  of  the  specimens  do  not  enable  me  to  determine 
this,  although  for  other  contact  zones  in  other  regions  it  has  already  been 
determined  that  the  adinoles  are  next  to  the  contact,  while  the  spilosites 
(and  desmosites)  are  intermediate  between  them  and  the  clay  slates. 

COMPARISON   Ot"   THIS   ANALYSES   OF   THE    NORMAL    MANSFIELD    CLAY   SLATES   AND   THE    CONTACT 

PRODUCTS. 

In  a  series  of  analyses  designed  to  illustrate  the  chemical  changes 
which  accompany  the  increasing  metamorphism  of  a  rock,  it  is  of  great 
importance  that  the  various  phases,  from  the  unmetamorphosed  to  the  most 
metamorph(5sed  form  of  the  rock,  be  represented.  Moreover,  the  order  of 
succession  from  the  unmetamorphosed  to  the  most  metamorphosed  foi-m  of 
the  rock  should  be  definitely  known.  In  the  present  case  pertain  pha  es 
of  the  metamorphosed  rocks  are  represented,  but  it  has  been  impossible, 
owing  to  poor  exposures,  to  determine  in  this  locality  the  order  of  succes- 
sion. This  has,  however,  been  done. so  satisfactorily  by  Lessen  and  others, 
and  the  characters  of  each  fades  in  the  progression  have  been  so  well 
described,  that  I  have  no  hesitation,  after  a  microscopical  study  of  the  thin 
sections  of  the  specimens  analyzed,  in  presenting  the  series  of  analyses  in 
the  following  table  as  illustrative  of  the  changes  which  have  taken  place 
in  a  clay  slate,  in  the  contact  zone  of  dolerite,  in  its  passage  to  spilosite 
and  adinole.     The   analyses   are  given  in   the   order  of  approach  to  the 

dolerite  as  determined  by  the  character  of  tlie  rocks.     No.  1  is  the  uimieta- 
MON  xxxvi 14 


210 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 


morphosed  form  of  clay  slate ;  Nos.  2  and  3  are  the  intermediate  phase,  and 
No.  4  is  the  jnost  metamorphosed  phase. 

Comparison  of  analyses  of  clay  slate,  spilosites,  and  adinole. 


1. 

2. 

3. 

'4. 

SiO, 

60.28 

.69 

22.61 

52.51 

1.70 

19.00 

None. 

3.31 

7.19 

Trace. 

1.55 

Trace. 

Trace. 

3.29 

.70 

6.72 

Trace. 

X.34 

+3.26 

.15 

None. 

57.77 

.92 

19.35 

None. 

1.29 

3.37 

Trace. 

1.71 

None. 

Trace. 

4.35 

.22 

8.22 

None. 

X.18 

+2.34 

.04 

None. 

74.16 

.37 

11.85 

TiO, 

Al.,03     

Cr>0" 

Fe  O3 

2.53 

.45 

Trace. 

.13 

.04 

.82 

1.66 

.06 

2.10 

None. 

FeO                       

MnO                      

CaO                  

BaO              

SrO 

M"-0       

1.35 

5.73 

.54 

2.10 

.15 

6.57 

KG     

Na.O 

Li,0                                     

H  0  at  100° —             . .             

.60 

3.62 

.03 

None. 

.97 

.05 
.52 
.08 
.09 
.18 

HoO  at  100°+               

PjO,                       

CO,            

c        .               

SandSOi 

None. 
None. 
Trace. 

None. 
None. 
None. 

CI 

F                                                       .   . 

Total 

99.57 

99.72 

99.76 

100.  76 

XHoO  at  110°. 

+HiO  above  110°. 

No.  l  =  Clay  slate  (.Sp.  32497).     Sec.  17,  T.  43  N.,  R.  31  W.,  450  N.,  1620  W. ;  George  Steiger. 

No.  2  =  Spiiosite  (Sp.  32861).     Sec.  7,  T.  43  N.,  E.  31  W.,  750  N.,  1380  W. ;  H.  N.  Stokes. 

No.  3=Spilosite  (.Sp.  32827).     Sec.  7,  T.  43  N.,  R.  31  W.,  250  N.,  325  W. ;  H.  N.  Stokes. 

No.  4  =  Aillnole  (Sp.  32465).     Sec.  8,  T.  43  N.,  R.  31  W.,500  N.,  475  W. ;  George  Steiger. 

In  these  analyses  the  usual  increase  of  silica  as  the  dolerite  is 
approached  is  at  once  noticeable,  and  hand  in  hand  with  it  goes  the  diminu- 
tion in  percentage  of  alumina  and  iron  oxides.  The  content  of  water  and 
carbonaceous  matter  also  suffers  a  diminution.  The  most  iKiteworthy  differ- 
ence between  the  clay  slate  and  the  contact  rocks  is  shown  in  the  relations 
of  potassa  and  soda.  This  is  well  Ijrought  out  in  an  examination  of  analyses 
Nos.  1  and  2.  It  will  be  seen  that  there  is  only  about  one-eighth  as  much 
potassa  in  the  contact  rocks  as  in  the  normal  clay  slate;   while,  on  the  con- 


CONTACT  METAMOUPHISM  BY  INTKUSIVES.  211 

triir\',  aljout  12  times  as  much  soda  as  there  was  in  tlie  shitt^  has  hceii  added 
to  the  contact  rock.  This  causes  a  reversal  of  the  relations  of  the  soda  and 
potassa,  so  that,  whereas  in  the  clay  slate  there  is  present  10  times  as  nuicli 
potassa  as  soda,  we  find  in  the  contact  rock  taken  as  a  sample  very  nearly 
1»)  times  as  nuich  soda  as  potassa.  The  very  considerable  change  in  chemi- 
cal t-omposition,  especially  in  the  amount  of  silica  and  soda,  seems  to  lend 
great  weight  to  the  supposition  that  in  such  contacts  an  actual  transfer  of 
material  (soda-silicate)  takes  place  from  the  basic  intrusive  to  the  slate. 
This  idea  is  upheld  by  Roth,^  Zirkel,"  and  others.  W.  Maynard  Hutchings^ 
advocates  this  view,  and  has  described  some  interesting  products  as  a  result 
of  the  contact  of  the  Whin  Sill  which  still  further  support  it. 

XO    KNDOMUUPIIIC    EFFKCTS    OF    DOLKRITE    IN'TUUSION. 

Although  the  exoraorphic  contact  effects  of  the  dolerite  intrusion  were 
so  obvious,  no  evidence  is  found  that  the  dolerite  itself  suffered  any  change 
consequent  upon  its  intrusion. 

METABASALT. 

Basalt  has  been  described  at  length  under  the  volcanics,  where  it  plays 
an  exceedingly  important  role.  Basalt  as  a  dike  has  been  found  in  only 
two  places,  and  therefore  very  little  remains  to  be  added. 

The  two  basalt  dikes  occur  within  a  very  short  distance  of  each  other, 
in  sees.  15  and  16,  T.  42  N.,  R.  31  W.,  and  are  found  penetrating  the  crystal- 
line schists  of  the  Upper  Huronian.  Their  reUitions  to  the  other  intrusive 
rocks  of  the  same  region  are  not  known.  The}'  are  probably  of  the  same  age 
as  the  dolerites,  of  whicli  they  should  most  likely  be  considered  offshoots. 

These  dikes  are  a  porphyritic  basalt.  The  phenocrysts  were  of  augite, 
olivine,  and  labradorite.  They  were  in  a  very  fine  groundmass  of  feldspar, 
augite,  and  iron  oxide.  However,  the  former  existence  of  the  augite  and 
olivine  phenocrysts  is  determinable  only  by  means  of  their  outlines.  They 
are  in  very  small  quantity  and  are  entirely  altered  to  pilite.  The  feldspar 
phenocrysts  are  in  coarse,  heavy  crystals  and  are  remarkably  fresh.  The 
groundmass  is  very  fine  grained,  and  ranges  from  an  exceedingly  fine  micro- 

'  Chem.  Geol.,  by  .1.  Roth,  Berlin,  1890,  Vol.  Ill,  p.  145. 

^  Lehrbuch  tier  PetrograpUie,  by  F.  Zirkel,  Vol.  II,  1894,  p.  722. 

'  Notes  (III  thu  composition  of  clay  slates,  etc.,  and.  on  some  points  in  their  contact  metamorpliism, 
by  W.  Maynard  Hutchiugs :  Geol.  ila','.,Vol.  I,  Dec.  4,  1894,  p.  75.  Cheni.  Geol.,  Vol.  Ill,  p.  145.  An 
interesting  contact  rock,  with  notes  on  contact  metamorphism,  by  W.  Maynard  Hutchlngs :  Geol.  Mag., 
Vol.  II,  1895,  pp.  122-131,  163-169. 


212  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

ophitic  texture  to  the  pilotaxitic  texture.  The  feldspars  in  it  are  in  small  lath- 
shaped  individuals,  and,  like  the  phenocrysts,  are  fresh.  The  augite  of  the 
groundmass  is  to  a  great  extent  altered  to  uralite,  and  the  iron  ores  to  spliene. 
One  of  the  dikes  is  about  5  feet  wide.  In  the  center  it  is  a  moderately 
fine-grained  rock;  on  the  edges  it  is  a  dense  aphanitic  basalt.  Even  in  thin 
section  the  gradation  from  the  rock  with  microophitic  groundmass  to  the 
one  with  a  dense  pilotaxitic  groundmass  is  well  shown.  A  dike  of  larger 
size  might  readily  have  cooled  sufficiently  slowly  to  have  crystallized  at  its 
center  as  a  dolerite. 

UliTRA-BASIC  INTRUSIVES. 

'^ Under  this  head  are  the  descriptions  of  the  picrite-porphyries  (porphy- 
ritic  limburgites). 

PICRITE-PORPHYRY    (PORPHYRITIC    LIMBURGITE). 
GEOGRAPHICAL    DISTRIBUTION    AND   EXPOSURES. 

The  picrite-porphyries  occur  in  isolated  outcrops  of  comparatively 
small  size  in  sees.  9,  22,  and  27,  T.  44  N.,  R.  32  W.,  in  the  area  supposed 
to  be  underlain  by  the  Lower  Huronian  Hemlock  volcanics.  They  are 
surrounded  by  outcrops  of  the  altered  poikilitic  dolerites,  but  the  exposures 
are  not  such  as  to  allow  their  relations  to  be  determined.  Their  occurrence 
points  to  an  intrusive  character.  It  is  on  account  of  their  field  occui-- 
rence  alone  that  we  feel  justified  in  describing  them  here  under  the  general 
heading  for  this  chapter,  "Intrusives,"  instead  of  under  the  volcanics  with 
the  basalts,  their  proper  place  from  a  strict  petrographical  standpoint. 

PETROGBAPHICAL  CHARACTERS. 

The  picrite-porphyries  are  medium-grained  rocks,  which  varj^  in  color 
from  gray  to  dark  green  and  almost  black.  In  general  they  have  a  por- 
phyritic  character.  This  is,  however,  not  so  well  marked  in  the  gray  as  in 
the  darker-colored  rocks.  The  gray  ones  have  a  spotted  appearance.  The 
sppts  are-  gray  in  color,  fibrous,  ver}"  rarely  larger  than  3  or  4  millimeters 
in  length,  and  lie  in  a  finely  fibrous,  dark-green  matrix.  In  the  dark  rocks 
the  porphyritic  crystals  reach  a  length  of  1  centimeter,  and  are  bluish  to 
black,  with  silky  luster.  They  lie  in  a  fine-grained,  more  or  less  fibrous, 
green  groundmass.  In  one  of  the  dark  rocks  the  magnetite  is  very  notice- 
able. The  crystals  project  from  the  weathered  surface  and  the  rock  is 
strongly  polar-magnetic. 


ULTHA  BA81G  INTIUTSIVBS.  213 

Tlic  rocks  ()rii;iiiall\'  cousisti.'il  l;ii';ic'ly  i>t  (ilixiiic,  i)\i'().\ciic,  linruhlciiiU', 
biotite,  inaynetitt',  and  ilinenite.  Tliey  now  contain  alsso,  in  consideralile 
quantity,  a  cliloriti<'  product  wliicli  seems  to  lia\e  been  dei'ived  from  the 
alteration  of  an  original  vitreous  base.  All  of  the  specimens  are  exceed- 
in<j'lv  altered.  The  orig-inal  mineral  constituents  have  to  a  great  extent  been 
determined  from  their  form,  which  in  some  cases  has  been  preserved  by  the 
})rodncts  of  alteration,  and  by  certain  structures  in  the  i)seudomorphs.  The 
minerals  now  com])Osing  the  rock  are  original  hornblende,  biotite,  apatite, 
magnetite,  and  ilmenite,  with  secondary  amphibole,  serpentine,  chlorite, 
calcite,  sphene,  and  rutile. 

The  two  kinds  of  rocks,  the  gray  and  the  dark-colored  ones,  were 
evidently  derived  from  rocks  of  essentially  the  same  composition.  They 
have  undergone  difterent  processes  of  alteration,  and  upon  this  depends  the 
difference  in  color.  As  the  study  of  these  picrites  is  chiefly  one  of  the 
alteration  products  of  the  minerals  which  composed  them,  it  seems  best 
to  describe  sej^arately  the  two  rocks  showing  the  different  products  of 
alteration. 

GRAY   TUEMOLITIZED   riCUITE-POUPHYRY. 

In  the  gray  rocks  the  spots  which  are  macroscopicallj^  observed  are 
found  under  the  microscope  to  consist  of  an  aggregate  of  minerals.  Exami- 
nation of  these  aggregates  shows  them  to  be  entirely  secondary.  A  careful 
study  of  these  aggregates  .shows  them  to  consist  of  amphibole,  magnetite, 
ilmenite,  and  serpentine,  the  first  being  pi'edominant.  No  trace  of  the  origi- 
nal minerals  remains.  The  aggregates  are  the  same  in-  all  of  the  crystals, 
and  the  only  clue  to  the  original  mineral  is  the  form  of  the  pseudomorphs 
and  certain  structures  in  the  aggregates.  By  means  of  the  fomi  the  pheno- 
crysts  are  readily  divisible  into  three  kinds.  The  first  kind  has  a  long 
prismatic  habit,  with  pyramidal  faces  meeting  at  rather  an  acute  angle. 
The  iron  oxide  is  arranged  along  certain  lines,  giving  the  characteristic 
mesh  structure  of  Serpentinized  olivine.  The  second  kind  is  a  short,  thick 
prism,  for  the  most  ])art  with  rounded  ends,  in  some  cases  the  pyramidal 
faces  meeting  in  a  rather  obtuse  angle.  The  iron  oxide  in  some  of  these 
cases  marks  an  imperfect  parting  perpendicular  to  the  long  direction  of  the 
prism.  These  are  supposed  to  be  pseudomorphs  after  a  pyroxene.  The 
third  kind  consists  of  round  and  irregular  grains  or  plates,  some  of  which 
may  be  referred  to  py^roxene,  others  to  olivine. 


214  THE  CRYSTAL  FALLS  IKON-BEARING  DISTRICT. 

The  amphibole  is  in  small  needles.  It  has  a  very  faint  greenish  tinge. 
In  cross  section  it  shows  marked  prismatic  development.  The  character  of 
the  needles  is  plus  in  the  long  direction.  The  maximum  angle  found 
between  c  :  c  is  18  degrees.  The  needles  appear  to  be  tremolite  containing 
some  iron,  and  thus  approaching  actinolite  in  composition.  Usually  the 
needles  have  no  regular  arrangement,  but  in  some  of  the  pseudomorphs 
with  rectangular  outlines  there  is  a  parallel  arrangement  of  such  a  number 
of  the  needles  parallel  to  the  long  axis  of  the  pseudomorphs  as  to  give  to 
the  pseudomorjih  a  distinctly  uniform  polai-ization  effect. 

Isolated  crystals  of  magnetite  and  brownish  transparent  plates  of 
ilmenite  are  scattered  among  the  actinolite  needles.  By  far  the  greater  part 
of  the  iron  oxide  is  collected  in  aggregates  of  small  crystals  and  irregular 
grains.  The  formation  and  arrangement  of  tliese  aggregates  has  in  some 
cases  taken  jjlace  along  fracture  and  cleavage  planes  of  the  original  minerals, 
and  thus  in  the  pseudomorphs  we  see  the  mesh  structure  of  olivine  and 
transverse  parting  of  pyroxene  clearly  brought  out.  In  other  cases  the  iron 
oxide  is  in  irregular  masses  collected  at  the  center  or  outlining  the  periphery 
of  the  pseudomorphs  or  scattered  in  small  masses  through  them. 

Between  the  tremolite  needles  and  the  iron  oxide  is  a  small  quantity 
of  minute  fibers.  They  have  a  greenish  tinge  and  low  double  refraction. 
Their  extinction  is  parallel  to  the  long  c  axis,  which  is  also  the  axis  of  least 
elasticity.  They  are  believed  to  be  serpentine  fibers.  No  definite  arrange- 
ment of  these  fibers  could  be  discerned  over  the  greater  part  of  the  pseudo- 
morphs, but  in  one  crystal,  on  the  edge  of  the  section,  where  it  is  esjjecially 
thin,  the  arrangement  of  these  needles  perpendicular  to  the  long  direction 
of  the  iron  aggregates  outlining  the  meshes  is  unmistakable.  Calcite  is  in 
considerable  quantity  in  some  of  the  pseudomorphs.  It  is  highly  probable 
that  it  owes  its  origin  to  the  alteration  of  the  original  mineral,  though  some 
of  the  calcium  went  into  the  amphibole. 

Besides  the  above-described  pseudomorphs  after  olivine  and  pyroxene, 
a  few  large  prismatic  and  irregularly  bounded  areas  were  found  among  the 
phenocrysts,  which  now  consist  chiefly  of  chlorite,  with  grains  of  calcite, 
titanite,  magnetite,  and  minute  plates  of  ilmenite  scattered  tlu-ough  them. 
It  is  clear  that  the  chlorite  is  derived  from  a  hornblende,  as  shown  by  the 
presence  of  ragged  remnants  of  hornblende  which  possesses  uniform  orien- 
tation throughout  each  area.     This  hornblende  shows  weak  pleochroism  in 


ULTUAHASIG  INTRUSIVES.  215 

tlu^  light-VLillow  to  yreenisli  tones  of  actinolite,  ulthough  its  character  is 
more  that  of  the  compact  hornblende.  In  one  case  its  secondary  natm-e 
was  shown  l)y  the  presence  of  a  small  irreg'ular  area  of  brown  hornblende 
lyinji'  in  a  mass  of  the  green.  The  two  have  the  same  orientation.  In  this 
case  the  chlorite  is  apparently  a  tertiary  product,  the  orig'inal  mineral 
1  )eing  the  brown  hornblende,  from  which  w.as  formed  the  light-green  variety, 
which  in  its  turn  alters  to  the  chlorite. 

Between  the  various  pseudomorphs  are  irregular  plates  of  compact, 
dark-brown  hornblende,  plates  of  biotite,  large  crystals  of  magnetite,  and 
roug'h  branching  aggregates  of  ilmenite.  These,  while  molded  on  the 
phenocrysts,  themselves  lie  in  the  chloritic  mass  already  mentioned,  which 
also  often  completely  surrounds  the  j)henocrysts,  and  which  is  probabl}"  an 
altered  vitreous  base. 

The  pieces  of  brown  hornblende  which  remain  unaltered  show  moder- 
ately strong  pleochroisin,  reddish  brown  for  c  and  tt  and  light  brownish 
yellow  for  a.  Czrb>>a.  This  hornblende  contains  inclusions  of  iron  oxide 
and  has  all  the  appearance  of  an  original  mineral.  By  alteration  it  passes 
through  a  compact  greenish  amjjhibole  to  a  much  lighter  colored,  reedy, 
actinolitic  variety  of  amphibole.  In  the  secondary  amphibole  occur  certain 
golden-brown  grains  with  high  single  and  double  refraction,  which  are 
supposed  to  be  rutile  formed  from  the  hornblende,  and  also  some  brown 
transparent  plates  of  ilmenite.  The  orientation  of  the  secondary  horn- 
blende is  the  same  as  the  original.  No  further  alteration  of  this  amphibole 
was  observed,  but  it  is  believed  that  the  prismatic  crystals  altered  to 
chlorite,  calcite,  and  magnetite,  as  described  above,  are  the  extreme  cases 
of  alteration  of  an  automorphic  form  of  a  brown  hornblende  very  similar 
to  the  part  described. 

The  biotite  between  the  phenocrysts  is  in  ragged  areas  either  surround- 
ing iron  oxide  or  associated  with  it  or  with  the  hornblende.  It  is  very 
pleochi'oic,  the  absorption  jjarallel  to  the  basal  cleavage  being  so  strong  as 
to  render  the  section  opaque.  Perpendicular  thereto  the  color  is  a  dai-k 
chocolate  brown.  The  mica  does  not  show  its  usual  bright  polarization 
colors  in  sections  cut  parallel  to  crystallographic  c.  This  may  be  due  in 
some  measure  to  the  very  strong  absorption.  In  some  cases  the  biotite  is 
seen  to  have  a  strong  blue  to  violet  metallic  luster  in  incident  light.  The 
biotite  has  partly  altered  to  chlorite.     The  alteration  proceeds  along  the  basal 


216  THE  CRYSTAL  FALLS  IROJf -BEARING  DISTRICT. 

cleavage.  As  this  alteration  progresses  there  is  a  lighteuiiig  of  the  color  of 
the  biotite,  and,  as  a  consequence  of  this  the  whole  cause  of  the  metallic 
luster  and  the  partial  cause  of  the  color  of  the  biotite  is  disclosed.  In  the 
lighter  biotite  one  by  careful  examination  can  see  innumerable  small  plates 
of  a  brown  or  smoky  color.  At  first  sight  they  remind  one  strongl}-  of  the 
inclusions  so  common  in  man}'  hj^perstlienes.  Closer  examination  only 
emphasizes  this  reseiiiblance,  and  they  are  believed  to  be  micaceous  ilmenite 
plates.  These  inclusions  were  studied  by  means  of  an  oil  immersion 
objective  givmg  a  magnification  of  about  1,250,  and  were  found  to  have 
mainly  a  roundish  or  hexagonal  outline.  In  addition  to  these,  some  plates 
of  long,  irregular  form  were  observed.  These  are  all  isotropic  and  lion- 
pleochroic.  These  minute  plates  lie  parallel  to  the  biotite  lamellse.  The 
consequence  of  this  is  that  in  sections  parallel  to  c  one  sees,  for  the  most 
part,  onl}^  short  black  streaks — the  edges  of  the  plates — whereas  in  the  basal 
sections  of  the  biotite  one  can  determine  the  irregular  or  rounded  contours 
of  the  plates.  The  plates  ai-e  too  small  to  allow  the  metallic  luster  to  be 
seen  on  an  isolated  one.  En  masse  they  j^roduce  a  very  decided  blue 
metallic  shimmer,  as  seen  in  some  of  the  biotite  fragments. 

Numerous  apatite  crystals  occur.  They  are  usually  clear  white,  but 
one  crystal  was  seen  showing  a  dichroism  from  faintest  brownish  for  rays 
perpendicular  to  crj-stallographic  c  to  light  smoky  brown  for  rays  parallel 
thereto.     This  crystal  contains  a  core  of  brown  glass. 

•  Some  of  the  iron  oxide  is  in  roughly  rectangular  masses,  and  appears 
to  be  magnetite.  This  is  associated  with  an  iron  oxide,  which  occurs  in 
opaque,  ragged  masses  formed  of  long,  irregular,  and  knotty  stringers. 
These  at  places  are  parallel  to  one  another  and  at  other  places  cut  one 
another  at  various  angles,  and  at  still  other  places  meet  at  a  common  cen- 
ter, forming  an  opaque  mass  of  varying  dimensions,  but  usually  small. 
Now  and  then  one  of  the  large  magnetite  masses  constitutes  a  center  from 
which  extend  the  knotty,  irregular  stringers.  The  general  appearance  of 
these  ragged  masses  is  that  described  by  Grerman  petrographers  as  mrhackt. 
When  these  stringers  pierce  the  section  at  an  oblique  angle,  the  ends 
are  translucent,  with  a  brown  color,  becoming  more  opaque  as  the  section 
gets  thicker.  Such  masses  have  all  the  appearance  of  ilmenite,  and  are 
believed  to  be  that  mineral.  Similar  ilmenite  stringers  are  included  in  the 
chlorite,  wliich  results  from  the  alteration  of  the  biotite 


ULTRA  BASIC  INTUUSIVES.  217 

The  cliloritc  in  tlio  iiiirainorplis  after  liiotite  sliows  extrenielv  low  blue 
polarization  color,  and  the  characteristie  pleoehroism — yellowish,  tinged 
with  red,  when  the  rays  vibrate  perpendicular  to  the  cleavage,  and  green 
when  i)arallel  thereto.  Apatite  needles  included  in  the  l)iotite  are  unaltered 
in  the  secondar}'  chlorite. 

Some  of  the  minute  octahedral  crystals  in  the  amphibole  pseudomorphs 
after  olivine  appear  to  be  slightly  pellucid,  with  a  brown  color.  If  so,  thev 
might  be  referred  to  picotite,  but  there  is  doubt  of  the  correctness  of  the 
observation,  in  view  of  the  high  power  used,  the  oil  immersion  lens,  and 
the  fact  that  the  search  was  for  picotite.  Close  search  was  also  made  for 
perovskite,  but  none  could  be  found,  unless  the  transparent  crystals  very 
doubtfully  referred  to  picotite  are  realh'  iierovskite. 

Forming  the  matrix  in  which  the  pseudomorphs  after  hornblende, 
olivine,  and  pyroxene  of  these  rocks  lie,  and  frequently  surrounding 
isolated  crystals,  one  sees  an  aggregate  composed  chiefly  of  a  fine  felt  of 
chlorite  fibers.  This  alteration  product  contains  a  few  apparently  original 
apatite  needles,  some  secondary  grains  of  magnetite,  and  crystals  of  amphi- 
l)ole  which  are  colorless  or  else  show  but  the  faintest  tinge  of  green,  and  are 
larger  than  the  amphibole  crystals  in  the  pseudomorphs.  It  is  a  secondary 
amphibole  very  poor  in  iron,  probably  highly  calcareous,  and  approaching 
tremolite  in  composition.  This  chlorite  aggregate  shows  no  indication 
whatever  of  crystal  forms.  It  seems  to  be  the  product  of  a  homogeneous 
mass,  such  as  would  result  from  the  decomposition  of  a  vitreous  base.  Such 
a  base  the  agg-regate  is  presumed  to  represent,  althoug'h  no  trace  whatever 
of  the  glass  has  been  observed  in  the  rock,  nor  in  view  of  the  altered  con- 
dition of  the  rock  could  such  a  glass  be  reasonably  expected  to  still  remain. 


DARK    SERI'ENTINIZED   PICRITE-rOIiPH YRY. 


The  second  variet}'  of  the  jncrite-porphyries  is  very  dark  greenish- 
black,  and  represents  the  results  of  a  slighth'  different  process  of  alteration 
from  that  by  which  the  gray  forms  just  described  were  produced.  These 
dark  picrite-porphyries  show  a  very  much  better  developed  porphvritic 
structure  than  do  the  gray  ones.  This  is  due  to  the  fact  that  the  oliAines  in 
these  rocks  were  well  developed  and  reached  a  length  of  a  centimeter.  The 
olivines  are  completely  altei-ed,  serpentine,  pilite,  and  magnetite,  being  the 
products  which  form  the  pseudomorphs.     The  characteristic  mesh  structure 


218  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

of  altered  olivine  is  well  broug-lit  out  l^y  the  serpentine  and  iron  ore.  In 
the  centers  of  the  meshes  there  remain  small  masses  of  a  felt  of  tremolite 
needles  (pilite).  This  alteration  of  the  olivine  corresponds  to  that  first 
described  by  Lewis/  and  more  recently  by  Professor  Bonney  and  Miss 
Raisin,^  from  a  rock — kimberlite — very  similar  to  the  picrite-porphyries 
here  described.  He  writes  as  follows:  "It  frequently  happens  that  while 
sei'pentinization  begins  at  the  outside  of  a  crystal,  iibrous  tremolite  begins 
growing-  within,  finally  forming  a  mass  of  asbestiform  fibers  surrounded  by 
a  zone  of  green  serpentine." 

The  minerals  which  composed  these  black  picrite-porphyries  were  the 
same  as  those  constituting  the  gray  ones.  These  minerals  were  olivine, 
pyroxene,  hornblende,  biotite,  magnetite,  and  ilmenite.  They  were  cemented 
by  a  glass  matrix.  The  glass  is  completely  altered.  All  of  the  minerals 
are  represented  by  pseudomorphs.  Remnants  of  the  original  hornblende 
and  biotite  alone  are  preserved. 

The  contours  of  the  original  pyroxene  crystals  are  filled  with  pilite, 
serpentine,  and  magnetite.  The  serpentine  is  present  in  greater  quantity 
in  these  pyroxene  pseudomorphs  than  it  was  in  tlie  pyroxene  pseudomorphs 
in  the  o-ray  picrite-porphyries.  The  alteration  of  the  hornblende  results 
in  the  production  of  an  aggregate  of  chlorite  inclosing  grains  of  caleite, 
some  sphene,  and  iron  oxide,  similar  to  that  in  the  gray  picrite-porphyries. 
The  biotite,  magnetite,  and  ilmenite  also  show  those  characters  which  have 
been  described  for  the  same  minerals  in  the  first-described  picrite-porphyries. 

Between  all  of  the  foregoing  minerals  we  find  a  fine  felty  chlorite 
mass  eontaimng  grains  and  dendritic  masses  of  iron  ore  and  a  few  needles 
of  tremolite.  This  corresponds  to  the  material  forming  the  cement  for  the 
minerals  in  the  gray  porphyries,  and,  hke  that,  is  believed  to  represent  an 
original  vitreous  matrix. 

In  one  of  the  dark  picrite-porphyries  the  magnetite  is  present  in  large 
quantity  and  is  very  noticeable,  crystals  of  it  standing  out  upon  the  weath- 
ered surface.  This  rock  did  not  affect  the  magnetic  needle  very  powerfully, 
though  it  was  expected  that  it  would  do  so.     However,  another  one  of 

1  Ou  a  dlamondiferous  peridotite  and  the  genesia  of  the  diamond,  hy  H.  C.  Lewis:  Geol.  Mag., 
3d  ser.,  Vol.  IV,  1887,  p.  22. 

Papers  and  notes  on  the  genesis  and  matrix  of  the  diamond,  hy  the  late  Henry  Carvill  Lewis, 
edited  hy  Prof  T.  G.  Bonney,  London,  1897,  p.  14. 

2  Notes  on  the  diamond-hearing  rock  of  Kimherly,  South  Africa,  Part  11,  hy  Prof  T.  G.  Bonney 
and  Miss  C.  A.  Raisin  :  Geol.  Mag.,  4th  series,  Vol.  II,  1895,  p.  496. 


ULTKA-BASIC  INTRITSIVES. 


219 


tliosc  porphyrios,  in  wliicli,  by  tlio  way,  tlic!  iron  content  is  relatively  low, 
is  unitpie,  in  tiiat  it  is  very  stronj^ly  \)o\av  niag-netio,  and  in  this,  as  well  as 
its  probable  original  niineralogical  composition,  may  be  cdinpanMl  with  the 
polar  magnetic  wehrlite  from  the  Frankenstein,  Hesse-Darmstadt,  Germany.' 
The  German  rock  shows  tremoliti?  scattered  through  the  serpentine  result- 
in"-  from  the  olivine.  It  is  a  coarse,  evenly  granular  rock,  differing  in  this 
respect  from  the  Crystal  Falls  rocks  which  are  porphyritic. 

Au  analysis  (No.  1)  of  the  polar  magnetic  serpentinized  picrite- 
porphyry,  in  which  great  abundance  of  olivine  was  originally  present,  is 
here  given,  and  there  is  placed  with  it  for  comparison  an  analysis  (No.  2) 
of  a  very  similar  rock  described  by  Darton  and  Kemp,"  from  New  York. 
Both  analyses  were  made  by  Dr.  H.  N.  Stokes,  United  States  Geological 
Survey.     In  No.  1  Ba,  Sr,  Li,  CI,  S,  and  SO'  Avere  not  looked  for. 

Analyses  of  picrite-porphyry. 


SiO- 37.36 

TiO, : "t^ 

Al.O:, ITO 

Cr,03 62 

Fe,03 n.61 

FeO 6.12 

MnO Trace. 

XiO ;  .04    I 

CaO [        1.19 

BaO 

SrO 

MgO I      31.11 

K^O ;1  Trace,  i 

Na^O 1  I 

P.O5 1  -06 

CO. None. 

SO3 ! 

S  I 

HiOatllO^ -65 

HnO  above  110° 10-37 


36.80 

1.  2G 

4.16 

.20 


Less  0=S  . . 
Total 


8.33 
.13 
.09 

8.63 

.12 

Trace. 

2.5. 98 

2.48 
.17 
.47 

2.95 
.06 
.95 
.51 

6.93 


99.68 


100.22 
.47 


99.75 


1  Der  raagnetstein  von  Frankenstein  an  der  Bergstrasse,  by  Audreae  and  Kfinig:  Abhaudl.  der 
Senkenberg.  naturf.  Gesell.,  Frankfort  a  Main,  1888,  pii.  59-79. 

Cf.  Above  article,  p.  66,  footnote,  for  references  to  other  occurreuces  of  tremolite  as.soclated 
Tvith  serpentine. 

-  Newly  discovered  dike  at  Dewitt,  near  Syracuse,  New  York,  by  N.  H.  Darton  and  J.  F.  Kemp: 
Am.  .Jonr.  Sci.,  Vol.  XLIX,  1895,  p.  461. 


220  THE  CRYSTAL  FALLS  IRON-BEAKIiSG  DlSTKiUT. 

CLASSIFICATION. 

These  rocks  just  described,  from  their  miiieralogical  composition,  if  we 
admit  the  presence  of  a  vitreous  base,  would  belong-  with  the  picrite- 
porphyrites  of  Rosenliusch.'  This  designation  does  not  seem,  however,  to 
be  appropriate,  as  he  states"  that  he  uses  the  term  "porphyrite"  onlv  for 
certain  textural  phases  of  rocks  containing  lime-soda  feldspar.  He  has 
evidently  extended  that  definition  so  as  to  be'  able  to  iise  it  for  these 
picrites,  considering  that  the  glass  possesses  the  necessary  ingredients  for 
the  formation  of  such  lime-soda  feldspar,  provided  the  conditions  under 
which  it  cooled  had  been  favorable  for  the  feldspar  development. 

The  porphyritic  texture  of  these  Crystal  Falls  picrites  and  the  presence 
of  a  vitreous  base^  show  them  to  be  closely  related  to  rocks  of  effusive  char- 
acter. Those  which  they  most  closely  resemble  among  the  younger  basaltic 
lavas  are  the  porphyi-itic  forms  of  the  limburgites  (magma  basalts). 

One  of  the  best-known  rocks  with  which  this  may  be  closely  compared, 
as  far  as  association  is  concerned,  is  the  rock  first  described  by  H.  Carville 
Lewis  as  a  saxonite-porphyry,*  later  called  kimberlite.  This  was  described 
by  him  as  volcanic,  and  as  associated  with  dolerites  and  melaphyres.  He 
described  it  as  a  basic  lava.^  Other  occurrences  of  very  closely  related  basic 
rocks  havino-  a  vitreous  base  have  been  described  from  the  United  States  bv 
Diller, Williams,  Merrill,  Brainier  and  Brackett,  Kemp,  and  Darton  and  Kemp." 

'  Microscopische  Physiograpliie,  by  H.  Roseiibusch:  3fl  ed.,  Stuttgart,  Vol.  II,  1896,  p.  1191. 

-Op.  cit.,  p.  436. 

'Should  the  vitreous  baso  be  considered  as  not  having  been  pre.sent  and  the  rocks  be  pnt  among 
the  peridotites,  then  they  would  correspond  very  closely  to  the  wehrlite  described  on  p.  2."i4. 

••Papers  and  notes,  cit.,  p.  .">0. 

'■The  genesis  of  the  diamond,  by  H.  C.  Lewis :  Science,  Vol.  VIII,  1886,  p.  345. 

On  a  diamon<liferous  peridotite  and  the  genesis  of  the  diamond,  by  H.  C.  Lewis:  Geol.  Mag., 
3d  series,  A'ol.  IV,  1887,  p.  22. 

"Dikes  of  jieridotite  cutting  the  carboniferous  rocks  of  Kentucky,  by  J.  S.  Diller:  Science.  l,S8.i, 
p.  65;  Notes  on  the  peridotite  of  Klliot  County,  Kentucky,  by  ,J.  S.  Diller:  Am.  .lour.  Sci.,  Vol.  XXXIl, 
1886,  p.  188;  Hull.  U.  S.  Geol.  Survey,  No.  38, 1887. 

The  serpentine  (peridotite)  occurring  in  the  Onondaga  salt  group,  at  Syracuse,  New  York,  by 
G.H.  Williams:  Am.  .lour.  Sci.,  Vol.  XXXIV,  1887,  p.  137;  Proc.  Geol.  Soc.  Am.,  A'ol.  I,  1889,  p.  533; 
Perowskit  in  serpentin  von  Syracuse,  New  York,  by  G.  H.  Williams :  Neues  Jahrb.  Vol.  II,  1887,  p.  263. 

On  a  peridotite  from  Little  Deer  Isle,  in  Penobscot  Bay,  Maine,  by  G.  P.  Merrill :  Proc.  U.  S.  Nat. 
Mus.,  1888,  p.  191. 

The  peridotite  of  Pike  County,  Arkansas,  by  J.  C.  Brauner  and  R.N  Brackett:  Am.  Jour.  Sci.. 
Vol.  XXXVIII,  1889,  p.  50. 

Peridotite  dikes  in  the  Portage  sandstone  of  Ithiica,  New  York,  by  .1.  F.  Kemp:  Am.  ,Iour.  Sci.. 
Vol.XLII,  1891,p.  410. 

A  newly-discovered  dike  at  Dewitt,  near  Syracuse,  New  York,  by  N.  H.  Darton  and  .1.  F.  Kinij) 
Am.  Jour.  Sci.,  Vol.  XLIX.  189.3,  p.  456. 

riie  rock  described  liy  F.  L.  Rausome  as  a  fourchite  should  i>erhaps  also  be  compared  with  these 


ULTRA-BASIC  INTRUSIVES.  221 

Hatch'  lias  also  described  a  very  similar  pre-Tertiary  rock  from  Kug- 
laiid  as  a  limburgite.  Kemp-  emphasizes  the  resemblance  of  the  Dewitt 
dike  to  liiiil)urgite,  and  states  that  it  should  be  called  limburgite.^  If  we 
attempt  to  extend  the  use  of  the  term  "  limburgite"  to  include  the  pre-Tertiary 
vitreous  basalts,  we  shall  have  to  include  under  it  the  rocks  heretofore  desig- 
nated as  picrite  and  picrite-porphyrite.  Rosenbusch  has  now  put  the  jiicrites 
and  picrite-porphyrites  with  the  effusive  rocks,  and  if  of  these  two  sets  of 
terms  there  is  one  to  be  discarded,  it  should  be  the  name  "limburgite."  It 
seems  preferable  luider  the  rules  of  priority  to  retain  the  name  "picrite."  It 
would  then  seem  very  suitable  to  apply  to  these  pre-Tertiary  porphyritic 
limburgites  Hussak's  old  term,  "picrite-porphyry,"  using  the  term  "por- 
phyry" simply  witli  a  textural  significance.* 

SECTION  II.— A  STUDY  OF  A  ROCK  SERIES  RANGING  FROM 
ROCKS  OF  INTERMEDIATE  ACIDITY  THROUGH  THOSE  OF 
BASIC  COMPOSITION  TO  ULTRA-BASIC  KINDS. 

Beginning  near  the  town  of  Crystal  Falls,  in  isolated  knobs,  and 
extending  southeast  toward  the  Micliigamine  River,  where  the  exposures  are 
larger  and  better  connected,  there  is  found  a  series  of  rocks  wnose  charac- 
ters are  of  such  interest  petrogeiietically  as  to  warrant  a  detail  descrijjtion 
of  them. 

These  rocks  are  all  intrusive  in  character,  with  few  exceptions  are 
medivim  to  coarse  grained,  and,  while  the  granitic  texture  is  predominant, 
there  are  certain  facies  in  which  the  texture  is  porphyritic  and  even  parallel. 
They  have  been  only  slightly  affected  by  dynamic  action,  and  these  cases 
are  purely  local.  Analyses  show  them  to  vary  in  chemical  composition 
from  those  of  intermediate  acidity  to  those  of  ultra-basic  character. 

The  prevailing  rocks  are,  on  the  one  hand,  diorites  of  intermediate 
acidity  ranging   to    more   acid    rocks,   tonalites,   quartz-mica-diorites,   and 

rocks,  re[)re8enting  as  it  probably  does  the  oliviae-free  form  of  the  limburgite  (augitite).  Geology  of 
Angel  Island,  by  F.  L.  Ransoine :  Bull.  Geol.  Dept.  Univ.  of  Califoruia,  Vol.  1, 1894,  p.  200. 

'  The  Lower  Carboniferous  volcanic  rocks  of  East  Lowthian  (Carlton  Hills),  by  F,  Hatch:  Trans. 
Royal  Acad.  Edinburgh,  Vol.  XXXVII,  1892,  p.  115. 

-Op,  cit.,p.460. 

"'Taking  plutonic  rocks  as  practically  the  granitoid,  and  volcanic  as  the  porphyritic,  the 
Dewitt  rock  is  a  basaltic  dike  of  the  same  composition  and  texture  as  limburgite,  and  should  be  called 
limburgite,  even  if  it  is  not  a  surface  flow."     (Loc.  cit.,  p.  460. ) 

■•I  believe  E.  Hussak  was  the  first  to  use  this  term  for  a  somewhat  similar  rock.  Pikrit-pliorphyr 
von  Steierdorf  im  Banat,  by  E.  Hussak :  Verhandl.  K.-k.  geol.  Reichsanstalt,  1881,  pp.  258-262. 


222  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

granite  (plagioclastic),  and,  on  the  other,  hornblende  gabbros,  gabljros, 
norites,  and,  kxstly,  peridotites  of  varying  mineralogical  character.  These 
rocks  thus  resemble  in  their  -sanations  those  Scottish  plutonic  rocks  so  well 
described  by  Messrs.  Dakyns  and  Teall.^ 

The  rapid  changes  in  mineralogical  composition  and  texture  in  a  single 
rock,  and  the  changes  from  one  kind  to  another  through  intermediate  facies, 
show  verv  clearly  the  intimate  relationship  of  these  rocks  to  one  another, 
and  warrant  the  assumption  that  they  all  belong  to  a  geologic  unit,  a  con- 
clusion reached  a  luimber  of  years  since  by  Williams  for  a  somewhat  sim- 
ilar series,  the  Cortlandt  series,  from  New  York. 

Granite  is  present  as  a  local  facies  of  the  diorite.  Howevei',  it  is  very 
subordinate  in  quantity  and  not  altogether  typical,  and  as  no  analysis  has 
been  obtained,  its  jjosition  is  still  more  or  less  doubtful. 

In  the  following  pages  only  those  kinds  of  rocks  of  Avhich  analyses 
have  been  obtained  will  be  included  in  the  final  discussion.  Others  will  be 
described  in  detail  or  merely  mentioned,  as  representing  facies  of  the  main 
types,  according  to  their  petrological  interest.  The  rock  tyj^es  of  which 
analyses  have  been  made  are  as  follows:  Diorite,  gabbro,  norite,  and 
peridotite. 

DIORITE. 

NOMENCLATURE. 

Diorite,  according  to  the  generally  accepted  definition,  is  a  granular 
rock  consisting  essentially  of  hornblende,  which  must  be  primary,  and  a 
soda-lime  feldspar.-  The  term  has  been  used  in  a  difi^erent  sense  by  many 
writers  on  the  Lake  Superior  and  other  regions.  It  has  been  used  to  com- 
2irise  rocks  which  contain  hornblende  and  plagioclase  as  preponderating 
constituents,  it  is  true,  but  in  which  the  hornblende  is  secondary,  therein 
diff'ering  from  a  true  diorite.  These  so-called  diorites  have  been  regarded 
as  derived  from  an  original  dolerite  (diabase)  by  uralitizatiou  of  the  pyrox- 
ene. By  some  writers  these  rocks  have  been  classed  ^yh]^  the  epidiorites, 
thus  recognizing  their  secondary  nature,  but  by  this  name,  epidiorite, 
unfortunately  implying  a  false  relationship. 

In  this  paper,  following  Brogger,  I  restrict  tlie  name  to  the  granitic 

'  On  tlie  jiltitonic  rocks  of  Garabal  Mil!  and  Meall  Breac,  by  J.  K.  Dakyns,  es(|.,  M.  A.,  and  J.  J. 
H.  Teall,  esq.,  M.  A.,  F.  R.  S.,  F.  C.  S.:  Quart,  .hmr.  Geol.  Soc,  Vol.  XLVIII,  181W,  pp.  104-121. 
-Lehrbuch  der  Petrojjrapbic,  by  F.  Zirkel,  Leipzig-,  Vol.  II,  1891,  p.  465. 


DIOKITE  INTUUSIVES.  223 

rocks  of  iiitermediiite  acidity,  in  which  the  feldspar  is  ])hiy'iochiso  and  the 
bisilicate  constituent  is  mica  or  primary  hornblende.  The  feldspar  is  a  lime- 
soda  plagioclase.' 

DISTRIBUTION    AND    EXPOSURES. 

The  dist.ril)ation  of  the  diorite  is  limited  to  a  few  localities,  all  of  which 
are  in  the  area  underlain  by  Upper  Huronian  sedimentaries.  The  most 
typical  occurrences,  and  those  showing  greatest  variations,  form  knobs 
beginning  near  Crystal  Falls  and  continuing  to  the  south  and  south- 
east. Especially  large  outcrops  form  the  hills  in  sees.  19  and  20,  T.  43  N., 
R.  31  W.  The  smaller  occurrences  are  not  indicated  on  the  map.  These 
diorite  exposures  are  always  good,  so  far  as  getting  fairly  fresh  specimens 
are  concerned,  but  their  contacts  with  other  rocks  are  almost  invariably 
deeply  covered  with  drift;  hence  their  relations  in  many  cases  are  doubtful. 

PETROGRAPHICAL  CHARACTERS. 

The  diorites  are  holocrystalline  rocks  of  medium  to  coarse  grain.  In 
texture  they  show  some  variation  from  those  which  are  granitic  to  those  in 
which  the  texture  is  imperfectly  ojjhitic.  The  color  is,  on  the  whole, 
moderately  light  gray  or  reddish,  but  at  times  when  the  dark  minerals 
become  more  prominent  in  the  basic  facies,  especially  where  we  g"et  basic 
schlieren,  the  rock  is  very  dark  gray  or  greenish  brown. 

The  important  mineral  constituents  are  feldspar,  quartz,  biotite,  and 
hornblende.  The  accessory  minerals  are  epidote,  apatite,  zircon,  sphene, 
and  iron  oxides.  The  secondary  niinerals,  white  and  larown  mica,  chlorite, 
biotite,  epidote-zoisite,  calcite,  and  rutile  are  also  present. 

Feldspar. — Plagioclasc  feldspar,  orthoclase,  and  microcline  occur  together. 
The  plagioclase  is  found  in  individuals  which  are  fairly  automorphic.  In 
the  ophitic  textured  diorites,  the  plagioclase  is  the  best  developed  of  all  the 
essential  constituents.  In  the  granular  rocks  the  degree  of  automorphism 
is  highest  where  orthoclase  and  quartz  are  present  in  the  largest  quantity, 
and  diminishes  as  these  diminish,  when  the  plagioclase  individuals  beghi  to 
interfere  with  one  another's  development.     For  the  most  part  the  plagioclase 


'  Die  EruptivgeBteine  des  Kiiatiauiagebietes.  I.  Die  Gesteine  dor  Grorudit-Tiiigiiait-Soiic, 
by  W.  C.  Brogger,  1894,  No.  i,  p.  93.  II.  Die  Eruptionsfolge  der  tiiadischen  Eruptivgesteine  bei 
Predazzo  in  Siidtyrol,  1895,  No.  7,  p.  35.     Videnskabsselslvabets  Skrifter,  I  Mathematiskuaturv.  Klasse. 


224  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

gives  rather  thin  sections,  thoug-li  they  can  hardly  be  correctly  called  lath- 
shaped.  No  other  form  of  plagioclase,  showing  a  uniformly  better  or 
poorer  development,  or  any  other  difference  in  character  indicating  the 
presence  of  two  kinds  of  lime-soda  feldspar,  was  observed. 

The  plagioclase  sections  almost  invariably  show  polysynthetic  twinning 
according  to  the  albite  law,  with  twinning  lamelhie  which  vary  fi'om  very 
thin  to  moderately  thick  plates,  the  thinner  being  the  more  connnon  form. 
Very  common  is  the  combination  of  the  albite  and  Carlsbad  twinning  laws 
in  one  individual.  Less  commonly  we  find  individuals  twinned  according 
to  the  pericline  as  well  as  the  albite  law,  and  sometimes  a  Carlsbad  twin  is 
made  up  of  individuals  twinned  according  to  the  albite  and  pericline  laws. 

In  determining  the  character  of  the  feldspar,  the  Levy  method  was 
followed.'  A  great  number  of  measurements  made  on  the  zone  perpen- 
dicular to  010  gave  equal  extinction  angles,  varying  chiefly  around  15 
degrees,  but  running  as  a  maximum  to  19  degrees.  From  this  it  appears 
that  the  plagioclase  is  andesine,  probably  a  somewhat  basic  kind.  That 
these  andesines  vary  slightly  in  composition  is  shown  by  a  very  slight  but 
noticeable  zonal  structure,  the  moi-e  basic  character  of  the  center  of  the 
individuals  being  most  admirably  brought  out  by  the  more  advanced  con- 
dition of  alteration  of  the  center  as  compared  with  the  periphery. 

The  andesine  is  for  the  most  part  very  much  altered,  to  such  an  extent  ~ 
that  in  many  sections  the  boundaries  of  the  twinning  lamell:3e  are  so  blurred 
that  measurements  are  rendered  impossible.  Muscovite,  which  appears  in 
minute  rectangular  sections  showing  good  cleavage,  is  the  chief  secondary 
product  from  the  feldspar,  with  epidote-zoisite  next  in  importance.  Calcite 
and  biotite  are  present,  but  in  comparatively  small  quantities.  In  some 
cases  muscovite  almost  replaces  the  feldspar;  in  others  epidote-zoisite  does 
so.  In  such  a  case  one  sees  in  the  center  of-  the  feldspar  only  a  mass  of 
secondary  mineral.  As  the  examination  is  carried  from  the  center  toward 
the  outside,  the  original  feldspar  material  is  distinguished  as  a  thin  film 
between  the  secondary  minerals.  This  increases  in  mass  until  we  reach 
the  ontside  narrow  rim  of  practically  unaltered  feldspar. 

orthociase. — This  is  prescut  in  large  quantity  in  irregular  plates  which 
form  a  part  of  the  mesostasis  for  the  plagioclase  and  bisilicates.  Less  com- 
monly we  find  it  in  micropegmatitic  intergrowth  with  the  quartz.      It  is 

'  fitude  sur  la  determination  des  feldspaths,  by  A.  Michel  Lf'vy,  Paris,  1894. 


DIORITE  INTRFSIVES.  225 

iiivariahly  more  or  less  (IcconiiKiscd,  and  shows  innuiiicralile  mimite  dark 
specks  scattered  through  it.  The  (juantity  (if  orthocdase  varies  in  tliese 
dioritic  rocks  consideral)ly;  at  times  it  almost  equals  or  even  exceeds  the 
plagioclase,  when  the  rocks  apjjroacdi  the  granites,  and  at  times  it  sinks  to 
a  few  large  plates  in  each  section,  when  the  rocks  are  a  normal  diorite. 

Microciine. — Thig  mineral  is  not  abundant.  It  is  in  individuals  which 
frecpiently,  though  not  in  all  cases,  are  automorphic  with  re.spect  to  the 
orthoclase  and  (piartz.     It  is  remarkably  fresh. 

Quartz. — Quartz,  at  times,  is  an  essential  constituent,  and  again  it  dimin- 
ishes in  amount  until  it  is  present  only  in  a  few  grains,  or  even  disappears 
altogether.  Like  the  orthoclase,  it  is  completely  xenomorphic,  and  with  the 
orthoclase  constitutes  the  mesostasis.  Undulatory  extinction  in  the  quartz 
gives  indication  of  slight  pressure. 

Biotite. — The  original  bit)tite  in  the  granular  dioritic  rocks  is  automorphic 
with  respect  to  all  minerals  but  the  hornblende.  In  the  ophitic  forms  it 
has  a  development  about  equal  to  that  of  the  hornblende.  It  shows  a  dark 
rich  chocolate-browu  or  greenish-brown  color  for  h  and  C,  and  a  lio-ht 
yellowish-brown  for  a.  The  biotite  includes  small  ei)idote  crystals  with 
})leochroic  courts  and  some  grains  of  sphene.  Both  of  tliese  are  original. 
The  biotite  is  almost  invariably  more  or  less  altered,  lileaching  in  s(ime 
cases  to  a  very  light  colored  mica  with  exceedingly  high  polarization  colors. 
This  bleaching  follows  along  the  laminae  of  the  biotite  and  results  in  givino- 
sections  parallel  to  the  vertical  axis  a  banded  appearance  resembling  parallel 
intergrowths  of  muscovite  and  biotite  lamiiuie.  More  commonly  it  alters 
to  chlorite,  rutile  (often  present  as  sagenite),  sphene,  epiclote-zoisite,  and 
calcite.  There  is  also  a  distinct  banding  of  the  biotite  and  the  chlorite  in 
places.  In  the  alteration  of  the  biotite  we  very  commonly  find  lenses  of 
calcite  produced  between  the  lamiure.  In  some  cases  the  epidote-zoisite  is 
clearly  a  product  of  the  alteration  of  the  biotite,  for  in  many  cases  it  is  found 
in  the  rectangular  shape  of  the  biotite  section,  and  in  other  instances  in  spaces 
between  the  feldspars  in  the  ophitic  rocks,  which  in  fresher  specimens  are 
found  to  be  occupied  by  the  biotite.  Moreover,  in  the  epidote-zoisite  are 
minute  grains  of  sphene  similar  to  those  contained  in  the  original  biotite. 

Where  it  is  present  as  a  secondary  product,  it  occurs  with  the  musco- 
vite and  is  xenomorphic  with  respect  to  it.  The  green  tone  is  absent  from 
the  secondary  biotite. 

MON  xxxvi 15 


226  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

Hornblende. — Tlic  lioriibleude  in  the  diorites  shows  a  most  excellent  devel- 
opment ill  the  prism  zone;  very  nmch  less  well  developed  are  the  termi- 
nating planes.  The  color  varies  from  dirty  green  to  a  reddish-brown.  The 
brown  hornblende  occupies  the  center  of  the  crystals,  while  the  green  occu- 
pies the  outside,  the  green  agreeing  perfectly,  optically,  with  the  brt)wn.  A 
zonal  structure  is  indicated  bv  the  difference  in  the  character  of  the  horn- 
blende, though  the  zones  are  not  sharply  delimited,  but  grade  into  one 
another.  In  a  few  cases  the  greenish  hornbleude  grades  into  one  which  is 
almost  colorless.  The  pleochroism  is  as  follows:  Brown  hornblende :  a, light 
yellow  or  light  reddish-yellow;  b,  light  reddish-brown;  C,  darker  reddish- 
brown.  Green  hornblende:  a,  light  yellow;  Hj,  bright  green;  c,  dull  or 
olive  green.  This  green  hornblende  is  clearly  original  and  not  to  be  con- 
sidered as  a  secondary  product  after  the  brown  hornblende.  Both  kinds 
are  free  from  inclusions. 

Accessory  minerals, — Tlic  cpidote  Is  observed  very  frequently  inclosed  in  the 
altered  biotite,  and  is  surrounded  by  pleochroic  halos.  In  such  cases  it  is 
considered  a  primary  constituent.  The  accessory  minerals,  apatite,  sphene, 
and  zircon,  show  none  other  than  their  usual  characters.  Titaniferous 
magnetite  is  present  in  the  diorites  in  very  small  quantity. 

According  to  the  relative  proportion  of  the  important  minerals  just, 
described — plagioclase,  orthoclase,  quartz,  hornblende,  and  biotite — com- 
posing the  diorites,  we  get  the  following-  varieties:  Mica-diorite,  quartz- 
mica-diorite,  quartz-diorite,  and  tonalite.  These  grade  into  one  another,  as 
stated  above;  and,  as  will  be  shown  later,  certain  of  them  grade  into 
o-ranites.  On  account  of  these  variations  these  dioritic  rocks  are  especially 
interesting. 

DESCRIPTION   OF  INTERESTING  VARIATIONS. 
SEC.    15,    T.   42   N.,  R.    31    W. 

A  dike  of  rock  4  feet  wide,  occurring  at  425  paces  N.,  1050  W.,  sec. 
15,  T.  42  N  ,  R.  31  W.,  near  Norway  portage,  shows  the  following  min- 
eraloo-ical  variation.  A  specimen  taken  from  the  center  of  the  dike  shows  the 
rock  to  be  there  a  typical  fine-grained  granitite  with  little  or  no  plagioclase. 
(Photomicrograph,  fig.  A,  PI.  XXXIX.)  Along  the  sides  the  dike  rock 
is  a  mica-diorite  consisting  of  mica  and  plagioclase  without  any  (piartz. 
Measm-ements  on  zone  perpendicular  to  010  gave  equal  extinction  angles 


DIORITE  INTRUSIVES.  227 

with  ii  luiixiiiuiiu  (it"  If)  (legrties.  Only  oiw  kind  of  plajiioclase  i«  distin- 
•fuisliable  by  its  mode  of  development,  and  this  is  rich  in  Ca( ),  as  shown  by 
its  alteration  products.  The  feldspar  rang'es  at  most  from  all)ite  to  andesine. 
No  chemical  anahsis  has  been  obtained  of  either  the  "ranitite  or  flic  mica- 
diorite  phase,  but  the  mineralogical  composition  is  sufficiently  marked  to 
show  conclusively  that  we  have  here  a  gradation  from  a  granitic  to  a  dioritic 
rock  rich  in  CaO.  The  idea  has  been  suggested  by  Johnston-Lavis  ^  that 
in  some  cases  the  variation  in  chemical  composition  of  intrusive  rocks, 
especiall}'  where  this  variation  is  one  between  the  center  and  the  peripher^- 
of  an  intrusive  mass,  may  l)e  due  to  resorption  by  the  intrusive  of  parts  of 
the  rock  intruded.  The  sharp  line  of  demarcation  which  exists  between 
the  dike  and  the  intruded  hornblende-gabbro  in  the  occurrence  described 
above  seems  to  preclude  the  possil)ilitA'  of  a  fusion  and  mingling  of  the  two 
rocks. 

ACROSS  RIVER  FROM  CRYSTAL  FALLS. 

Near  Crystal  Falls,  just  across  the  river  from  the  town,  are  a  number 
of  small  knobs  of  granite  grading  into  quartz-mica-diorite.  They  are 
medium-grained  rocks,  reddish  to  gra}-  in  color.  They  take  a  very  fine 
polish  and  are  well  adapted  to  ornamental  stonework,  as  is  shown  by  the 
columns  made  from  them  which  are  used  in  the  court-house  at  Crystal 
Falls.  When  examined  under  the  microscope,  the  rocks  are  found  to  con- 
sist of  autoinorphic  biotite  and  plagioclase,  with  xenomorphic  orthoclase 
and  quartz,  these  last  forming  the  cement.  Some  of  the  slides  show 
beautiful  micropegmatitic  intergrovvths  of  quartz  and  feldsjjar.  The  amount 
of  quartz,  plagioclase,  and  orthoclase  varies  so  that,  depending  upon  the 
specimen  examined,  one  would  call  the  rocks  forming  the  knobs  granite  or 
quartz-mica-diorite.  Most  coimnonly  the  rock  is  a  plagioclase-bearing' 
granite.  No  analysis  has  been  obtained  of  the  granite,  but  it  is  confidently 
believed  that  the  chemical  composition  would  sustain  the  microscopical 
diagnosis.  Within  the  granite  there  are  found  lenticular  schlieren  of 
considerably  darker  color  than  the  main  mass,  in  which  the  plagioclase  is 
the  preponderant  feldspathic  constituent.  The  rock  of  these  lenses  is 
essentially  a  quartz-mica-diorite. 

'  The  basic  eruptive  rocks  of  Grau  (Norway)  and  their  interpretation ;  a  criticism  by  H.  .1. 
Johnston-Lavis:  Geol.  Mag.,  4th  ser.,  VoL  I,  1894,  p.  252. 

The  causes  of  variation  in  the  composition  of  igneous  roclis,  by  H.  J.  Johnston-Lavis :  Natural 
Science,  Vol.  IV,  1894,  pp.  134-140. 


228  THE  CEYSTAL  FALLS  lEOI^-BEARma  DISTRICT. 

These  knobs  are  cut  bv  a  nuinl^er  of  small  dikes  from  a  fraction  of  au 
incli  to  3  inches  in  width.  In  all  of  these  dikes  the  rock  shows  the  same 
characters.      It  is  very  light  gray  Xo  pink  in  color,  and  aphanitic. 

An  examination  under  the  microscope  enables  the  separation  of  each 
dike  into  a  verv  compact  fine-grained  saalband  and  a  somewhat  coarser- 
grained  porphyritic  central  portion.  In  the  central  part  of  the  dike  pheno- 
crvsts  of  quartz,  feldspar,  and  biotite  lie  in  a  very  tine  groundmass  of 
quartz  and  feldspar.  Tlie  texture  of  this  groundmass  is  microgranitic. 
The  saalband  is  composed  of  the  microgranitic  groun(hnass  without  the 
phenocrysts.  The  (juartz  phenocrysts  show  the  usual  characters.  The 
feldspar  phenocrysts  are  in  most  cases  so  completely  replaced  by  a  musco- 
vite  aggregate  as  to  preclude  any  exact  determination  of  their  original 
character.  In  some  cases  indistinct  remains  of  poly  synthetic  twinning  are 
seen.  Even  when  the  main  mass  of  the  feldspar  phenocrysts  is  entirely 
altered,  there  is  a  narrow  zone  of  very  fresh  feldspar  material  surrounding 
it.  Twinning  in  the  center  is  also  continuous  through  this  zone.  More- 
over, this  zone  itself  shows  a  very  noticeable  zonal  structure  by  the  change 
in  extinction  angle  observed  in  passing  from  the  inner  to  the  outer  portion. 
This  less  altered  zone  of  feldspar  contains  numerous  inclusions  of  quartz 
from  the  groundmass.  The  character  of  the  feldspar  phenocrysts  could  not 
be  determined,  but  the  presumption  is  that  they  are  of  the  same  character 
as  the  feldspar  in  the  coarser  main  mass — that  is,  andesine — with  a  more 
acid  feldspar,  possibly  oligoclase,  surrounding  them.  The  further  presump- 
tion is  tlien  wan-anted  that  the  feldspar  of  the  groundmass  agrees  with  this 
outer  feldspar  zone  in  character — that  it  is  also  oligoclase,  or  at  least  is 
more  acid  than  the  phenocrysts.  Automorphic  biotite  plates  are  now  repre- 
sented bv  chlorite  pseudomorphs,  with  here  and  there  some  secondary  epidote. 

The  groundmass  consists  chiefly  of  quartz  and  feldspar,  but  contains 
disseminated  through  it  many  minute  plates  of  white  mica  and  a  few 
crystals  of  zircon.  The  feldspar  of  the  groundmass  is  too  small  to  permit 
of  its  accurate  determination.  A  plagioclase  feldspar  in  sections  indicating 
an  approach  to  automorphism  was  observed.  Its  character  as  oligoclase  (f), 
( )r  at  least  a  feldspar  of  a  more  acid  character  than  that  of  the  centers  of 
the  ithenocrysts,  is  sunnised  for  reasons  given  above.  Microcline  in  sec- 
tions showing  characteristic  twinning  and  in  more  or  less  rectangular  out- 
lines was  obsei-ved  in  considerable  quantity.     An  untwinned  feldspar  was 


DIORITE  INTKUSIVKS.  229 

deterniined  as  ortlioclasc  l)y  tlic  (lirtereiicci  shown  ))v  its  i-efractive  index 
and  tliat  of  the  twiuucid  plajriorlase.  Quartz  was  also  recognized  in  tliis 
way  in  the  g-roundniass.  The  quartz  and  ortliorlase  form  tlie  cement  for 
tlie  other  constituents.  The  muscovite  in  tlie  groundmass  is  ])resumably 
secondary,  as  is  tliat  in  the  phenocrysts.     (Figs.  A  and  />',  I'l.  XL.) 

The  rock  is  here  inserted  as  showing  an  exceedingly  line  grained  j)or- 
l)hyritic  form  of  the  quartz-mica-diorite.  It  may  compare  to  tliis  mica- 
diorite  as  does  the  tonalite-porphyrite  of  Becke'  to  the  tonahte  described 
by  Inm,  and  one  miglit  call  it  a  quartz-mica-diorite-porphyry. 

No  analysis  of  this  rock  has  thus  far  l)een  obtainable.  Possibly  its 
chemical  composition  may  indicate  it  to  be  more  closely  allied  to  the  true 
granites  than  is  believed  to  he  the  case,  judging  from  its  mineralogical 
composition  and  its  association  with  the  rocks  on  the  border  line  between 
granites  and  diorites. 

SOUTHEAST  OF  CRYSTAL  FALLS. 

Southeast  of  Crystal  Falls,  in  sec.  16,  northwest  of  Lake  Tobiu,  and 
extending  southwest  into  sec.  28,  T.  42  N.,  R.  32  W.,  is  a  range  of  hills 
upon  which  are  numerous  exposures  of  a  uniformly  medium-grained  rock. 
The  main  mass  of  the  knobs  is  of  tonalite,  which  shows  several  facies.  A 
miarolitic  texture  is  very  common  in  this  massif.  The  cavities  are  now 
filled  with  calcite,  quartz,  and  epidote-zoisite  alone  or  togetlier.  This  last 
mineral  occurs  in  single  large  individuals  or  in  tufts  of  individuals,  which 
radiate  from  one  side  of  the  cavity.  Li  one  case  a  cavity  incompletely 
filled  by  such  a  tuft  has  been  completely  filled  by  a  later  infiltration  of 
quartz.  The  color  of  the  rock  varies  from  light  pink  to  very  dark  greenish 
gray.  The  areas  of  the  lighter-colored  rocks  may  be  seen  extending  in 
finger-like  projections  into  the  darker-colored  phases.  There  are,  however, 
no  sharp  lines  between  these  varieties,  but  a  gradual  passage  from  the  lio-hter 
to  the  darker  rock.  These  diff"erent  phases  evidently  belong  to  a  single 
rock  mass.  Under  the  microscope,  however,  important  variations  in  the 
textural  and  mineralogical  character  of  the  rock  masses  are  seen.  The 
main  mass  of  the  rock  is  granular  tonalite.  The  essential  constituents  are 
plagioclase,  orthoclase,  quartz,  hornblende,  and  mica.  The  most  common 
association  of  minerals  is  hornblende  and  mica  in  automorphic  crystals, 

'  Petrographische   stndien   am  Tonalit  der  Rieserferner,  by  F.   Hecke:  Tschermak.s  mineral 
Mittheil.,  Vol.  XIII,  1892,  p.  435. 


230  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

witli  plag-Ioclase  somewhat  less  well  developed.  Between  these  there  is 
found  the  quartz,  with  some  accessory  orthoclase,  and  microcline  as  the 
last  products  of  crystallization.  In  some  cases  these  two  minerals  are 
present  in  micropegmatitic  interg-rowths.  A  textural  variation,  which  the 
facies  mentioned  below  also  undergo,  is  from  a  granular  to  an  im]:)ei'fectly 
ophitic  texture.  In  such  cases  the  order  of  crystallization  of  the  hornblende- 
mica  and  plagioclase  is  reversed,  the  plagioclase  being  most  automorphic  in 
the  ophitic  varieties. 

The  rock  resembles  the  tonalite  described  by  Becke  from  the  Rieser- 
ferner.^  It  also  closely  resembles  some  slides  of  the  typical  Adamello  tonalite 
with  which  I  have  been  able  to  compare  it.  The  chief  difference  between 
them  is  that  the  plagioclase  and  hornblende  have  a  better  crystallographic 
development  in  the  Crystal  Falls  rocks  than  in  the  Adamello  tonalite,  and 
that  the  accessory  allanite  of  the  Adamello  rock  is  wanting  in  the  Crystal 
Falls  tonalite,  though  the  normal  epidote  may  represent  it.  The  horn- 
blende also  differs  slightly  from  that  of  the  Adamello  rock  in  that  it  is  not 
throughout  reddish  brown.  Tlie  central  portion  of  some  of  the  crystals 
shows  this  color,  but  the  outer  portion  is  a  dirty  green,  even  grading  into 
an  almost  white  hornblende. 

The  tonalite  grades,  by  diminution  of  biotite,  with  corresponding 
increase  of  hornblende,  into  a  quartz-diorite,  and  by  diminution  or  disap- 
pearance of  the  hornblende  and  increase  of  the  biotite  into  a  quartz-mi ca-diorite. 

Hornblende  never  occurs  alone  in  the  rocks,  whereas  biotite  may 
occur  as  the  only  l^isilicate  C(instituent.  It  is  a  very  common  thing  to  find 
in  the  diorites  rounded  basic  segregations  consisting  chiefly  of  mica  with 
hornblende  suljordinate  and  just  a  little  accessory  feldspar.  When  the 
orthoclase  and  quartz  diminish,  we  get  the  mica-diorites.  Orthoclase  is 
always  present  in  all  of  these  dioritic  rocks.  In  certain  facies  orthoclase 
and  quartz  are  very  abundant  and  the  plagioclase  is  correspondingly  dimin- 
ished. Such  rocks  are  clearly  plagioclase-bearing  granites,  and  represent 
gradations  between  the  ordinary  tonalite  and  granite,  and  point  to  close 
relationship  of  this  occurrence  with  the  occurrences  nearer  Crystal  Falls 
alread}'  described,  in  which  the  granitic  facies  predominates  and  the  dioritic 
facies  is  subordinate. 

'  Petrographlsche  studien  am  Tonal  it  iler  Rieserferner,  by  F.  Becke:  Tschermaks  mineral.  Mitt- 
heil.,  Vol.  XIII,  1892,  pp.  364-379. 


BIORITE  INTKUSIVES. 


231 


Similar  "Tiuliitions  have  l)eeu  noted  \)y  lierke  in  tlic  tonalito  Iruni  the 
Riesei-t'einu'i-.'  Tlie  diorite  massif  (if  the  Crystal  Falls  district  seems  to  cor- 
respond very  closely  to  the  granodiorite  masses  of  Becke,  Turner,  and  Lind- 
gren,-  which  on  the  one  hand  grade  into  the  granitites  and  <ai  the  other  into 
the  diorites. 

ANALYSIS  OF   DIORITE. 

It  has  not  been  found  possible  thus  far  to  obtain  analyses  of  all  these 
varieties.  The  more  acid  facies  of  the  diorites  seem  in  their  mineralogical 
composition  to  show  very  clearly  their  gradations  toward  the  tonalites  and 
granites.  This  being  the  case,  it  was  deemed  of  more  importance  to  study 
the  relations  of  the  more  basic  dioritic  facies  in  order  to  determine  tlie  rela- 
tionship of  these  rocks  to  those  of  tlie  more  basic  gabbro  and  peridotite 
families  which  are  found  iu  association  with  them.  To  this  end  an  analysis 
of  one  of  the  mica-diorites  was  obtained. 

This  rock  contains  the  dark  constituents,  biotite  and  hornblende,  in 
large  quantity,  and  of  these  the  mica  predominates.  Plagioclase  predomi- 
nates among  the  white  silicates,  with  orthoclase  and  quartz  very  subordinate. 
The  mica  is  considerably  altered,  but  on  the  whole  the  rock  is  fairly  fresh. 
Fio-.  B,  PI.  XXXIX,  is  a  photomicrograph  of  the  rock  and  shows  its  general 
characters.  The  following  analysis  was  made  by  Dr.  H.  X.  Stokes,  in  the 
laboratory  of  the  United  States  Geological  Survey: 

Analysis  of  diorite. 


SiO:-. 
TiO,  . 

ALO:,. 

Fe,0, 
FeO-. 
MnO 
CaO  . 
MgO. 


Per  cent. 


58.51 

.72 

16.32 

2.11 

4.43 

Trace. 

3.92 

3.73 


KjO 

Na,0 

H;0  at  100° 

H;0  above  100= 

PjOfS 

Co, 

Total 


4.08 
3.11 

.23 
2.00 

.30 
None. 


99.17 


iPetrographische  studieu  aai  Tonalit  der  Rieserferner,  by  F.  Becke:  Tschermaks  mineral.  Mitt- 
heil..  Vol.  XIII,  1892,  p]).  379-464. 

-The  granodiorite  of  California  appears  from  Lindgren's  description  (Granitic  rocks  of  Cali- 
fornia, by  W.  Lindgren  :  Am.  Jour.  Sci.,  4th  series,  Vol.  Ill,  1897,  p.  308 ;  where  can  be  found  references 
to  mention  and  descriptions  of  granodiorite)  to  correspond  very  closely  to  tonalitc,  though  Turner 
nses  the  name  as  synonymous  with  quart/.-mica-diorite  (Geology  of  the  Sierra  Nevada,  by  H.  W.  Turner: 
Seventeenth  Ann.  Kept.  U.  S.  Geol.  Survey,  Part  I,  1896,  pp.  636,  724). 


232  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

The  altered  character  of  the  rock  is  readily  seen  in  the  large  content 
of  water.  It  is,  nevertheless,  not  so  marked  as  to  render  the  analysis  use- 
less for  purposes  of  determination. 

The  character  of  the  plagioclase  feldspar  is  clearly  indicated  by  the 
relatively  high  percentage  of  lime.  This  high  content  in  lime  and  the  large 
amount  of  alkalies  present,  7.18  per  cent,  clearly  show  its  relationship  to  the 
diorite  family.  The  content  in  potash  feldspar  and  the  possible  derivation 
of  the  rock  from  a  granitic  magma  is  shown  by  the  high  content  in  potash. 
Possibly  a  considerable  amount  of  the  potash,  with  the  greater  part  of  the 
mao-uesia,  should  be  deducted  for  the  biotite  which  is  so  abundant. 

This  rock  is  one  which  it  is  somewhat  difficult  to  place  definitely  in 
the  existing  division  of  rock  families.  The  large  amount  of  lime  and  the 
relatively  low  percentage  of  alkalies  prevent  the  placing  of  the  rock  with 
the  syenites.  On  the  whole  it  approaches  close  to  the  monzonite  group 
according  to  the  chemical  composition  of  the  group  given  by  Brogger. 
But  it  differs  from  this  in  that  the  lime  (3.92  per  cent)  is  too  low  to  bring 
the  rock  within  his  limits  (4.52  to  10.12  per  cent).'  However,  if  we  con- 
sider the  total  of  the  alkaline  earths  (7.65  per  cent)  in  the  rock  under  dis- 
cussion, we  find  that  it  comes  well  within  Brogger's  range  (6.05  to  17.52 
per  cent)  for  a  total  of  magnesia  and  lime.  Moreover,  the  alkali  total  (7.19 
per  cent)  is  about  right  to  warrant  its  classification  in  the  monzonite  grouj) 
as  a  representative  of  the  type  of  biotite-monzonite. 

On  comparing  the  analysis  with  that  of  trne  normal  diorites.  we  find 
that  the  relative  proportions  of  the  alkalies  are  abnormal.  The  lime  con- 
tent is  also  too  low  for  rocks  of  this  character,  and  the  magnesia  is  too  higli. 
The  above  considerations  seem  to  make  clear  the  relationship  of  the 
rock  to  the  monzonites  and  diorites.  However,  it  is  so  intimately  asso- 
ciated with  and  so  evidently  but  a  facies  of  the  tonalite  which  is  the  domi- 
nant type  where  this  rock  occurs,  that  it  is  considered  to  be  more  closely 
related  to  the  lime-soda  feldspar  rocks,  in  which  the  orthoclase  is  but  acces- 
sory, than  to  the  monzonite  family  of  orthoclase-plagioclase  rocks.  It  is 
therefore  considered  to  be  a  mica-diorite. 


'  Op.  cit.,  Part  II,  p.  51. 


GABBRO  AND  NOIilTE  INTltUSlVES.  233 

GAUnno    AND    NDRITE. 

PETROGRAPHICAL    CHARACTERS. 

The  <;iil)l)r()s  and  iiorites  are  liolocrystalline  rocks  of  luoderatel}^  fine 
to  coarse  grain.  They  show  a  considerable  variation  in  texture.  Some,  the 
finest-grained  forms,  possess  a  very  good  parallel  texture  (PI.  XLIJ,  figs. 
A  and  B);  others  are  noticeably  porphyritic.  A  few  have  poikilitic  tex- 
tures (PI.  XLI,  figs.  A  and  B);  less  common  is  an  approach  to  the  ophitic 
texture  of  the  dolerites.  Most  connnon  of  all  the  rocks  are  the  hypidio- 
morphic  granular  ones  (PI.  XLIV,  fig.  A,  and  PI  XLIII,  fig.  A). 

The  rocks  vary  from  a  light  grayish-green  color  for  some  of  the  coarse- 
grained ones,  through  darker  greenish  colored  rocks  to  those  of  a  dark 
brownish  or  greenish-black  color  for  the  finest-grained  forms. 

The  important  origiiial  mineral  constituents  are  feldspar,  mica,  horn- 
blende, pyroxene,  and  olivine.  Apatite,  sphene,  zircon,  rutile,  octahedrite 
(anatase),  brookite  (?),  and  iron  oxide  occur  as  accessory  minerals.  White 
and  brown  mica,  chlorite,  hornblende,  talc,  serpentine,  sphene,  rutile,  and 
calcite  occur  as  secondary  minerals. 

Feldspar. — Both  plagiocksc  and  orthoclase  are  present  in  the  gabbros  and 
norites.  Plagioclase  is  by  far  the  most  important.  It  occurs  normally  in 
the  coarse-grained  kinds  of  rock  as  broad  tabular  individuals.  In  the  finer- 
grained,  especially  the  })orphyritic  and  poikilitic  facies,  the  feldspar  sections 
aiisume  a  broad,  lath-shaped  character.  The  sections  show  the  character- 
istic polysynthetic  twinning.  Twins,  according  to  the  albite,  pericline,  and 
Carlsbad  laws,  are  present,  usually  the  albite  and  Carlsbad  or  the  albite  and 
pericline  being  combined;  in  some  cases  all  three  occur  together.  Twin- 
ning lamellse  vary  in  breadth,  but  on  the  whole  are  moderately  narrow. 
Measurements  made  on  the  zone  normal  to  0 1 0  give  equal  extinction  angles 
against  the  twinning  plane,  which  reach  a  maximum  of  34  degrees.  The 
feldspar  is  evidently  labradorite.  A  zonal  structure  is  noticeable,  and  is 
especially  shown  by  the  alteration  being  more  advanced  in  the  centers  of 
the  individuals.  It  is  possible  that  the  labradorite  is  accompanied  by  a 
small  amount  of  more  acid  feldspar,  andesine  in  zonal  growth  with  it. 
The  alteration  of  the  feldspar  results  in  the  production  of  the  same  sec- 
ondary products  formed  from  the  slightly  more  acid  feldspar  of  the  diorites. 


234  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

mica  (both  muscovite  and  biotite),  epidote-zoisite,  and  calcite.  The  plagio- 
clase  shows  very  beautifully  tlie  effects  of  dynamic  action  in  local  granu- 
lation of  the  peripheries  of  the  indi\T[dual.  Such  lines  of  granulated  feldspar 
can  be  followed  through  the  sections,  probably  indicating  shearing  planes. 
Inclusions  are  common.  Some  stout  rutile  crystals  were  observed  in  the 
feldspar.  In  scTme  cases  minute  hair-like  needles,  which  in  a  few  instances 
were  of  a  size  sufficient  to  admit  of  their  ready  determination  as  rutile, 
were  also  found  penetrating  the  plagioelase.  Crystals  of  apatite  and  iron 
ores  are  also  commonl)'  included  in  it.  There  have  also  been  found  in  a 
few  cases  minute  hexagonal  plates,  which  are  translucent,  with  brown  color, 
and  are  presumably  micaceous  ilmenite. 

Mention  of  the  presence  of  oi'thoclase  in  these  rocks  is  made  with  con- 
siderable doubt.  Here  and  there  a  few  plates  of  untwinned  feldspar, 
showing  a  character  somewhat  different  from  that  of  the  plagioelase  plates, 
were  observed.  As  will  be  seen,  the  possibility  of  its  presence  is  indicated 
by  the  potash  shown  in  the  analysis. 

Biotite. — The  biotite  in  the  coarsely  granular  rock  is  in  irregular  plates. 
They  are  frequently  included  in  and  attached  to  the  outside  of  the  horn- 
blende. Its  period  of  crystallization  thus  overlaps  that  of  the  hornblende, 
though  on  the  whole  being  contemporaneous  with  it.  In  the  fine-grained 
rocks  biotite  is  better  developed  than  the  hornblende,  and  is  apparently 
for  the  most  part  older  than  it.  In  color  it  vai'ies  from  a  rich  reddish  brown 
for  rays  vibrating  parallel  to  the  cleavage  to  a  light  yellow  for  those  per- 
pendicular thereto.  It  includes  crystals  of  zircon  and  apatite,  which  are 
surrounded  by  pleochroic  halos. 

Hornblende. — In  most  of  the  sectlous  hornblende  is  the  most  striking-  com- 
ponent.  It  is  present  in  the  gabbros  in  three  different  varieties.  The  most 
prominent  kind  is  a  reddish-brown  hornblende,  which  has  a  dirty  green 
hornblende  commonly  associated  with  it  and  frequentl}'  intergrown  with  it 
zonally.  This  liornblendo  occurs  without  the  green,  but  the  green  is 
invariably  associated  with  the  brown.  The  two  are  optically  continuous  in 
the  intergrowths.  It  is  possible,  though  not  susceptible  of  proof,  that  the 
green  is  the  result  of  the  incipient  alteration  of  the  brown.  The  second 
kind  is  compact,  strongly  pleochroic,  common  green  hornblende,  and  the 
third  is  a  noncompact,  reedy  variety  of  light-green  hornblende.     The  first 


GABBRO  AND  NORITE  INTRUSIVES.  235 

two  kinds  of  lioniblondc^  ;ire  presumed  to  l)e  primary.  The  tliird  variety  is 
secoiidarA',  hut  secondary  after  tlie  orio-inal  liornblende,  thus  not  atfectiii"- 
the  diaracter  of  tlie  rock. 

The  first  variety,  the  reddisli-browii  hornblende,  occurs  in  the  g-abbros 
in  anh('(ba.  A  zonal  .structure  is  marked  l)y  l)rown  liornblende  occupying 
the  centers  of  the  crystals  and  by  dull  green  hornblende,  which  agrees 
optically  with  the  brown,  occupying  the  outsides.  The  brown  hornblende 
is  somewhat  lighter  colored  than  basaltic  hornblende.  The  pleoehroism  is 
strong  in  the  following  colors:  Brown  hornblende:  tl,  light  yellow  or  red, 
with  tinge  of  green;  ft,  red  brown;  c,  same  or  darker  red  bi'own,  excep- 
tionall}'  yellowish-brown;  C^b>a.  Green  hornblende:  a,  greenish- 
yellow;  Jjs,  yellowish-  or  brownish-green;  c,  dull  olive  green,  frequently 
with  bluish  tinge;   C>I)>-a. 

This  hornblende,  with  respect  to  its  rather  exceptional  pleochroi.sm 
and  its  general  characters,  seems  to  agree  very  well  with  that  described  b}^ 
van  Horn  from  ver}^  similar  rocks  from  Italy,  and,  like  that,  is  probably  a 
very  basic  hornblende.^  Twinning  parallel  to  100,  co  P  oo,  is  very  common. 
An  imperfect  parting  parallel  to  the  orthopinacoid  100,  oo  P  oc,  was  also 
observed  in  some  cases.  It  is  also  indicated  by  the  jjlaty  inclusions  which 
lie  in  this  plane  In  some  of  the  sections  where  the  green  hornblende  is 
not  intergrown  with  the  brown  the  green  kind  shows  very  commonly  a 
system  of  fine  striations  parallel  to  the  positive  orthodome  101,  P  oo.  In  rare 
cases  the  brown  hornblende  is  intergrown  with  almost  colorless  hornblende, 
one  end  of  a  crystal  being  brown,  the  other  faintly  yellowish.  Irregular 
mottled  intergrowths  of  the  two  were  also  found. 

The  normal  brown-green  hornblende  is  rendered  poikilitic  in  some 
specimens  by  a  few  rounded  grains  of  perfectly  fresh  pyroxene,  and  also  by 
plagioclase  crystals  which  it  includes.  This  same  kind  of  hornblende  is 
frequently  rendered  very  dark  by  the  number  of  exceedingly  small  inclu- 
sions which  it  contains,  and  in  this,  and  also  in  its  reddish-brown  color, 
resembles  so  strongly  many  hypersthenes  as  to  be  readily  mistaken  for  them 
upon  cursory  examination.  These  inchisions  are  of  several  kinds,  all  dis- 
tributed throughout  the  same  individuals.    It  is  impossible  in  studying  them 


'  Petrographische  Untersuchungcn  fiber  die  noritischen  Gesteine  der  Umgegend  von  Ivrea  in 
Oberitalien,  by  F.  R.  van  Horn:  Tschermaks  mineral  Mittheii.,  Vol.  XVII,  1897,  Paxt  V,  p.  s.J9. 


236  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

to  get  any  optical  tests,  except  that  of  extinction,  owing  to  the  minute  size 
of  the  inchisions  and  to  the  fact  that  where  large  enough  for  exaniinatiou 
the  tests  were  vitiated  b^s'  the  presence  of  the  hornblende. 

Of  these  inclusions  some  are  readily  distinguishable  as  rutile.  Some 
of  the  larger  of  the  crystals  reach  a  leng-th  of  0.04.5  nun.  and  a  thickness 
of  0.0125  mm.  Numbers  of  them  show  the  characteristic  heart-shaped 
and  geniculated  twins  of  rutile,  so  that  there  is  no  doubt  as  to  the  determi- 
nation. Associated  with  the  rutile  are  other  crystals  0.019  mm.  long,  which 
show  the  typical  2)ointed  pyramidal  development  of  octahedrite  (anatase). 
Still  others  show  a  flat  tabular  development  somewhat  similar  to  that  of 
brookite,  though  these  could  not  be  jjositively  determined  as  that  mineral. 
The  hexagonal  plates  of  clove-brown  color  so  frequent  in  hornblende  and 
also  in  hypersthene  occur  also  in  this  hornblende.  They  are  believed  to  be 
micaceous  ilmenite.  The  thin  ^ilates  are  translucent,  thicker  ones  are  less 
so,  and  those  which  are  still  thicker  are  opaque  and  metallic.  The  thin 
plates  appear  when  on  edge  as  fine,  hairlike  streaks.  The  thick  ones 
appear  in  the  same  position  as  more  or  less  rectangular  bars  or  rods.  Often 
these  small  plates  are  associated  with  masses  of  iron  oxide,  also  included  in 
the  hornblende.  This  iron  ore  occurs  in  the  plates  and  bars  characteristic 
for  ilmenite.  These  ilmenite  masses  are  translucent  only  on  the  edges, 
where  the  slide  has  cut  the  mass  in  such  a  manner  as  i  o  give  an  exceedingly 
thin  section  of  the  ore.  At  such  places  the  ore  is  translucent  with  the 
same  brown  color  as  the  thin  plates.  Another  rare  variet}'  of  the  inclu- 
sions occurs  in  round  grains  of  rich  green  color,  and  may  possibly  be  a 
spinel. 

In  those  sections  in  which  both  original  brown  and  original  green 
hornblende  occur  the  inclusions  are  confined  to  the  brown  kind.  Where 
the  brown  kind  is  surrounded  by  the  green  hornblende  the  inclusions  grad- 
ually diminish  in  quantity  as  we  approach  the  green  zone.  With  this  goes 
also,  hand  in  hand,  a  lightening  of  the  color  of  the  including  mineral 
(brown  hornblende),  and  there  is  thus  an  imperceptible  change  from  the 
brown  to  the  green  hornblende.  Where  the  green  hornblende  occu.rs  alone 
it  is  frequently  as  full  of  inclusions  as  is  the  brown  hornblende  of  other 
sections.  Individuals  of  the  same  sections  difter  from  one  another  with 
respect  to  the  quantity  of  the  inclusions,  some  being  crowded  with  them, 
while  others  are  practically  free  from  them. 


GABBRO  AND  NOKITE  INTRUSIVES.  237 

Thi.s   lirowii   li(.nil)lcU(lLs  on  alturatiou,  breaks   up  into  aggregates  of 
ei)i(lote-zoisito  and  light-green  chlorite. 

The  second  kind  of  hornblende  is  the  perfectly  fresh,  compact,  coni- 
nidu    dark-green   kind,   with    pleochroism    varying  from    yellow   for  a,  to 
yelloAvish-green  for  Jji,  and  to  bluish-green  for  C;  C>b>a.     This  is  found 
in  very  few  cases.     It  ap])ears  in  every  instance  to  be  a  primary  constituent. 
The  third  kind  of  hornblende  may  ht-.  primary,  although  the  evidence 
obtainable  points  to  its  secondary  nature.     It  has  a  light-green  color,  and 
when    examined   for    pleoclu-oism    exhibits    a  scarcely  noticeable   change. 
This    hornblende    differs    very    nuich    from    the    other    two    hornblendes 
described,  in  that  it  is  not  compact,  but  occurs  in  aggregates  of  coarse  reed- 
like (schilfaehnliche)  individuals.     Such  aggregates  do  not  at  all  resemble 
uralite.     The  individuals  are  far  too  coarse  and  wedge  out  at  short  distances 
within  the  aggregates.     The  aggregates  occupy  irregularly  shaped  areas. 
The  aggregates  consequently  have  a  coarse  patchy  polarization.     They  are 
frequently  surrounded  by  ragged  pieces  of  biotite,  just  as  are  the  plates  of 
compact  hornblende.     Moreover,  they  occur  in  rocks  which  show  pressure 
effects,  and  are  best  developed  in  those  in  which  such  effects  are   most 
marked.     The  aggregates  rather  frequently  occur  with  irregular  pieces  of 
the  greenish  or  brown  compact  interposition-bearing  hornblende  bordering 
the  aggregates  or  in  the  midst  of  them.     The  light-green  reedy  hornblende 
never  contains  such  interpositions,  but  does  have  associated  with  it  fairly 
large  grains  of  rutile,  which  may  perhaps  be  considered  as  having  been 
derived  from  the  various  titanium-bearing  microlites  in  the  original  brown 
hornblende.     The  general  appearance  of  these  aggregates  and  their  asso- 
ciation with  the  original  hornblende  seem  to  point  toward  their  secondary 
origin  from  the  latter  through  the  effects  of  pressure. 

Pyroxene. — Thc  pyroxeue  comprises  both  a  monoclinic  and  an  ortho- 
rhombic  kind.  These  are  the  first  of  the  bisilicates  to  crystallize  in  these 
o-abbros.  The  monochnic  kind  is  of  two  varieties.  The  first  is  in  colorless 
to  faint-pink  grains  included  in  large  plates  of  original  brown  hornblende. 
These  grains  have  a  well-developed  prismatic  cleavage.  One  basal  section 
shows  very  nicely  the  characteristic  pyroxene  cleavage.  The  extinction 
measured  against  the  prismatic  cleavage  reached  as  high  as  50  degrees. 
This  pyroxene  is  presumed  to  be  augite.  It  never  shows  diallagic  parting. 
In    other    sections    the    monoclinic  pyroxene    is  a  clear  white  to    faintest 


238  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

green  malacolite  or  diopside.     This  is  in  roundish  grains  included  in  the 
original  green  hornblende,  which  it  equals  in  quantity. 

The  orthorliombic  pyroxene  occurs  in  individuals  which  show  fairly 
good  prismatic  development,  but  with  rounded  terminal  faces.  The  pris- 
matic cleavage  is  very  well  developed.  A  transverse  parting  is  also  com 
mon.  The  pyroxene  is  usually  colorless,  but  in  some  cases  a  scarcely 
noticeable  pleochroism  was  observed,  varying  from  a  faint-greenish  tinge 
for  rays  ^'ibrating  parallel  to  C,  to  a  yellow  for  those  parallel  to  a  and 
h.  It  contains  small,  dark,  streak-like  intrusions,  some  of  which  under 
exceptionalh'  favorable  conditions  are  transparent,,  with  a  faint-greenish 
tinge  The  exit  of  the  bisectrix  in  basal  sections,  as  Avell  as  the  parallel 
extinction,  renders  it  easily  distinguishable  from  the  monoclinic  pyroxene. 
The  optical  angle  could  not  be  measured,  but  was  clearly  very  large,  as  the 
hyperbolas  passed  completely  out  of  the  field  of  view.  The  orthorliombic 
pyroxene  is  evidently  enstatite  or  bronzite,  and  the  pleochroism  clearly 
points  to  its  position  near  bronzite.  A  few  crystals  of  rutile  and  also  some 
of  the  ilmenite  plates,  so  common  in  hypersthene,  were  found  occurring  in 
the  bronzite.  The  ilmenite  plates  are  in  rather  rare  irregular  patches  in  the 
crystals. 

In  many  cases  along  the  cleavage  lines  or  around  the  edges  of  the 
crystals  or  along  the  transverse  parting  planes  are  narrow  zones  of  a  sec- 
ondary yellowish-green,  finely  fibrous,  serpeutinous  mineral.  Beyond 
these  zones  is  a  pure  white  aggregate  of  secondary  talc  scales  (Fig.  B,  PI. 
XXXVIII).  Among  these  scales  are  a  few  minute  rutile  crystals,  and  also 
a  few  Ijlack  ferruginous  specks,  these  products  being  probably  derived  from 
the  inclusions  in  the  bronzite,  and  the  ferruginous  material  possibly  to  some 
extent  from  the  mineral  itself  In  some  cases,  instead  of  the  intermediate 
serpentine  zone,  the  rather  rare  occurrence  is  observed  of  the  passage  of 
the  bronzite  directly  into  the  talc  aggregate. 

Olivine. — The  determination  of  the  original  presence  of  olivine  in  the 
gabbroic  rocks  is  based  upon  very  slight  evidence.  In  some  of  the  sections 
containing  augite  almost  every  individual  of  this  augite  has  near  its  center 
a  rounded,  very  rarely  irregular  area  of  yellowish-green  fibrous  serpentine. 
These  areas  are  sharply  delimited  from  the  surrounding  pyroxene,  and  the 
conclusion  seems  warranted  that  it  resulted  from  the  alteration  of  some 
mineral  included  in  the  pyroxene.     Tlie  only  bisilicate  found  in  the  rocks 


GABBliO  AND  NOltlTE  INTKUSIVES.  239 

of  the  district,  which  ciystalhzud  before  the  jjyroxene  is  ohviiie.  In  the 
peridotite,  to  l)c  described  in  the  next  section,  this  is  usunllv  surrounded 
bv  nionochnic  or  orthorlioinl)ic  pyroxene.  This  idtered  niinend  is  not 
important  in  (punitity. 

Iron  oxide. — Jlinouite  aud  titaniferous  magnetite  occur  in  some  of  tlie  rocks 
in  considerable  quantity.     Both  alter  to  sphene  and  rutile. 

Apatite. — Among-  the  accessory  minerals  apatite  is  jjerhaps  the  most 
common,  and,  as  usual,  one  of  the  very  earliest  minerals  to  crystallize.  It 
is  contained  in  all  the  essential  constituents,  and  in  biotite  is  surrounded  by 
pleochroic  hales.  In  some  cases  it  has  even  crystallized  before  sphene.  It 
is  noticeable  in  some  sections  that  great  numbers  of  apatite  crystals  are 
arranged  along  lines  representing  sections  of  planes  between  the  jjlagioclase 
plates,  thus  practically  outlining  the  feldspar  individuals. 

Sphene. — In  many  cases  in  these  gabbros  sphene  is  found  contained  in 
some  of  the  freshest  rocks  as  an  original  accessory  constituent.  It  is  present 
in  largest  quantity  in  the  very  finest-grained  gabbros,  which  «how  a  parallel 
texture.  In  these  rocks  sphene  in  some  cases  surrounds  an  iron  ore,  which, 
to  judge  from  the  rod-like  sections  which  are  so  common,  is  ilmenite.  One 
might  be  led  to  think  that  the  sphene  was  secondary  in  such  cases,  but  the 
iron  ore  is  perfectl}'  fresh,  and,  considering  that  in  the  same  thin  section 
crystals  of  apatite  are  also  surrounded  by  sphene,  it  seems  clear  that  we 
may  consider  such  sphene  as  original.  It  thus  appears  that  a  portion  of  the 
titanium  oxide  combined  with  the  iron  oxide  first,  forming  the  titanic  iron 
ore.  This  was  followed  by  the  crystallization  of  the  calcium-titanium 
compound,  thus  giving  the  sphene.  In  these  rocks  sphene  is  not  in  crystals, 
but  in  grains.  These  grains  are  arranged  in  long  chains  hing  between  the 
other  mineral  constituents  and  with  the  long  direction  of  the  individual 
grains,  as  well  as  of  the  lines  of  grains,  parallel  to  the  long  directions  of 
the  other  constituents  of  the  rock. 

Zircon  and  rutile. — Zircou  is  iu  vcry  small  quantity.  Rutile  shows  its  usual 
characters.  It  is  most  commonly  associated  with  the  octahedrite  (anatase) 
and  brookite  (?)  as  inclusion  in  the  hornblende.  The  iron  oxide  is  chiefiy 
present  as  ilmenite,  with  some  titanic  magnetite. 

The  secondary  minerals  have  alread}^  been  mentioned  and  their  char- 
acters described  under  tlie  description  of  the  minerals  from  which  they  are 
derived. 


240  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

DESCRIPTION    OF    INTERESTING    KINDS    OF    GABBRO. 

The  minerals  described  above  as  the  leading  essential  constituents  of 
the  rocks  to  be  described  may  be  combined  in  varying  quantities.  Accord- 
ing to  these  combinations  a  number  of  different  mineralogical  types  of  rock 
may  be  produced.  The  wide  range  in  mineral  composition  of  the  gabbroic 
rocks  is  equaled,  if  not  surpassed,  by  similar  variations  noted  by  Fairbanks 
in  certain  rocks  from  Point  Morrito,  California.'  It  may  cause  the  further 
description  to  be  more  readily  understood  if  we  preface  it  by  the  statement 
that  all  of  these  types,  however,  are  simple  facias  of  a  single  magma.  The 
important  phases  which  will  be  described  are,  in  the  order  of  their  impor- 
tance, hornblende-gabbro,  consisting  essentially  of  liornblende  and  lab- 
radorite;  gabbro,  consisting  of  monoclinic  pyroxene  and  labradorite,  and 
bronzite-norite,  consisting  essentially  of  bronzite  and  labradorite.  The 
various  mineralogical  types  exhibit  very  interesting  ranges  in  texture  in 
certain  cases,  t-o  which  attention  will  be  called. 

HORNBLENDB-GABBRO    IN    SEC.    1.5,    T.    i'2   N.,  R.  31    W. 

A  hornblende-gabbro  forms  a  large  knob  in  sec.  15,  T.  42  N.,  R.  31  W., 
just  at  the  foot  of  the  Norway  Rapids,  on  the  west  bank  of  the  Michigamme 
River.  This  exposure  shows  very  prettily  a  change  in  texture.  The  change 
in  texture  is  also  accompanied  by  a  slight  mineralogical  change.  The  knob 
is  composed  partlv  of  a  fine-grained  granular,  but  more  largely  of  a  coarse- 
grained porphyritic,  gabbro.  The  fine-grained  portion  is  a  pure  gabbro 
composed  of  plagiocla^e  and  brown  hornblende,  with  very  little  brown 
mica.  No  quartz  was  observed,  nor  was  any  orthoclase  definitely  deter- 
mined. The  plagioclase  is  in  fairly  well-de  'eloped  automorphie  plates. 
The  hornblende  is  the  brown  variety,  with  numerous  minute  inclusions, 
which  has  already  been  described,  and  is  not  always  so  well  developed  as 
is  the  plagioclase.  In  places  it  plays  rather  more  the  role  of  a  cement. 
This  relation  of  the  two  minerals  results  in  forming  an  imperfect  ophitic 
texture  in  places,  though  on  the  whole  the  two  minerals  are  about  equally 
developed,  and  produce  a  granular  structure.     (Fig.  ^,  PI  XLIV.) 

'  The  geology  of  Point  Sal,  by  H.  W.  Fairbanks  :  Bull.  Dept.  Geol.,  Univ.  California,  Vol.  II, 
1896,  p.  56  et  seq. 


GABBRO  AND  NORITE  INTRUSIVES.  241 

The  coarsse-iiTniiU'd  })()r])liyriti('.  gabbro  foriiiing-  tlie  greater  ])art  of  the 
knol)  consists  of  phigioclase,  liornbleiide,  biotite,  and  iron  oxide,  witli  a  very 
small  amount  of  pyroxene.  The  hornblende  occurs  in  phenocrysts  which 
have  irregular  rounded  shaj)es  instead  of  being  well  crystallized.  Some  of 
the  largest  phenocrysts  have  a  diameter  of  slighth^  less  than  1  centimeter. 
They  are  poikilitic,  rendered  so  by  inclusions  of  lath-shaped  plagioclase  and 
rounded  grains  of  pyroxene.  (Photomicrograj)h,  fig.  A,  PI.  XLI.)  This  por- 
phyritic  liornblende  is  a  dark  reddish-brown  -variety  containing  such  great 
numbers  of  minute  inclusions  as  to  be  opaque  in  many  places,  which  grades 
over  into,  and  is  in  many  places  in  optical  continuity  with,  a  dirty  green 
hornblende.  This  green  hornblende  is  in  anhedra  and  forms  the  cement 
for  the  feldspar,  and  the  two  together  the  groundmass  for  the  brown  horn- 
blende phenocrysts.  Tlie  plagioclase  is  most  commonly  in  broad,  well- 
developed  crystals,  which  frequently  give  quadratic  sections.  Some  few 
grains  of  a  pink  monoclinic  pyroxene  are  included  by  the  hornblende. 

SECS.  15,  22,  28,  AND  29,  x.  42  n.,  b.  31  w. 

Exposures  of  a  hornblende-gabbro  with  interesting  facies  associated 
with  it  occur  in  the  southeastern  corner  of  sec.  15,  at  tlie  southeastern  corner 
of  sec.  22,  extending  east  and  west  through  the  northern  part  of  sec.  28,  at 
the  southeastern  corner  of  sec.  28,  and  on  the  west  bank  of  the  Micliigamme 
River  in  sec.  29,  T.  42  N.,  R.  31  W.,  at  the  location  N.  100,  W.  1,250  paces. 
This  is  medium  to  coarse  grained  and  of  a  gray  color  from  a  short  distance. 
Examined  at  moderately  close  quarters,  one  distinguishes  very  readily  a 
milky  white  feldspar  and  a  black  or  dark-green  hornblende  in  about  equal 
quantities.  The  microscopical  examination  adds  to  these  two  minerals  in  a 
very  subordinate  quantity  biotite,  pyroxene,  and  orthoclase.  The  labra- 
dorite  plagioclase  is  in  medium-broad,  irregular  plates,  though  at  times 
approaching  a  very  distinctly  lath-shaped  form.  The  orthoclase  is  present 
in  a  few  rare  individuals  in  the  form  of  irregular  plates.  The  hornblende 
constituent  is  in  irregular  plates  and  varies  in  character.  It  may  be  the 
brown  or  the  green  variety  ah-eady  described,  or  the  two  together  in  sepa- 
rate individuals,  or  even  the  brown  grading  into  the  green.  This  green  is 
original  and  not  the  alteration  product  of  the  brown.  Biotite  is  the  normal 
reddish-brown  kind  in  irregular  plates.     The  pyroxene  is  usually  absent 

MON  XXXTI 16 


242 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 


from  the  sections  of  these  rocks,  but  when  present  it  is  ver}-  rare,  and  occurs 
in  small  irregular  grains  not  uncommonly  intergrown  with  the  hornblende 
and  evidently  older  than  the  hornblende.  It  is  light  green  in  color, 'with  a 
scarcely  noticeable  pleoclu-oism.  Its  monoclinic  character  was  readily  deter- 
minable; but  a  more  exact  determination  was  not  made.  It  does  not,  how- 
ever, show  diallagic  parting,  and  is  diagnosed  as  possibly  diopside. 

The  feldspar  shows  the  best  development  of  the  accessory  minerals. 
It  can  rarely,  however,  be  said  to  be  automorphic.  The  texture  is,  on  the 
whole,  granular. 

From  a  mineralogical  study  of  the  rocks  alone,  one  would  unhesitat- 
ingly place  them  with  the  diorites,  especially  if  those  facies  were  seen  in 
which  the  pyroxenic  constituent  was  wanting. 

The  following  analysis  (Sp.  23354),  obtained  from  Mr.  George  Steiger, 
of  the  United  States  Geological  Survey,  shows  the  chemical  composition 
of  one  of  these  rocks  : 

Analysis  of  hornhlendegahhro. 


SiO,  . 
TiO,  . 

Al.Oa 
FeiOa 
FeO  . 
MnO. 
CaO  . 
MgO. 


Per  cent. 


49.80 
.79 

19.96 

6.32 

.49 


11.33 
7.05 


K,0   

NajO 

H.O  100^— 
H2O  100^-f, 

P2O5 

CO2 

Total 


Per  cent. 


.61 
2.22 

.13 
1.71 

.07 

.15 


100.63 


An  examination  of  the  analysis  shows  that  the  microscopical  determi- 
nation of  the  rock  as  a  diorite  would  be  incorrect  if  we  accept,  as  has  been 
done  in  the  preceding  pages,  Brogger's  characterization  of  the  diorite  and 
gabbro  families.^ 

The  rock  analyzed  is  hornblende-gabbro,  as  shown  by  the  relatively 
high  content  in  the  characteristic  alkaline  earths,  esj^ecially  magnesia, 
which  usually  appears  in  inverse  proportion  to  the  silica,  and  in  the  low 
percentage  of  alkalies. 


'  Op.  cit.,  Part  II,  pp.  35, 39. 


c 


GABBRO  AND  NOIUTE  INTKUSIVEB.  243 

HOUNBLKNDK-CSABUKO    DUCES. 

One  (tt"  tlic  exposures  of  the  above-descTibod.  liornlilende-g-abln-o — tlie 
one  on  the  west  bank  of  the  Michigamnie  River  in  section  29— is  very 
interesting  on  aeeount  of  at  least  two  different  series  of  dikes  which  cut  it. 
The  coarse-grained  liornblende-gabbro  forms  the  main  mass  of  the  knob. 
The  dike  rocks  may  be  divided  into  the  fine-grained  hornblende-gabbro 
forming  the  earhest  dikes  and  the  coarse-grained  bronzite-norite  forming 
the  latest  dike. 

Some  of  the  specimens  of  the  fine-grained  rock  are  massive,  but  the 
o-reater  portion  possess  a  distinctly  parallel  texture.  These  are  distinctly 
micaceous.  The  rock  of  the  dikes  has  in  general  very  much  the  macro- 
scopical  appearance  of  a  biotite-mica-schist.  The  dikes  are  very  narrow, 
never  more  than  18  inches  wide.  The  larger  ones  send  off  branches,  and 
in  places  inclose  angular  pieces  of  the  coarse  diorite.  Thus  the  relation  of 
this  fine-grained  rock  to  the  main  gabbro  mass  is  perfectly  clear,  though  in 
places  it  so  closely  resembles,  macroscopically,  pieces  of  mica-schist  that  in 
spite  of  the  branching  of  these  dikes,  indicating  their  intrusive  nature,  they 
were  supposed  by  one  observer  to  be  curiously  shaped  stringers  of  the 
mica-schists  included  in  the  main  mass  of  the  gabbro. 

The  notes  do  not  indicate  from  just  what  portions  of  the  dike  the 
specimens  were  taken;  hence  it  is  impossible  to  state  positively  that  the 
more  schistose  parts  are  nearest  the  edges  and  the  more  granular  portions 
nearer  the  center,  as  one  would  naturally  expect.  However,  in  all  but  one 
of  the  specimens  which  show  a  contact  between  the  dikes  and  gabbro 
which  they  penetrate,  the  rock  nearest  the  contact  shows  a  parallel  struc- 
ture. Hence  it  may  be  stated  that  in  some  cases  the  edges  of  the  dikes 
are  the  more  schistose  portions.  The  one  specimen  referred  to  above  is 
granular  and  much  finer  grained  along  the  contact  than  farther  from  it. 

The  microscope  shows  the  rock  of  these  dikes  to  be  a  gabbro  differing 
little  in  character  from  the  main  mass.  The  plagioclase  is  well  developed 
in  rectangular,  more  or  less  lath-shaped  crystals.  Mica  of  a  rich  brown 
color  is  rather  more  abundant  than  usual,  and  is  in  about  equal  quantity 
with  the  hornblende.  The  hornblende  is  brown  or  of  a  dirty  greenish  color, 
containing  the  inclusions  mentioned  in  the  detail  descriptions  of  the  minerals 
of  the  gabVjros.  Some  irregular  grains  of  a  light-green  augite  (diopside) 
were  observed  in  the  sections.     Orthoclase  (?)  in  grains  is  present  simply 


244  THE  CEYSTAL  FALLS  lEON-BEAElNG  DISTEICT. 

as  an  accessory.  Ilmeuite  occurs  in  irregular  masses  in  rod-shaped  pieces: 
Sphene  is  present  in  original  grains  and  also  as  a  secondary  ])roduct  after 
ilnienite.     Apatite  occurs  and  is  sometimes  included  in  the  sphene. 

The  most  striking  feature  of  the  rock  is  its  textural  variation.  Some 
of  the  sections  show  very  good  granular  texture;  others  have  a  fair  ophitic 
texture ;  the  most  common  is  a  striking  parallel  texture  which  macroscopic- 
ally  gives  to  the  rock  a  schistose  appearance.  This  may  be  seen  even  in 
the  same  section  with  the  ophitic  texture,  the  two  grading  into  each  other. 
The  parallel  texture  is  occasioned  by  tlie  arrangement  in  a  common  direc- 
tion of  the  long  diameters  of  nearly  all  of  the  minerals  (figs.  A  and  B,  PI. 
XLII).  The  grains  of  sphene  often  lie  in  long  trains  agreeing  with  this 
general  parallelism.  One's  first  idea  would  probably  be  that  the  texture  was 
due  to  the  cause  which  has  produced  parallel  structures  in  most  other  ancient 
rocks — pressm'e.  However,  it  can  not  be  referred  to  this,  as  the  minerals — 
with  some  individual  exceptions — show  slight  or  no  pressure  efi"ects. 
Apparently  the  only  explanation  borne  out  by  the  facts  in  this  case  is  that 
we  have  to  do  with  a  fluidal  texture,  produced  by  the  movement  in  the 
magma  consequent  upon  its  injection  along  the  fissures  in  the  gabbro,  the 
parallelism  of  the  minerals  agreeing  with  the  bounding  sides  of  the  fissure. 

BRONZITE-NORITE    DIKE. 

The  main  hornblende-gabbro  and  the  fine-grained  dikes  just  described 
are  cut  by  a  dike,  about  3  feet  wide,  of  coarse  rock  which  resembles  that 
forming  the  main  mass  of  the  knobs  in  every  way  except  that  it  contains  a 
very  much  altered  orthorhombic  pyroxene  (bronzite)  in  greater  quantity 
than  the  hornblende.  The  rock  is  a  very  pure  kind  of  bronzite-norite  (fig. 
B,  PI.  XLIV).  The  following  analysis  (No.  1,  Sp.  23755),  by  Mr.  George 
Steiger,  shows  the  gabbro  affinities  of  the  rock.  The  high  percentage  of 
magnesia  gives  a  clear  indication  of  the  important  role  played  by  the  bron- 
zite in  the  composition  of  the  rock. 

With  the  analysis  of  the  bronzite-norite  there  is  placed  for  comparison 
an  analysis  (No.  2)  of  a  norite  from  Ivrea,  Italy,  which  is  essentially  the 
same  as  the  above  in  mineralogical  as  well  as  chemical  composition.  In 
the  Italian  rock  hypersthene,  instead  of  bronzite,  is  the  chief  pyroxenic 
constituent.^ 

'  Petrographische  Uutersuchiingen  iibcr  die  uoritischeu  Gesteine  der  Umgegcnd  von  Ivrea  in 
Oberitalien,  by  F.  E.  van  Horn:  Tschermaks  mineral,  Mtttbeil.,  Vol.  XVII,  1897,  Part  V,  p.  404. 


GABBKO  AND  NORITE  INTRUSIVES. 

Andh/scs  of  iiorites. 


245 


SiO, 

TiOj 

ALO, 

Fe,0., 

FeO 

MnO 

CaO  

MgO 

KaO 

Na^O 

H;0  100°—. 
HiO  100°  +  . 

PjOa 

CO, 


1. 


48.23 
1.00 

18.26 
1.26 
6.10 


Total 


9.  .39 
10.84 

.73 
1.34 

.26 
2.00 

.07 

.43 


99.91 


49.95 

.69 

19.17 

4.72 

6.71 

Trace. 

9.61 

.5.03 

.74 

3.13 

.09 

Trace. 


9.84 


SEC.  29,  T.  42  N.,  R.  31  W.,  1,200  N.,  200  W. 

On  the  east  bank  of  the  Michigamme  River  at  1,200  paces  N.,  200  W., 
sec.  29,  T.  42  N.,  R.  31  W.,  there  is  an  outcrop  which  shows  even  better 
than  the  occurrence  just  described  the  variation  in  mineralogical  character 
and  the  relative  ages  of  these  varieties.  At  tliis  place  there  is  a  knob  com- 
posed of  hornblende-gabbro  essentially  like  the  general  type  described  as 
typical  for  this  district.  This  knol)  is  cut  by  a  rock  which  is  coarser  in 
grain  and  a  trifle  darker  than  the  variety  which  it  intrudes.  Examined 
under  the  microscope,  it  is  seen  to  differ  from  the  normal  phase  also  in  min- 
eralogical  composition,  and  resembles  rather  closely  in  this  respect  the 
porphp-itic  variety  described  on  p  240.  Like  that,  the  hornblende  is  reddish- 
brown,  containing  a  large  number  of  inclusions  and  grading  over  into  the 
green  hornblende.  This  hornblende  includes  rounded  grains  of  a  white  to 
pinkish  monoclinic  pyroxene  in  sufficient  quantity  to  be  characteristic.  The 
pyroxene  is  never  automorphic,  as  one  would  perhaps  expect  to  find  it, 
though  it  was  evidently  one  of  the  first  minerals  to  crystallize.  Contained  in 
the  pyroxene  individuals  are  rounded  areas  of  yellovvish-greeu  serpentine. 
These  areas  are  sharply  outlined  from  the  pyroxene  and  do  not  appear  to  be 
the  result  of  its  alteration;   cousequenth'  the  conclusion  is  reached  that  the 


246  THE  CRYSTAL  FALLS  IROX-BBARING  DISTRICT. 

serpentine  results  from  the  alteration  of  a  mineral  older  than  the  pyroxene. 
Most  'irobably  this  mineral  was  olivine,  though  no  positive  statement  to 
that  effect  can  be  made.  This  pyroxene  I'ock  also  contains  an  exceedingly 
large  quantity  of  apatite.  No  analysis  was  obtained  of  this  facies,  liut  the 
microscopical  characters  enable  us  to  place  it  as  a  gabbro  (possibly  olivine- 
bearing)  and  to  consider  it,  like  the  bronzite-norite,  as  a  facies  of  the  pre- 
dominant hornblende-gabbro. 

This  same  exposure  of  gabbro  is  cut  by  a  coarse  peridotite  (wehrlite), 
a  description  of  which  will  be  found  on  p.  253.  In  this  peridotite  there  is 
found  a  narrow  strip  of  rock,  about  2  inches  wide,  which  is  presumed  to  be 
either  an  inclusion  or  a  yerj  narrow  dike  in  the  peridotite.  The  exposure 
does  not  admit  of  its  relations  being  determined  more  accurately.  The 
presumption  is  that  it  is  a  dike.  Whether  an  inclusion  or  a  dike,  it  is 
younger  than  the  massive  hornblende-gabbro  forming  the  main  exposure. 
In  this  respect  it  corresponds  to  the  gabbro  just  described  (p.  243)  as  cutting 
the  normal  gabbro. 

This  dike  rock  is  macroscopically  a  fine-grained,  granular,  dark-gray 
rock.  The  microscope  shows  it  to  be  very  fresh,  porphyritic  in  texture, 
and  composed  of  phenocrysts  of  bronzite  lying  in  a  finely  granular  ground- 
mass  of  plagioclase,  hornblende,  and  orthorhombic  and  monoclinic  pyroxene. 
No  quartz  whatever  was  found  associated  with  these  minerals.  The  pla- 
gioclase is  the  usual  kind,  labradorite.  Some  unstriated  feldspar,  possibly 
orthoclase,  was  also  observed,  though  in  very  small  quantity.  The  bronzite 
is  in  narrow  prisms  which  reach  a  length  of  1.23  mm.  Commonly  they 
have  well-developed  transverse  partings.  It  is  worthy  of  note  that  a  few 
of  the  crystals  contain  the  brownish  platy  inclusions  so  common  in  hyper- 
sthene.  There  is,  however,  no  relation  between  the  color  and  pleochroism 
of  the  mineral  substance  and  these  brownish  plates.  The  bronzite  is  very 
clear,  with  weak  color,  showing  a  scarcely  distinguishable  greenish  tinge 
for  the  rays  vibrating  parallel  to  C,  with  yellowish  for  the  rays  parallel  to  a 
and  I).  In  one  case  the  bronzite  was  seen  altering  to  a  greenish-yellow 
fibrous  aggregate  of  serpentine.  In  the  groundmass  this  same  orthorliombic 
pyroxene  is  represented  by  irregular  grains.  The  hornblende  is  the  usual 
reddish-brown  kind,  but  differs  frOm  that  seen  in  the  other  gabbros  of  this 
district  in  that  it  contains  ilmenite  inclusions  only,  without  any  rutile  oi* 
anatase.     It  is  in  anhedra.     A  faint-greenish  pyroxene  occurs  in  irregular 


p 


GA15BKO  AND  NOHITR  INTUUSIVES.  247 

xenoniorpliic  individiuils,  ioi'iiiiiiy  a  soiiu'wliat  smaller  proportion  of  the 
"Touiuliiiass  than  does  the  hornblende,  l)ut  i.s  more  abundant  than  the 
bronzite  of  the  groixndmass.  Sphene  is  pi-esent  in  considerable  (quantity, 
and  likewise  ilmenite  (fig'.  A,  PI.  XLV). 

This  rock  stands  between  the  hornblonde-gabbros  and  the  norites.  In 
texture  it  might  be  compared  with  the  norite-porphyrite  (enstatite-porphyrite 
of  Ivosenbusch)  with  holocrystalline  groundmass.  But  it  differs  essentially 
from  this  rock  in  the  presence  of  a  large  quantity  of  hornblende.  This 
hornblende  connects  it  with  tlie  hornblende-gabbros,  from  which  its  content 
of  pyroxene,  both  orthorhombic  and  monoclinic,  tends  to  separate  it.  Owing 
to  its  obvious  relation  to  the  bronzite-norites,  which,  like  it,  occur  as  differ- 
entiation facies  of  and  dikes  in  the  hornblende-gabbro,  I  shall  call  it  a 
"bronzite-norite-porphyry,"  using  the  term  "porphyry"  purely  in  a  textural 
sense. 

The  rocks  may  be  compared,  in  their  variation,  to  those  described  by 
G.  H.  Williams  from  Maryland,'  by  Chester  from  Delaware,"  and  by  Fair- 
banks from  California.'  A  series  of  basic  rocks  similar  in  many  respects 
to  those  of  Crystal  Falls  has  also  recently  been  described  in  two  interest- 
ing papers  by  Van  Horn  *  and  Schaefer.^ 

DYNAMICALLY  ALTERED  GABBRO. 

Near  the  junction  of  sees.  26,  27,  33,  34,  T.  43  N.,  R.  31  W.,  there  is 
a  large  gabbro  mass  which  shows  raai-ked  evidence  of  dynamic  action,  and 
may  well  be  cited  as  an  example  of  a  metamorphosed  gabbro,  or,  perhaps 
more  clearly,  as  a  rock  intermediate  between  a  hornblende-gabbro  and  a 
hornblende-gneiss.  None  of  tlie  gabbros  thus  far  described  show  any  evi- 
dence of  having  taken  part  in  very  extensive  orogeuic  movements.  The 
minerals  of  but  few  of  them  show  more  than  the  common  phenomenon  of 
slight  wavy  extinction.     Hence  it  is  clear  that  their  intrusion  took  place 

'The  gabbros  and  associated  hornblende  rocks  occurring  near  Baltimore,  by  G.  H.  Williams: 
Bull.  U.  S.  Geol.  Survey  No.  28,  1886 ;  Oiitliue  of  geoIo;,ry  of  Maryland,  Baltimore,  1893,  p.  39. 

-The  gabbros  and  associated  rotks  in  Delaware,  by  F.  D.  Chester:  Bull.  U.  S.  Geol.  Survey 
No.  59, 1890. 

^The  geology  of  Point  Sal,  by  H.  W.  Fairbanks:  Bull.  Dcp.  Univ.,  California,  Vol.  XI,  1896, 
p.  56  et  seq. 

^Petrographische  Uutersuchungen  iiber  die  noritischeu  Gesteine  der  Umgegend  von  Ivrea  im 
Oberitalieu,  by  F.  R.  van  Horn:  Tscliermaks  mineral.,  Mittheil.,  Vol.  XVII,  1897,  Part  V,  pp.  391-420. 

^  Der  basische  Gesteinzug  von  Ivrea  im  Geliiet  desMastalloue-Thals,  by  K.  W.  Schaefer:  Tscher- 
maks  mineral.,  Mittheil.,  Vol.  XVII,  1898,  Part  VI,  pp.  495-517. 


248  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

after  the  occun-ence  of  the  inountaiu-building  inovemeuts  to  which  the 
intruded  rocks  have  been  subjected.  It  is  not  beheved  that  the  mass 
referred  to  in  this  description  is  an  exception  to  the  general  rule,  but  that 
its  metamorphosed  condition  is  referable  to  local  causes,  such  as  have  pro- 
duced the  shearing  planes  to  which  allusion  has  been  made  (p.  234).  It  may 
perl  laps  occur  near  or  along  some  minor  fault  plane,  though  no  indications 
of  a  fault  have  been  observed. 

The  outcrop  is  composed  in  part  of  massive  gabbro  and  in  part  of  the 
metamorphosed  kind.  The  gabbro  in  its  typical  massive  form  (fig.  A, 
PI.  XLIII)  is  a  medium  to  coarse  grained  granular  rock,  composed  essentially 
of  plagioclase  and  dirty  brown-green  compact  liornblende,  the  latter  being 
quite  full  of  the  inclusions  mentioned  above  as  commonly  occurring  in  the 
hornblende  of  the  rocks  in  this  region.  In  the  metamorphosed  rock  the 
grain  is  much  finer,  the  rock  possesses  imperfect  schistosity,  and  the  color 
is  a  lighter  green  than  in  the  original.  The  component  minerals  are  chiefly 
hornblende  and  feldspar,  with  some  quartz,  chlorite,  epidote,  calcite,  and 
rutile.  The  hornblende  has  a  light-green  color.  It  occurs  mainly  in  aggre- 
gates of  small  irregular  grains.  In  some  cases  these  surround  an  angular 
nucleus  of  dirty  brown-green  original  hornblende  containing  the  same 
inclusions  and  in  every  way  similar  to  that  of  the  coarse-grained  uncrushed 
rock.  The  light-green  hornblende  contains  none  of  these  minute  inclusions 
and  onlv  here  and  there  grains  of  rutile,  which,  if  the  diagnosis  of  the 
interpositions  as  titanium  minerals  is  correct,  may  possibly  be  considered  as 
havino-  been  derived  from  them.  This  hornblende  is  believed  to  be  sec- 
ondarv.  It  is  produced  by  a  process  of  mechanical  Ijreaking  down  of  the 
original  hoi'nblende,  accompanied  by  recrystallization.  The  process  is 
somewhat  similar  to  the  crushing  of  ordinary  plagioclase  and  the  produc- 
tion of  a  more  acidic  variety.  As  may  be  seen  from  the  photomicrograph 
(fig.  B,  PI.  XLIII)  of  a  section,  the  feldspar  exhibits  signs  of  intense  crushing. 
The  twinning  lamellse  are  strongly  bent,  and  the  pieces  possess  wavy 
extinction.  Many,  in  fact  most,  of  the  feldspars  are  fractured.  Wherever 
these  fractures  occur  the  feldspar  along  the  edges  of  the  fractured  portions 
has  been  altered,  producing  secondary  epidote,  muscovite,  plagioclase  feld- 
spar, and  quartz;  the  last,  however,  in  small  quantity.  The  biotite  has 
been  crushed.  This  has  partly  altered  to  chlorite,  and  the  latter  contains 
many  grains   of   epidote.     Some  granular    calcite  and   considerable   iron 


GABBttO  AND  NOUITE  INTRUSIVES.  249 

pyritt's  are  also  toiind  in  the  altered  rocks.  These  secondary  products  are 
easily  distinguished  from  the  original  minerals  by  the  total  absence  m  them 
of  wavy  extinction.  The  effect  of  the  crushing  upon  the  texture  has  been 
to  render  it  more  or  less  schistose,  the  resulting  rock  resembling  in  its 
mineralogical  character,  and  in  texture,  an  ordinary  honiblende-gneiss,  or, 
when  (piartz  is  present  in  but  small  quantity,  a  plagioclase  amphibolite. 

RELATIVE  AGES  OF  GABBROS. 

An  attempt  to  determine  the  relations  of  the  varieties  described  resulted 
in  the  establishment  of  the  following  order  of  intrusion:  The  typical 
coarse-grained  hornlilende-gabbro  was  the  first  formed,  and  seems  to  be  the 
most  widely  distributed.  This  was  then  intruded  by  dikes  of  gabbro,  which 
contain  both  monoclinic  pyroxene  and  hornblende  in  association  and  rep- 
resent a  gradation  to  the  normal  gabbro.  These  hornblende-gabbros  and 
normal  gabbros  were  then  cut  by  the  dikes  of  bronzite-norite  and  bronzite- 
norite-porphyry. 

PERIDOTITES. 

Until  recently  all  of  the  ultrabasic  rocks  were  included  by  most 
petrographers  under  the  family  name  of  peridotite.  ■  In  the  last  edition  of  his 
Mikroskopische  Physiographie,  Rosenbusch  has  separated  these  rocks  into 
the  plutonic  rocks  and  the  volcanic  rocks.  The  term  "  peridotite "  is 
restricted  to  the  first,  and  the  last  are  called  the  "picrites."  The  charac- 
teristic differences  between  these  two  types  are  very  clearly  shown  in  the 
specimens  from  the  Crystal  Falls  district  which  I  have  had  the  oppor- 
tunity of  studying.  The  rocks  here  described  agree  with  Rosenbusch's 
narrower  definition  of  the  peridotites.^ 

DISTRIBUTION,  EXPOSURES,   AND   RELATIONS. 

The  peridotite  dikes  were  all  found  near  the  Michigamme  River,  in 
sees.  29  and  22,  T.  42  N.,  R.  31  W.  Typical  wehrlite  with  very  little  green 
amphibole  is  found  on  the  east  bank  of  the  Michigamme  river,  near  the 
center  of  sec.  22,  T.  42  N.,  R.  31  W.  It  shows  no  relations  to  the  other 
rocks.  The  amphibole-peridotite,  exhibiting  marked  variations  to  wehrlite 
and  olivine-gabbro,  forms  an  outcrop  on  the  east  bank  of  the  Michigamme 
River  in  the  NE.  ^  sec.  29,  T.  42  N.,  R.  31  W.     This  dike  cuts  the  gabbro. 

'  Op.  cit.,  p.  343. 


250  THE  CRYSTAL  FALLS  lEON-BBAKIXG  DISTRICT. 

A  rock  to  be  described  last,  connecting  the  diorites,  gabbros,  and  peridotites, 
was  taken  from  near  the  northeast  corner  of  sec.  22,  T.  42,  B.  31,  where  it 
is  found  cutting  the  gabbro. 

PETROGRAPHICAL  CHARACTERS. 

The  peridotites  are  very  dark  green  to  black  coarse-grained  rocks, 
showing  in  almost  all  cases  a  granular  texture.  In  one  case  an  excellent 
poikilitic  texture  was  observed,  and  in  another  the  same  texture  in  a  very 
imperfect  condition  was  seen.  The  almost  total  absence  of  any  pressure 
phenomena  in  these  rocks  excludes  the  idea  of  their  having  been  subjected 
to  any  powerful  dynamic  action.  The  chief  mineral  constituents,  arranged 
in  order  of  relative  amounts,  are  pyroxene  (monoclinic  and  orthorhombic), 
olivine,  hornblende,  and  biotite.  The  following  minerals  also  occur  associ- 
ated with  these:   Feldspar,  apatite,  green  and  brown  spinel,  and  iron  oxide. 

Pyroxene  — Thcrc  is  prcscut  in  these  rocks  both  orthorhombic  and  mono- 
clinic  pyroxene.  The  orthorhombic  has  a  3'ellowish  tinge,  and  contains  a 
few  tabular  inclusions.  Only  a  few  grains  of  this  pyroxene  were  found, 
but  upon  one  section  a  figure  was  obtained  and  the  positive  character  of  the 
mineral  was  determined.  It  appears  to  be  bronzite.  The  bronzite  grains 
are  found  included  in  the  hornblende.  Its  relations  to  other  minerals  are 
not  shown  in  any  of  the  thin  sections. 

The  monoclinic  p\TOxene,  though  one  of  the  earliest  minerals  to  crys- 
tallize, is  likewise  in  xenomorphic  individuals,  many  of  them  twinned,  some 
polvsynthetically.  It  is  usually  of  a  light  yellowish  color,  and  is  then 
nonpleochroic;  in  some  sections,  however,  it  is  sufficiently  colored  to  show 
pleochroism  from  light  yellowish  to  brownish.  It  includes  a  number  of  the 
clove-brown  tabular  interpositions  like  those  in  the  hornblende,  and  these 
at  times  give  the  pieces  a  decided  violet  tinge.  Less  commonly  than  the 
tabular  interpositions  one  observes  green  needles  and  laths,  more  rarely 
rounded  grains  or  plates  of  larger  size  and  having  a  brownish-green  color. 
They  are  so  minute  as  to  defy  positive  determination,  but  are  presumed  to 
be  hornblende  microlites.  The  orthopinacoidal  parting  is  in  some  cases 
well  developed  in  the  augite.  This  diallagic  augite  is  in  large  quantity, 
and  though  usually  inchided  in  the  hornblende,  nevertheless  includes  both 
hornblende  and  biotite  in  a  few  ragged  plates.  Such  sections  are  pi'obably 
those  which  have  passed  tlu-ough  tlie  outer  edges  of  the  augite  crystals. 


PERIDOTITK  INTRTTSIVES.  251 

Olivine. — 'riiis  is  present  most  commonly  in  round  iuilit'dra,  and  is  usually 
almost  entirely  altered  to  yellowish-green  serpentine  tihers.  When  the  olivine 
is  completely  altered,  the  mesh  structure  affords  a  ready  means  of"  recogniz- 
ing the  original  mineral,  especially  when  taken  in  conjunction  with  grains 
of  a  rich-green  isotropic  mineral,  and  also  octahedra  and  grains  brown  in 
color,  probably  spinels,  which  are  found  included  in  the  ])seudomorphs. 

Hornblende. — Tlic  luost  of  tlic  hoHiblende  in  the  peridotites  is  a  brown 
variety  showing  strong  pleochi'oism.  a  is  light  cream-yellow,  c  is  yellowish 
brown,  and  h  is  reddish  brown;  tJ>>C>H.  Patton '  has  already  called 
attention  to  the  pleochroism  of  the  hornblende,  which  "is  exceptional, 
inasmuch  as  the  brownest  color  is  that  of  rays  of  light  vibrating  parallel  to 
the  orthodiagonal  axis."  The  brown  hornblende  is  accompanied  by  a  very 
small  quantity  of  green  hornblende.  Moreover,  the  brown  hornblende 
grades  over  into  a  light  green,  the  two  being  in  perfect  crystalline  conti- 
nuity. In  rare  cases  this  brown  hornblende  is  also  intergrown  with  a  light- 
green  pyroxene  in  such  way  as  to  give  a  mottled  polarization  effect.  The 
pinacoidal  cleavage  of  the  hornblende  continues  through  the  pyroxene,  and 
the  extinction  angle  of  the  hornblende  against  this  cleavage  is  19  degrees, 
while  that  of  tlie  pyroxene  runs  up  to  34  degrees.  The  hornblende  includes 
numerous  anhedra  of  pyroxene,  somewhat  fewer  grains  of  olivine,  and,  less 
commonly,  ragged  pieces  of  biotite,  giving  it  a  poikilitic  character.  It  is 
also  very  full  of  opaque  metallic  or  brownish  translucent  plates  of  ilmenite. 
In  some  individuals  minute  clear  microlites,  similar  to  those  described 
above  in  the  hornblende  of  certain  gabbros,  are  noticed  in  small  quantity. 
These  are  irregularly  distributed  in  the  hornblende,  giving  the  crystals  a 
patchy  appearance.  In  respect  to  its  color  and  these  inclusions,  this  horn- 
blende, as  noted  by  Patton,  bears  a  rather  striking  resemblance  to  hyper- 
sthene  on  superficial  examination.^  Iron  ore  in  large  masses  is  fairly 
frequent  as  an  inclusion,  and  it  is  very  commonly  noticeable  that  where 
such  inclusions  occur  the  zone  of  hornblende  immediately  surrounding 
them  is  free  from  the  platy  inclusions  mentioned  above.  Such  a  clear  zone 
is  also  observed  at  times  surrounding  the  inclusions  of  biotite  and  olivine, 
but  never  in  case  of  pyroxene.     Whei-e  these  clear  zones  surround  the 

'  Microscopic  study  of  some  Michigan  rocks,  by  H.  V.  Patton:  Rept.  State  Board  of  Geol.  Surv. 
for  1891-92,  1893,  p.  186. 
'  Loc.  cit.,  p.  186. 


252  THE  CliYSTAL  FALLS  IRON-BEARING  DISTRICT. 

included  biotite  or  olivine,  these  two  minerals  have  associated  with  them 
numerous  small  grains  of  ore,  which  probably  represent  the  iron  that 
would  have  been  incorporated  in  the  sun-ounding  hornblende  but  for  some 
selective  influence  exerted  by  the  olivine  and  biotite. 

Biotite. — This  mineral  is  present  in  flakes  of  very  irregular  outline.  The 
pleochroism  varies  from  cream  color  to  yellowish  red  or  brown.  Although 
one  of  the  last  minerals  to  crystallize,  its  crystallization  began  before  that  of 
the  pyroxene  or  hornblende  had  entirely  finished.  Hence  we  find  flakes  of  it 
included  in  these  minerals,  but  near  the  edges  of  the  crystals.  The  biotite 
itself  is  almost  free  from  inclusions,  containing  only  a  little  hematite  and  mag- 
netite.    It  alters  to  a  brilliant  green,  strongly  pleochroic,  chloritic  mineral. 

Feldspar. — Tliis  is  prGscut  lu  specimcus  from  two  outcrops,  and  in  these 
hardly  reaches  the  rank  of  an  essential  constituent.  It  was  the  last  mineral 
to  crystallize,  and  is  consequently  in  anhedra,  forming  the  mesostasis.  All 
of  the  feldspar  sections  were  tested,  but  no  determinative  measurements 
could  be  made.     It  is  probably  very  basic. 

Apatite. — Apatite  is  present  in  small  quantity.  It  exhibits  its  usual 
characters. 

Spinel. — There  is  a  spinel  found  in  round  grains  which  are  included  in 
the  olivine  (serpentine).  It  is  g-reen  in  color,  and  is  possibly  pleonaste.  A 
second  spinel,  probably  picotite,  occurs  in  small  brown  grains  and  octahedra 
in  the  olivine. 

caicite. — This  mineral,  derived  partly,  if  not  wholly,  from  the  altering 
minerals,  is  found  in  lenses  between  the  biotite  lamellse  and  in  minute  veins 
which  traverse  the  slide. 

Iron  ores. — Irou  Ore  IS  rcprcseuted  by  hematite,  magnetite,  and  pj^rite. 
The  hematite  is  in  blood-red  transparent  flakes  inclosed  in  the  biotite. 
Magnetite  is  included  by  all  of  the  chief  minerals,  and  is  in  irregular  masses 
without  good  crystal  development.  The  iron  pyrite  is  found  in  good  crys- 
tals, though  not  in  large  quantity,  and  is  scattered  here  and  there  through 
the  slides. 

PERIDOTITE  VARIETIES. 

The  relative  proportions  of  the  minerals  described  above  diff"er  very 
much,  and  we  have  different  kinds  of  rocks  corresponding  to  these  min- 
eralogical  variations.  These  kinds  are  not  sharply  sepai'ated,  but  are  seen 
under  the  microscope  to  grade  into  one  another. 


rEKlDOTlTE  INTKUSIVES.  253 

Tlie  purest  form  of  peridotite  is  welirlito,  which  is  composed  essentially 
of  olivine  and  augite.  When,  besides  these  minerals,  hornblende  is  present 
in  large  quantities,  the  rocks  Ixdong  to  the  ampliil)ole-peridotite  type.  In 
some  specimens  biotite  is  almost  in  sufficient  abundance  to  warrant  the 
naniini;-  of  them  biotite-peridotite.  Ag-ain,  in  other  specimens  feldspar  is 
present  in  considerable  quantity  and  the  rock  approaches  an  olivine-gabbro 
or  oliviue-hornblende-gabbro. 


WEHRLITE. 


This  is  represented  by  a  coarse-grained  rock  which  is  mottled  and  has 
a  dark-green  color.  (Specimen  23763,  from  sec.  22,  T.  42  N.,  R.  31  W., 
N.  1,500,  W.  900  paces.)  Under  the  microscope  the  mottling  is  seen  to 
be  due  to  the  association  of  very  dark  greenish-black  serpentine  pseudo- 
morphs  after  olivine  with  a  light-colored  augite. 

Olivine  and  augite  were  present  in  about  equal  quantities.  They  are 
in  anhedra,  and  therefore  must  have  crystallized  at  about  the  same  time. 
The  olivine  is,  with  very  few  exceptions,  completely  altered  to  serpentine. 
Augite  has  a  very  poor  development.  Between  the  olivine  and  augite  are 
small  quantities  of  irregular  plates  of  biotite.  A  few  small  irregular 
pieces  of  a  very  light  colored  greenish  hornblende  were  observed.  Thev 
are  intergrown  with  the  pyroxene  and  give  it  an  imperfect  poikilitic  texture. 

This  wehrlite  is  unquestionably  the  same  as  specimen  1247  of  the 
Geological  Survey  of  Wisconsin,  described  by  Dr.  A.  Wichmann  as  serpen- 
tine,  consisting   chiefly^  of  serpentine  with  some  unaltered   olivine   and 


augite. 


AMPHIBOLE-PERIDOTITE. 


This  variety  of  peridotite  was  obtained  from  the  outcrop  N.  1,260,  W. 
200  paces  from  the  southeast  corner  of  sec.  29,  T.  42  N.,  R.  31  W.,  on  the 
east  bank  of  the  Michigamme  River.  The  rock  is  very  coarse  grained, 
and  possesses  poikilitic  texture.  It  is  composed  of  hornblende,  pyroxene, 
olivine,  biotite,  and  iron  oxide.  The  hornblende  equals  in  quantity  all  of 
the  other  constituents.  Some  of  the  hornblende  individuals  measure  3  cm. 
in  length,  and  include  all  of  the  other  constituents  except  the  biotite.  The 
pyroxene  and  olivine  seem  to  have  crystallized  at  about  the  same  time,  as 

'Microscopical  observations  of  the  iron  bearing  (Huronian)  rocks  from  tbe  region  south  of  Lake 
Superior,  by  Dr.  Arthur  Wichmann,  Leipzig,  1876:  Gool.  of  Wisconsin,  Vol.  Ill,  1880,  p.  619. 


254  THE  CRYSTAL  FALLS  IRON-BEAKmG  DISTRICT. 

they  never  include  each  other.  They  are  both,  however,  included  in  the 
hornblende,  which  with  the  biotite  forms,  as  it  were,  the  mesostasis.  Biotite 
is  present  in  this  specimen  in  very  small  quantity,  and  is  essentially  the 
same  kind  as  that  above  described  (p.  252),  except  that  it  shows  a  trifle 
higher  absorption  parallel  to  the  cleavage  and  becomes  a  yellowish-red. 
The  rock  is  very  fresh  and  shows  scarcely  any  traces  of  alteration.  This  is 
partly  due  to  the  erosive  action  of  the  Michigamme  River  having  removed 
the  weathered  crust,  thus  making  fresh  specimens  obtainable. 

This  rock,  from  the  description  just  given,  would  be  classified  as  an 
amphibole-peridotite,  with  accessory  diallage,  bronzite,  and  biotite.  It 
approaches  Williams's  cortlandtite.  In  some  specimens  the  biotite  is  pres- 
ent in  very  large  quantity,  though  hardly  in  sufficient  quantity  to  waiTaut 
the  designation  of  any  of  the  rocks  as  biotite-peridotite. 

GRADATIONS   OF   AMPHIBOLE-PERIDOTITE   TO   WEHRLITE   AND   OLIVINE-GABBEO. 

There  were  taken  from  the  same  exposure  whence  the  above-described 
amphibole-peridotite  came  some  specimens  which  macroscopically  can  not 
be  distinguished  from  those  of  the  amphibole-peridotite  except  in  that  they 
are  a  trifle  finer  grained.  Examined  under  the  microscope,  however,  we 
find  difl^erences.  In  some  the  hornblende  is  very  much  reduced  in  quantity, 
and  varies  from  the  brown  kind  just  described  to  a  light-greenish  color,  the 
two  being  in  optical  continuity,  and  the  augite  and  olivine  are  increased  in 
quantity.  These  are  good  types  of  a  wehrlite.  In  some  of  the  wehrlites 
there  is  a  variable  percentage  of  feldspar.  In  certain  cases  it  reaches  an 
amount  which  would  almost  warrant  the  classing  of  the  rock  as  an  olivine  - 
gabbro.  Patton  described  a  rock  from  the  same  outcrop  in  which  the  horn- 
blende still  predominated,  but  in  which  there  was  also  a  certain  amount  of 
plagioclase.^  He  called  it  a  hornblende-picrite.^  According  to  the  ter- 
minology here  used,  if  the  plagioclase  is  to  be  neglected,  it  would  be  au 
amphibole-peridotite. 

The  thin  sections  of  the  feldspathic  phase  of  this  rock  seem  to  show 
that  it  approaches  more  closely  to  a  gabbro — that  is,  to  be  more  feld- 
spathic than  the  one  described  by  Patton.  They  certainly  contain  far  less 
hornblende  than  his,  judging  from  his  description,  and  more  feldspar.     The 


'  Mikroscopisctie  Physiographie,  by  H.  Roseiibiisch ;   3a  ed.,  Stuttgart,  Vol.  II,  1896,  p.  352. 
-Op.  tit.,  p.  186. 


PERIDOTITE   INTKUSIVES.  255 

constituents  ai'c  tlir  same  in  tlie  two  rocks,  and  with  some  few  modifications 
Ins  description  would  answer. 

Aiigite  is  the  chief  constituent,  and  following  it,  in  order  of  im])ortance, 
come  olivine,  hornblende,  biotite,  and  feldspar.  The  diallagic  augite  is  more 
automorphic  (see  fig.  B,  PI.  XLV)  as  the  feldspar  increases  in  quantity. 
It  is  the  only  one  of  the  minerals  which  shows  any  marked  degree  of 
autt)morphisni.  The  augite  present  in  the  sections  which  I  have  studied  has 
a  light-brownish  color,  differing  from  that  described  by  Patton,  which  is 
green  to  colorless.  The  augite  contains  the  inclusions  occurring  in  hyper- 
sthene,  as  well  as  the  green  (hornblende?)  ones  already  described.  It  is 
invariably  surrounded  by  a  narrow  rim  of  light-brown  hornblende,  and 
includes  in  places  on  the  edges  irregular  patches  of  the  same  brownish 
hornblende. 

J.  Romberg  describes  the  augite  in  Argentinian  gabbros,^  both  with 
and  without  olivine,  as  being  almost  always  surrounded  by  a  rim  of  green 
hornblende.  In  one  case,  however — that  of  the  olivine  gabbro  from  the 
island  of  Martin  Gai'cia,  in  the  La  Plata  River^ — both  brown  and  green 
hornblende  is  present  around  the  augite.  The  brown  hornblende  forms 
part  of  the  periphery  of  a  crystal;  the  green  the  remaining  portion.  Of 
the  green  hornblende  some  is  fibrous,  and  is  considered  by  Romberg  to  be 
certainly  secondary. 

The  olivine  possesses  its  usual  properties.  It  is  in  annedra,  with  the 
exception  of  three  or  four  individuals,  which  show  a  fair  degree  of  auto- 
morphism. The  olivine  includes  rounded  grains  of  a  brown  spinel,  and  is 
traversed  by  anastomosing  veins  of  the  iron  oxide.  It  shows  the  usual  alter- 
ation to  serpentine,  and  the  iron  oxide  is  the  result  of  this  serpentinization. 
The  olivine  is  of  exceptional  interest  on  account  of  the  fact  that  it  is  sur- 
rounded by  certain  zones  Avhere  it  is  close  to  the  feldspar  (figs.  A  and  B, 
PI.  XLVI).  The  characters  of  zones  observed  in  sections  from  this  same 
locality,  and  which  are  almost,  if  not  quite,  identical  with  these  which  I 
shall  proceed  to  describe,  have  already  been  described  by  Patton.'  There 
are  two  of  these  zones.  An  inner  one  is  composed  of  a  mineral  which  is 
probably  an  orthorhombic  pyroxene.     It  was  so  determined  by  Patton  in 

'  Untersuchungen  an  Diorit-Gabliro-und  Amphibolitgesteiuen  aus  dem  Gebiete  der  Argentini- 
Bcaen  Republik,  by  J.  Romberg:  Neues  Jabrbuch  fiir  Miueral.,  BB.  IX,  1894,  pp.  320-321. 
-Op.  cit.,  p.  322.  »0p.  cit.,  p.  168. 


256  THE  CEYSTAL  FALLS  IRON-BEARING  DISTRICT. 

the  specimens  collected  and  studied  by  him.  I  can  obtain  no  positive  proof 
for  or  against  this  statement.  If  it  is  an  orthorhombic  pyroxene,  it  agrees 
with  the  inner  zones  of  related  occurrences  wliich  have  been  described  by 
Tomebohm,  G.  H.  Williams,  Adams,^  Romberg,^  and  others.  This  zone  is 
at  any  rate  composed  of  a  colorless,  compact  mineral,  with  high  single  and 
moderately  high  double  refraction.  Its  single  refraction  is  nearly  equal 
to  that  of  olivine.  The  mineral,  as  a  rule,  extinguishes  parallel  to  the  lines 
of  cleavage.  In  a  few  instances  the  line  of  extinction  made  a  scarcely 
noticeable  angle  with  the  cleavage.  It  is  separated  from  the  olivine  by  a 
shai'p  line.  At  times  this  inner  zone  seems  to  disappear,  and  at  others 
becomes  considerably  broader  than  the  average.  The  width  is  usually 
about  0.02  mm.,  thoug-h  it  becomes  at  times  0.08  mm. 

Outside  of  this  pyroxene  zone  there  is  a  very  much  broader  zone  of 
light-green  hornblende.  This  is  compact,  and  is  in  optical  continuity  with 
the  ordinary  brown  hornblende,  which  is  the  dominant  hornblende  in  the 
rock.  This,  in  its  compact  nature  and  in  its  relation  to  the  compact  brown 
hornblende  of  the  rest  of  the  slide,  differs  from  the  short  fibrous  actinolite 
zone  ordinarily  described  as  taking  part  in  such  "  reaction  rims."  This 
hornblende  zone  reaches  an  extreme  width  of  0.15  mm.  The  outer  edge  of 
this  zone  is  penetrated  by  tubular  ramifying  growths  of  a  colorless  mineral, 
which  usually  extend  inward,  perpendicular  to  the  periphery,  and  which 
appear  to  be  continuous  with  the  feldspar.  This  portion  of  the  hornblende 
rim  is  about  0.05  mm.  wide  No  such  intergrowth  of  feldspar  with  the 
brown  hornblende  was  found,  nor  have  I  been  able  to  find  elsewhere  any 
description  of  such  an  outside  zone.^  However,  Romberg  describes  the 
iutei'esting  occurrence  in  an  olivine-gabbro  from  the  Argentine  Republic 
of  zones  around  the  hornblende  which  are  very  much  like  those  above 
described,  except  that  the  pseudopodia-like  growths,  as  he  describes  them, 
consist  of  a  dark-green  spinel  instead  of  a  clear  white  feldspar,  as  in  the 
Michigan  rock. 

In  some  cases,  where  the  olivine  and  augite  are  in  juxtaposition,  the 
inner  orthorhombic  pyroxene  zone  completely  surrounds  the  olivine.  The 
outer  hornblende  zone,  however,  surrounds  both  the  augite  and  the  olivine 

'  Uber  das  Norian  oder  Ober-Laurenti<an  vou  Canada,  by  F.  D.  Adams:  Neues,  Jahvbuch  fiir 
Mineral,  BB.  VIII,  1893,  p.  466,  where  refereuces  to  observations  made  previous  to  1893  may  be  found. 
-  Op.  cit.,  p.  322.  ^  Op.  cit.,  p.  323. 


PEKIDOTIl  H  INTRUSIVES.  257 

witli  its  ortliorlioinbic  pyroxene  zone.  Where  it.  is  in  contact  with  tlie 
an<j-ite  it  is  the  brown  variety  of  hornblende,  but  is  in  optical  continuity 
with  the  green,  which  is  the  kind  around  the  olivine  and  the  orthorhonibic 
pyroxene. 

Of  the  remaining  mineral  constituents  brown  hornblende  is  the  next 
one  in  importance.  It  has  in  it  patches  of  inclusions,  previously  described 
as  occurrino-  in  the  hornblende  of  these  ultrabasic  rocks.  It  includes  also 
the  augite  and  olivine.  This  brown  hornblende  is  comparativel}-  rarely 
found  in  large  plates,  but  usually  as  a  rim  of  varying  width  around  the 
augite  and  olivine,  as  already  described.  Where  it  occurs  in  large  plates  it 
is  in  that  part  of  the  section  which  is  free  from  feldspar,  and  more  closely 
resembles  the  araphibole-peridotite  phase. 

The  biotite  has  a  cream  to  light  yellowish-brown  color,  and  occurs  in 
in-egular  plates.  The  plagioclase  feldspar  is  in  irregular  broad  plates,  and 
forms  the  mesostasis.  The  feldspar  contains,  in  uot  verj-  large  quantity, 
small  microlites,  which  by  very  high  power  are  translucent  and  show  a 
greenish  tinge.     They  are  supposed  to  be  hornblende  microlites. 

PROCESS   OF   CRYSTALLIZATION. 

From  the  relations  described  as  existing  between  the  various  minerals 
it  seems  that  the  following  stages  may  be  outlined  in  the  progress  of  the 
consolidation  of  this  rock.  From  the  coarse  even-grained  character,  and 
from  the  fact  that  neither  a  fine-grained  groundmass  nor  glass  is  present, 
the  conclusion  seems  to  be  warranted  that  it  crystallized  under  high  pressure 
and  must  have,  of  course,  at  some  time  been  under  very  high  temperature 
also. 

The  augite  and  olivine  were  the  first  and  chief  silicate  constituents  to 
form,  and  crystallized  out  of  the  magma  at  apjjroximately  the  same  time. 
The  magma  soon  reached  a  condition  unfavorable  for  further  production  of 
olivine,  probably  on  account  of  increasing  acidity.  Immediately  around 
the  olivine  there  was  formed,  at  this  stage  for  a  short  while,  the  orthorhombic 
pyroxene.  The  monoclinic  p}  roxene  continued  to  grow  during  the  forma- 
tion of  this  orthorhombic  variety.  Finally,  however,  the  condition  was 
reached  when,  in  place  of  the  monoclinic  and  orthorhombic  pyi'oxenes,  the 
crystallization  of  hornblende  began. 

It  is  not  known  what  the  conditions  were  which  caused  the  formation 
MON  xxxvi 17 


258  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

of  the  hornblende  subsequent  to  and  in  such  intimate  association  with  the 
pyroxene  which  it  surrounds  in  zonal  growth.  An  explanation  of  such 
occurrences  has  been  attempted  by  Becke  in  a  recent  ai-ticle,  ^  in  which  the 
conclusion  is  reached  that  the  formation  of  the  hornblende  and  pyroxene 
depends  upon  changes  in  temperature  and  pressure.  His  explanation  is 
based  upon  the  facts  of  occurrence  of  pyroxene  and  hornblende  in  plutonic 
and  effusive  rocks,  and  also  upon  the  well-known  fact  that  under  high 
temperature  and  atmospheric  pressure  they  can  not  exist,  but  when  fused 
recrystallize  as  pyroxene;  and  in  addition  to  this,  upon  the  experimeiits  of 
Von  Chrustschoflf,^  who  has  obtained  hornblende  at  a  temperature  of  550  C. 
with  the  presence  of  water,  under  which  conditions  a  high  pressure  must  be 
developed.  However,  attention  should  be  called  to  the  fact  that  his  explana- 
tion does  not  take  into  account  other  important  factors  which  certainly 
influence  the  crystallization  of  minerals — for  example,  the  chemical  composi- 
tion of  the  magma  and  the  fusing  point  and  specific  gravity  of  the  minerals. 

Whatever  the  factors  are  which  determine  its  crystallization,  the  fact  is 
that  hornblende  began  to  crystfillize  from  this  peridotite  magma  in  the 
place  of  pyroxene. 

The  biotite  appears  to  have  been  formed  at  the  same  time  with  the 
hornblende.  The  production  of  these  two  minerals,  hornblende  and  biotite, 
then  continued  until  the  remaining  magma  had  reached  the  composition  of 
basic  feldspar,  which  then  crystallized  and  now  forms  the  mesostasis. 

A  zone  of  orthorhombic  pyroxene,  succeeded  by  one  of  hornblende, 
has  been  described  as  surrounding  the  olivine  in  this  peridotite.  The  term 
reaction  rim  has  been  applied  to  similar  zones  by  various  observers,  but  it 
seems  to  me  that  this  term  is  inapplicable  to  such  zones.  It  is  not  probable 
in  such  a  case  as  this  that  there  is  a  reaction  between  the  magma  and  the 
olivine.  Moreover,  the  zones  should  not  be  compared  to  the  resorption  rims 
found  so  commonly  in  certain  effusive  rocks,  where  from  the  fusion  of  the 
hornblende  crystals  pyroxene  has  been  produced. 

Such  a  zonal  growth  aroimd  the  olivine  seems  to  me  comparable  to 
such  a  case  as  that  described  by  Washington,^  where  colorless  diopside 


'  Gesteine  der  Colunibretes  ;  Anhang :  Einiges  iiber  die  Beziehung  von  Pyrosen  und  Amphibol 
in  Gesteinen,  by  F.  Becke:  Tschermaks  mineral.  Mittheil.,  Vol.  XVI,  1896,  pp.  327-336. 

=  Bull.  Acad.  imp.  sci.  St.-P^tersbourg,  1890,  p.  13.     Cf.  Becke,  Op.  cit.,  p.  337. 

'Italian  petrological  sketches;  4.  The  Eocca  Monfina  region,  by  H.  S.  Washington :  Jour.Geol., 
Vol.  V,  1897,  p.  254. 


PElilDOTlTE  INTEUSIVES. 


259 


pluniocrysts  are  sun-ouiulcd  1)a'  a  narrow  border  of  A-ellowish-o-reen  auo-ite 
wliicli  corresjionds  to  tlie  small  au<>-ites  in  the  g-roundmass,  or  to  those  cases 
which  are  so  roninion  in  plutonic  rocks — even  in  this  rock  described — where 
hornblende  is  found  surrounding-  the  pyroxene. 

A  general  explanation  which  would  account  for  tlic  successive  crys- 
tallization of  hornblende  and  pyroxene  in  this  rock  should  be  applicable  to 
such  a  zonal  g-rowth  as  occurs  around  tlie  olivine,  taking-  into  consideration, 
of  course,  the  jirobability  that  a  factor  of  slight  importance  in  the  one  case 
may  be  the  controlling  factor  in  the  other.  Such  occurrences  seem  clearly 
to  indicate  a  change  in  the  chemical  composition  of  the  mag-ma  as  the  chief 
factor  in  the  crystallization  of  the  diflPerent  minerals,  in  the  pressure,  in  the 
temperature,  and  also  in  other  factors,  either  one  alone  or  more  of  these 
combined. 


ANALYSI.S  Ol'    I'ERIDOTITE. 


The  peridotite  just  described  was  analyzed  by  Dr.  H.  N.  Stokes  of  the 
United  States  Geological  Survey,  and  his  results  are  here  given  (No.  1) : 

Analysis  of  peridotite. 


SiO;.. 
TiO,  . 

A1,0:,. 

Cr:0,. 
FejO, 
FeC. 
MnO  . 
NiC. 


CaO 

MgO 

KcO 

NajO...  

HjO  at  110^  . . . 
HjO  above  HO'-^ 

PiOs 

COi 

Total  


1  (23353). 


44.99 

.97 

.5.91 

.25 

3.42 

8.30 

Trace. 


8.79 

21.02 

.74 

.91 

.63 

3.19 

.05 

Trace  (f). 


2  (22981). 


37.36 

.79 

4.76 

.62 

6.61 

6.12 

Trace. 

.04 

1.19 

31.11 

Trace. 

.65 

10.37 

.06 

None. 


99.17 


99.68 


260  THE  CEYSTAL  FALLS  lEON-BEAEING  DISTEICT. 

It  will  be  seen  from  the  analvsis  that  the  silica  is  somewhat  too  hig-h 
for  the  typical  peridotites.  This  same  fact  is  also  emphasized  by  the 
teiidenc}'  manifested  in  some  facies  of  the  peridotite  for  feldspar  to  develop, 
and  thus  for  transitions  to  norite  and  g-abbro  to  be  produced. 

With  this  analysis  of  the  peridotite  there  is  placed  for  comparison  the 
analysis  (No.  2)  by  Dr.  H.  N.  Stokes  of  the  picrite-porphyry  already 
descril^ed.  The  close  resemblance  chemically  liecomes  at  once  manifest, 
althouiih  the  latter  is  more  nearly  a  typical  peridotite  in  composition.  It 
can  not  be  denied  that  possibly  this  picrite  is  but  a  further  differentiation 
product  of  the  same  magma  to  which  the  2:)eridotites  belong,  although  its 
occurrence  is  so  remote  from  these  that  it  is  impossible  to  connect  them  in 
the  field. 

PERIDOTITE   PROM   SBC.    22,   T.   42   N.,   E.   31   W.,   N.   1,990,   W.   150. 

Just  west  of  the  northeastern  corner  of  sec.  22,  T.  42  N.,  R.  31  W., 
there  is  a  bold  outcrop  of  hornblende  gabbro,  which  is  cut  by  a  dike,  about 
10  feet  wide,  of  a  very  massive,  coarse,  granular  black  peridotite.  Macro- 
scopically  one  can  readily  distinguish  in  the  peridotite  flakes  of  Ijiotite, 
poikilitic  plates  of  hornblende,  and  a  smaller  amount  of  white  feldspar. 
Under  the  microscope  the  constituents  are,  in  order  of  imjjortance:  Horn- 
blende, augite,  feldspar,  biotite,  bronzite,  olivine,  magnetite,  and  quartz.-' 

Hornblende. — Tlils  Is  thc  rlch  browu  kind,  full  of  inclusions,  g-rading  into 
the  green  variety  which  was  described  on  p.  234  as  occurring  in  the  gabbros 
of  this  district.  It  is  jiresent  in  anhedra  inclosing  liiotite,  pyroxene,  and 
olivine. 

Pyroxene. — Tliis  is  represented  by  mouoclinic  and  orthorhombic  varieties. 
The  mouoclinic  pyroxene,  augite,  is  most  abundant,  and  is  in  light-yellow 
to  pink-colored  anhedra,  except  where  it  touches  the  feldspar;  there  the 
augite  is  automorphic,  and  is  surrounded  by  a  narrow  border  of  light- 
brown  hornblende. 

The  orthorhombic  pyroxene  is  present  in  a  few  anhedra,  which  are 
colorless  or  have  a  faint  cream  tint.     It  is  presvimed  to  be  bronzite. 

Feldspar. — This  fills  tlic  intcrspaccs  between  the  other  constituents,  and 
occurs  in  grains  which  are  polysyntheticalh'  twinned  after  the  albite  law. 

'  Only  one  section  has  been  prepared  from  this  specimen,  and  it  may  not  give  a  correct  idea  of 
the  true  proportion  of  these  minerals  in  thr  rock  mass.  In  tlie  macroscopical  examination  of  the 
hand  .specimen  the  biotite  seemed  to  he  subordinate  only  to  the  hornblende. 


PERIDOTITK  INTKUSIVES.  261 

Mfiisurements  ji-avo  a,  syninietrical  t'xtiiiction  of  32  eaeli  side  of  the  twiniiiii"- 
plane  on  zoiie_L01(».      I  tlierefo.- ;  eonclude  tlie  feldspar  to  be  labradorite. 

Biotite. — This  is  the  ordinary  yellow  to  brownish  kind,  and  is  in  irre"'n- 
lar  plates.      It  shows  its  usnal  characters  and  is  inclnded  in  the  hornblende. 

Magnetite. — This  mineral  occnrs  in  (;rystals  and  gTains,  inclnded  in  all 
the  other  constitnents. 

Quartz. — A  few  grains  of  (jnartz  were  fonnd  associated  with  the  feldspar. 
The  presence  of  dihexahedral  liquid  inclusions  easily  gave  a  clew  to  the 
orientation  of  the  grains. 

The  rock  composed  of  the  above-described  nainerals  offers  a  good  illus- 
tration of  that  gradation  which  is  one  of  the  fundamental  laws  of  nature 
and  is  nowhere  better  exemplified  than  in  the  rocks.  On  the  one  hand, 
from  its  texture  and  from  the  presence  of  the  dominant  hornblende,  with  the 
small  quantity  of  quartz,  this  rock  may  perhaps  be  considered  to  be  closeh' 
related  to  the  diorites.  On  the  other  hand,  the  presence  of  the  pyroxene 
and  olivine  seems  to  point  toward  its  connection  with  a  gabbro. 

Its  geological  occurrence  points  most  satisfactorily  toward  its  corre- 
spondence in  age  and  its  intimate  relationship  to  the  peridotites  of  the  dis- 
trict. The  predominance  of  the  bisilicates  indicates  it  to  be  of  very  basic 
character,  and  for  these  reasons  I  have  called  it  "  peridotite,"  althouo-h  I 
have  not  succeeded  in  getting  an  analysis  to  prove  its  ultrabasic  nature. 

RELATIONS  OF  PERIDOTITES  TO  OTHER  ROCKS. 

The  peridotites  occur  in  such  small  quantity  that  general  conclusions 
concerning  their  relations  to  other  rocks  occurring  in  their  vicinity  are 
scarcely  warranted."  However,  from  the  fact  that  they  are  so  intimately 
associated  with  the  gabbro — cutting  it  in  two  cases  where  the  contact  ^\'as 
observed — and  from  the  fact  that  among  the  peridotites  themselves  certain 
phases  approach  in  mineralogical  composition  certain  of  the  gabbros  (see 
p.  254),  it  seems  advisable  to  conclude  that  they  represent  ultrabasic  differ- 
entiation products  of  the  same  magma  from  which  the  gabbro  types  were 
derived.  The  inappreciable  differences  in  grain  between  the  portion  of  the 
i-ock  nearest  the  contact  between  these  basic  rocks  and  the  gabbros  and 
those  farther  away  can  be  explained  by  supposing  their  intrusion  to  have 
taken  place  while  the  main  mass  of  the  gabbro  retained  considerable  heat 
and  thus  prevented  their  rapid  cooling. 


262  THE  CRYSTAL  PALLS  lEON-BEARING  DISTRICT. 

AGE    OF    PERIDOTITES. 

The  only  statement  which  can  be  made  concerning  the  age  of  the  peri- 
dotite  dikes  is  that  they  are  younger  than  some  of  the  gabbros,  and  that, 
not  having  suffered  the  defonnation  of  the  pre-Keweeuawan  orogenic  move- 
ments, they  are  Keweenawan  or  post-Keweenawan. 

GEKERAL   OBSERVATIOXS   ON  THE  ABOVE   SERIES. 

TEXTURAL  CHARACTERS    OF    THE    SERIES. 

There  are  represented  in  the  above  series  rocks  with  moderately  fine 
grain  as  well  as  those  of  very  coarse  grain.  They  vary  from  those  with 
parallel  texture,  through  those  vith  porphyritic,  poikilitic,  and  ophitic  texture, 
to  those  with  granular  texture.  There  is,  howevei",  tlu-oughout  a  clear 
preponderance  of  the  medium  to  coarse  granular  rocks.  The  rocks  are 
evidently  not  of  effusive  character,  though  some  jDOSsess  the  textures  prev- 
alent in  effusive  rocks. 

The  order  of  crystallization  of  the  minerals  in  the  rocks  of  granular 
.exture,  excluding  the  iron  ores  and  the  accessory  minerals,  is  as  follows. 
Tlie  order  in  the  imperfectly  ophitic  and  porphyritic  rocks  is  not  considered, 
as  those  are  rather  exceptional  occurrences. 

In  the  lists  those  minerals  are  hyphenated  of  which  it  has  not  been 
possible  to  determine  accurately  the  order  of  crystallization.  It  seems  that 
either  they  were  formed  at  the  same  time  or,  in  some  cases,  their  formation 
has  overlapped.  In  such  cases  the  one  placed  first  is  the  one  presumed  to 
have  begun  its  crystallization  first. 

DioRiTE.  Gabbroand  horn-        Bronzitb-noritb,  Pkridotite. 

BLKNDE-GABHRO. 

Hornblende.  Olivine.  Bronzite.  Olivine. 

Biotite.  Monoclinic  pjToxene.  Monoclinic  pyroxene.  Orthorhombic  pyroxene. 

Plagioclase.  Biotite-hornblende.  Blotite-bornblende.  Monoclinic  pyroxene. 

Microcline.  Plagioclase.  Plagioclase.  Biotite-hornblende. 

Orthoclase-nuartz.  Plagioclase. 

For  the  entire  series  the  order  may  be  arranged  as  follows:  Olivine, 
bronzite,  monoclinic  pyroxene,  mica-hornblende,  plagioclase,  orthoclase, 
quartz.  This  is  the  same  order  that  is  exhibited  by  the  most  basic  rock 
represented  in  the  series,  the  peridotite,  so  far  as  this  rock  contains  the 
minerals. 


SERIES  OF  INTKUSIVES. 


263 


The  order  of"  crystallization  of  the  minerals  throughout  the  series  is  due 
to  their  relative  solubility  in  the  eruptive  raagnui.  Among-  various  factors 
affecting  solubility  the  fusion  point  of  the  chemical  compounds  constituting 
the  different  minerals,  the  temperature  of  the  magmas,  and  the  pressure 
under  which  the  minerals  crystallized,  are  important.  The  porphyritic  and 
the  ophitic  textured  rock  facies,  having  crystallized  under  different  condi- 
tions of  pressure  and  of  temperature  from  those  under  which  the  granular 
rocks  were  formed,  show,  as  is  to  be  expected,  a  different  order  of  crystal- 
lization of  minerals. 

CHEMICAL   COMPOSITION    OF    THE    SERIES. 

In  the  following  tables  there  are  reproduced  the  analyses  which  have 
been  obtained  of  the  various  types.  They  are  arranged  according  to  dimin- 
ishing acidity.  Nos.  1  and  4  were  analyzed  by  Dr.  H.  N.  Stokes,  Nos.  2 
and  3  by  Mr.  George  Steiger,  both  of  the  United  Slates  Geological  Survey : 

Table  I. — Analyses  of  Crystal  Falls  rocks. 


SiOj  

TiOj 

Al,03  

CrjOa 

Fe^Os 

FeO 

MnO 

NiO 

CaO 

MgO 

K2O 

NaaO 

HjOatllOo  .. 
HjO  above  110' 

P2O5 

CO, 

Total... 


1  (26023). 


58.51 

.72 

16.  .32 

None. 

2.11 

4.43 

Trace. 

None. 

3.92 

3.73 

4.08 

3.11 

.23 

2.00 

.30 

None. 


2  (23354). 


3  (23755). 


49.80 

.79 
19.96 


6.32 
.49 


99.46 


11.33 

7.05 

.61 

2.22 

100°—  .  13 

1000+1. 71 

.07 

.15 


100. 63 


48.23 

1.00 

18.26 


1.26 
6.10 


9.39 
10.84 
.73 
1.34 
100°—  .26 
100° +2. 00 
.07 
.43 


99.91 


4  (23353). 


44.99 

.97 

5.91 

.25 

3.42 

8.30 

Trace. 

None. 

8.79 

21.02 

.74 

.91 

110°      .  63 

110°-j-3. 19 

.05 

Trace.  ( ?) 


99.17 


(1)  Mica-diorite  (quartzitic);  (2)  Hornblende-gabbro ;  (3)  Norite;  (4)  Peridotite  (wehrlite). 


264 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

Table  II. — I'ercentaijes  of  chief  oxides  reduced  to  100. 


1. 

2. 

3. 

4. 

SiO.            

60.36 
.75 

16.83 
2.17 
4.57 
4.04 
3.85 
4.21 
3.21 

50.52 

.80 

20. 25 

6.41 

.50 

11.50 

7.15 

.62 

2.25 

49.64 
1.03 

18.79 
1.30 
6.28 
9.67 

11.16 

.75 

1.38 

47.33 

1.02 

6.22 

3.60 

8.73 

9.25 

22.11 

.78 

.96 

TiO-                       

Al,Oi              

FeO,                         

FeO                      

CaO                      

UsO                      

K,0                          

Na.O 

Table  III. — Atomic  2}''oportions  of  metals. 


Si.. 
Ti  . 
AI  . 
Fe. 
Cii. 
Mg. 
K.. 
Na- 


55.85 

46.53 

.53 

.56 

18.41 

22.03 

5.08 

.834 

4.04 

11.42 

5.32 

9.85 

4.99 

.74 

5.78 

4.04 

45.27 

.71 

29.26 

5.70 

9.51 

15.22 

.88 

2.45 


42.48 

.70 

6.60 

9.02 

8.98 

29.67 

.91 

1.67 


The  analyses  show  that  all  of  the  rocks  contain  a  moderately  large 
amount  of  water.  Nevertheless,'  they  are  sufficiently  well  preserved  to 
warrant  a  discussion  of  their  analyses  for  classification  purposes.  This  is 
especially  true  of  No.  4,  which  is  remarkably  fresh  for  so  basic  a  rock. 

The  chief  rock-makiiio-  oxides  in  the  above  analyses  appear  in  Table 
II  reduced  to  100.  The  molecular  proportion  of  these  oxides  was  then 
obtained.  From  these  data  the  atomic  proportions  of  the  metals  were 
derived,  and  are  given  in  Table  III.  These  calculations  were  kindly  made 
for  me  by  Mr.  V.  H.  Bassett,  assistant  in  the  chemical  laboratory  of  the 
University  of  Wisconsin. 

If  we  examine  Table  II  we  see  that,  in  passing  from  the  more  acid  to 
the  basic  end  of  the  series,  in  correspondence  with  tliis  decrease  in  silica 
the  alumina  increases  rapidly,  then  decreases  until  it  reaches  the  extreme 
basic  rock,  when  it  cbops  suddenly  to  6.22  per  cent.  The  analyses  also 
show  an  increase  in  iron,  which  is  best  brought  out  in  Table  III.  The 
alkalies  decrease  with  diminishing  silica,  whereas  the  MgO,  which  for 
rocks  of  this  character  is  very  characteristic,  shows  a  decided  increase. 
Within  the  gabbro-norite-peridotite  series  (Nos.  2,  3,  and  4)  the  lime  shows 


SERIES  OF  INTKUSIVES.  265 

a  constant  diniinution  f()iTes))()ndin<i'  to  tlie  incTeasinj^'  nia<inesi;ui  cliaracter 
of  tla^  rocks.     The  potash  increases  as  the  soda  sliows  a  decrease. 

The  rocks  represented  l)y  tlie  analyses  are  believed  to  belong  to  a 
series  ranging-  troiu  a  diorite  on  the  one  liand,  through  horublende-gabbro 
and  norite,  to  peridotite  on  the  other.  It  should  be  borne  in  mind  that  the 
diorite  is  somewhat  excei)tioual,  representing  a  gradation  toward  the  ortho- 
clase  rocks.  On  the  acid  side  of  the  series  the  microscope  also  shows 
variations  to  tonalitic  and  even  granitic  rocks  very  rich  in  quartz  and  ortho- 
clase,  consequently  much  more  acid  iu  character  than  the  diorite  repre- 
sented in  the  analysis. 

It  is  a  difficult  matter  to  estimate  quantitatively  the  amount  of  the  one 
or  tlie  other  kind  of  rock  present  in  the  Crystal  Falls  district.  We  are  thus 
prevented  from  drawing  from  the  predominance  of  the  one  kind  or  the 
other  the  conclusion  that  those  represented  in  the  minority  are  the  results 
of  the  differentiation  of  a  magma  most  nearly  resembling  in  its  original 
constitution  that  which  predominates.  Moreover,  since  the  analyzed  rock 
types  were  not  selected  as  representatives  of  the  extremes  of  the  process  of 
differentiation,  it  would  not  be  wise  to  endeavor  to  give  the  mean  composi- 
tion of  the  parent  magma  from  the  analyses  of  the  differentiation  products 
Avhich  have  been  presented.  The  main  thesis,  however,  is  estaljlished  that 
the  separation  of  a  magma  into  the  various  products  described  has  taken 
place,  as  is  indicated  by  the  relations  in  the  field,  and  as  has  been  shown  by 
the  microscopical  and  chemical  analyses. 

RELATIVE  AGES  OF  ROCKS  OF  THE  SERIES. 

Study  of  the  relative  periods  of  eruption  of  the  various  rocks  results  in  the 
determination  of  the  hornblende-gabbro  as  the  rock  which  first  reached  its 
present  position.  It  was  followed  in  the  acid  part  of  the  series  by  the  diorite, 
which  in  one  place  cuts  it.     The  diorite  is  cut  by  the  diorite-porphyry. 

Along  the  basic  series  the  order  has  been  determined  as  hornblende- 
gabbro,  gabbro,  bronzite-norite,  peridotite. 

In  general  the  forces  of  differentiation  seem  to  liave  been  active  in  two 
directions,  tending  toward  increasing  acidity  and  increasing  basicity  of  the 
products  of  differentiation,  thus  agreeing  with  the  law  of  succession  of 
igneous  rocks  as  propounded  by  Iddings.^ 

■  The  ongiu  of  igueous  rocks,  by  J.  P.  Iddiugs:  Bull.  Philos.  .Soc.  Wash.,  Vol.  XII,  1892,  p.  195. 


PLATE   XIX. 


267 


PLATE     XIX. 

Fig.  .i. 

(Sp.  No.  32756.     Without  analyzer,  x90.) 

Photomicrograph  of  fractured  quartz  phenocryst  from  a  rhyolite-porphyry.  It  includes  num- 
berless liquid  inclusions,  which  diminish  in  quantity  as  the  distance  from  the  plane  of  fracture  is 
increased,  thus  indicating  their  close  connection  with  the  fracturing  of  the  cfuartz.  The  fracture  in 
the  quartz  phenocryst  continued  into  the  grouudmass,  as  may  be  seen  on  the  left-hand  side  of  the 
figure.     It  has  been  healed  with  secondary  quartz.     (Described,  p.  82.) 

Fig.  B. 

(Sp.  No.  32914.     With  analyzer,  x47.) 

Photomicrograph  of  a  section  of  rhyolite-porphyry,  designed  to  show  the  ihombohedral  part- 
ing, which  is  very  common  in  many  of  the  quartz  phenocrysts.     (Described,  p.  82.) 

268 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PL.   XIX 


(A-)    INCLUSIONS   IN    A   FRACTURED   QUARTZ    PHENOCRYST. 
(B-)    RHOMBOHEDRAL   PARTING   IN    A   QUARTZ   PHENOCRYST. 


The     MEftlDEM     GRAVURE    CO. 


PLATE  XX. 


269 


PLATE    XX. 
Fig.  a. 

(Sp.  No.  32119.     With  aualyzer,  x  90.) 

Micropoikilitic  rhyolite-porpbyiy,  showing  the  peculiar  texture  of  the  zones  which  invariably 
surround  the  quartz  phenocrysts  in  sections  in  which  the  texture  occurs.  The  same  texture  prevails 
in  the  groundmass.  The  irregular  white  areas  which  are  continuous  with  the  quartz  phenocrysts  and 
are  connected  with  each  other  represent  quartz.  Disconnected  dark  and  light  areas  between  the 
quartz  stringers  are  feldspar  grains.  These  do  not  possess  uniform  orientation;  hence  the  texture  is 
not  micropegmatitic.     (Described,  p.  84.) 

Fig.  R. 

(Sp.  No.  32137.     With  aualyzer,  x  90.) 

Photomicrograph  of  micropoikilitic  rhyolite-porphyry.  In  this  rhyolite-porphyry  the  micropo- 
ikilitic texture  is  much  finer  than  that  represented  in  Fig.  J,  and  the  quartz  in  the  zones  shows  a 
tendency  toward  spherulitic  development.  Owing  to  the  extreme  fineness  of  grain  it  is  difficult  to 
distinguish  the  (juartz  and  feldspar  in  many  cases.  The  greater  part  of  the  light  areas  shown  in  the 
photomicrograph  are  quartz.  The  dark  areas  between  the  quartz,  and  also  some  of  the  lighter  areas, 
represent  irregular  pieces  of  feldspar.     (Described,  p.  84.) 

270 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH  XXXVI     PL.    XX 


MICROPOIKILITIC   RHYOLITE-PORPHYRY. 


THE    MERIDEN     GRAVURE    CO. 


PLATE   XXI. 


271 


PLATE    XXI. 

Fig.  a. 

(Sp.  No.  32136.     Without  analyzer,  x  90.) 

Rhyolito-poriihyry  with  aureoled  phenocrysts.  The  finest-grained  type  of  micropoikilitic 
testiuf  is  here  represented.  Tbe  grouudmass  of  this  porphyry  consists  of  rounded  areas  of  material 
("quartz  ^pongeuse"),  corresponding  to  that  forming  the  zones  around  the  phenocrysts.  Between 
these  areas  there  may  be  found  in  places  small  feldspars.  These  photomicrographs,  represented  in 
figs.  .-(  and  B,  PI.  XX,  and  in  this  figure,  show  every  gradation  iu  the  micropoikilitic  texture,  from 
that  which  is  with  difficulty  distinguishable  as  such  to  the  coarser-graiued  unmistakable  variety. 
(Described,  p.  85.) 

Fig.  B. 

(Sp.  No.  32136.     With  analyzer,  x  90.) 

Rhyolite-porphyry  with  aureoled  phenocrysts.  This  is  the  same  section  as  is  represented  above 
■when  viewed  Itetweeu  crossed  nichols.  The  texture  of  the  groundmass  is  brought  out  somewhat 
better.  The  feldspars  especially  become  more  noticeable.  For  instance,  one  Carlsbad  twin  may  be 
seen  at  the  lower  right-hand  corner  of  the  phenocryst  partly  indenting  the  aureole.  Other  feldspars 
may  be  noticed  through  the  groundmass.  In  other  portions  of  the  section  from  which  this  photo- 
micrograph is  taken  the  (luartz  phenocrysts  have  no  aureoles  and  the  groundmass  possesses  an  imper- 
fect microgranitic  texture.  This  figure  brings  out  clearly  the  gradation  toward  that  texture. 
■(Described,  p.  85.) 

272 


U.S.   GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PL.    XXI 


{B) 


MICROPOIKILITIC    RHYOLITE-PORPHYRY. 


■ME     HERIDEN    GHAVURE     CO. 


PLATE  XXII 


MON  XXXVI 18  273 


PLATE    XXII. 

Fig.  a. 

(Sp.  No.  32732.     Without  analyzer,  x  18.) 

Aporbyolite  showing  beautifully  developed  peilitic  parting.  The  perlitic  cracks  are  brought 
out  clearly  by  the  chlorite  which  has  accumulated  in  them.     (Described,  p.  87.) 

Fig.  B. 

(.Sp.  No.  32732.     With  analyzer,  x  18.) 

Aporhyolite  showing  perlitic  parting,  when  viewed  between  crossed  nicols.  The  groundmaes 
lesolves  itself  into  a  fine  grained  mosaic  of  quartz  and  feldspar,  showing  microgranitic  characters. 
The  perlitic  parting  is  thereby  almost  completely  obscured.     (Described,  p.  87.) 

274 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PL.    XXII 


i^'srv- 


{^) 


(S) 


(.A)    PERLITIC    PARTING    IN    APORHYOLITE. 

(B-)    PERLITIC    PARTING    IN    APORHYOLITE    BETWEEN    CROSSED    NICOLS. 


The     MERinEN     GRAVURE    CO. 


PLATE  XXIII. 


275 


PLATE   XXIII. 

Fi(j.  A. 

(Sp.  No.  22953.     Without  analyzer,  x  38.) 

Rhyolite-porphyry  rcntleied  schistose  liy  crushing.  Granulation  of  the  feldspars  ami  the  result- 
ing production  of  schistose  aggregate's  of  secondary  quartz,  feldspar,  and  sericite  is  here  shown. 
The  two  large  areas  shown  near  the  center  of  the  figure  were  formerly  occupied  entirely  by  fcUlspar. 
The  greater  portion  of  this  has  now  become  altered,  mere  remnants  of  the  original  remaining.  This 
secondary  aggregate  has  especially  well-developed  parallelism.     (Described,  p.  9.S.) 

Pig.  B. 

(Sp.  No.  327-26.     With  analyzer,  s  18.) 

Photomicrograph  of  aporbyolite-porphyry  breccia  showing  the  fractured  character  of  the 
quartz  and  feldspar.  Certain  portions  of  the  section  show  the  perlitic  parting,  with  accumulations 
in  these  areas  of  chlorite.     (Described,  p.  93.) 

276 


U.  S.    GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PL.    XXIII 


CA^    SCHISTOSE   RHYOLITE   PORPHYRY. 
<B)    APORHYOLITE   BRECCIA. 


THE     MPRDE.-*    GRAVUftE     CO. 


PLATE  XXIT. 


27V 


platp:  XXIV. 

Fig.  a. 

(Sp.  No.  22968.     Without  analyzer,  x  18.) 

Schistdse  rbyolite-porphyry  with  well-developed  ilowage  structure.  A  feldspar  phenocvyst 
•which  has  been  more  or  less  rounded  by  crusliing,  occupies  the  center  of  the  figure.  There  is  also 
shown  iu  the  upper  left-hand  quadrant  of  the  figure  a  small  crushed  quartz  phenocryst.  (Described, 
p.  93.) 

Fig.  B. 

(Sp.  No.  22968.     With  analyzer,  x  18.) 

When  the  section  represented  iu  fig.  A  is  viewed  between  crossed  iiicols,  the  crushed  character 
of  the  feldspar  phenocrysts  is  therel)y  well  brought  out.  The  minutely  granular  character  of  the 
grouudmass  is  also  well  shown.     (Described,  p.  93.) 

278 


U.S.   GEOLOGICAL   SURVEY 


MONOGRAPH   XXXVI     PL.    XXIV 


::^is^ 


:**-'^ 


(B) 


(/»)    SCHISTOSE   RHYOLITE    PORPHYRY. 

(S)    SCHISTOSE    RHYOLITE    PORPHYRY    BETWEEN    CROSSED    NICOLS. 


TH=     MFRIDEN    QHAVUtiE     CO. 


PLATE   XXV. 


279 


PLATE     XXV. 
Fig.  a. 

(Sp.  No.  32116.) 

Photograph,  with  very  slight  enlargement,  of  the  polished  surface  of  a  Tery  fine  grained  but 
very  amygdaloidal  basalt.  The  amygdules  are  of  irregular  shape,  but  iu  general  with  .a  rounded 
or  tubular  character.  The  original  cavities  have  been  filled  vfith  chlorite  and  quartz.  The  chlo- 
ritic  amygdules  are  the  most  common.  A  few  of  the  white  quartz  amygdules  may  be  seen  on  the 
left-hand  side  of  the  figure.  It  should  be  noted  that  owiug  to  the  softness  of  the  chlorite  some  of  the 
amygdules  have  become  impregnated  with  the  powder  used  iu  pnli.shing  the  specimen.  This  could 
not  be  removed,  and  in  many  cases  may  be  seen  filling  as  well  as  outlining  the  chlorite  amygdules. 
(Described,  p.  95.) 

Fig.  B. 

(Sp.  No.  32903.     Without  analyzer,  x  18.) 

Photomicrograph  of  a  section  of  the  fine  grained,  possibly  vitreous,  amygdaloidal  basalt  repre- 
sented in  fig.  A  of  PI.  XXVII.  The  amygdules  consist  of  chlorite,  quartz,  and  feldspar.  The  greatest 
interest  centers  in  the  groundmass.  This  consists  of  a  fine  felt  of  ehlorite  with  minute  epidote  grains. 
Traversing  this,  one  sees  in  places  delicate  flowage  lines.  This  is  believed  to  represent  a  once  vitreous 
basalt,     t Described,  p.  102.) 

280 


U,    S.   GEOLOGICAL   SURVEY 


MONOGRAPH   XXXVI     PL.    XXV 


(A)  AMYCDALOIDAL    BASALT. 

(B)  AMYCDALOIDAL   BASALT. 


THE     MERIOEN     GHAVURE     CO. 


PLATE   XXVI. 


281 


PLATE    XXVI. 

FiCr.     A. 

(Sp.  No.  32541.     Without  analyzer,  x  IS.) 

Fine-grained  amygdaloidul  basalt.  The  only  recognizable  original  constituent  in  the  ground- 
mass  is  the  feldspar  in  microlites  which  luo.st  commonly  fringe  out  at  the  ends.  They  are  not  infre- 
quently arranged  in  sheaf-lilie  aggregates.  These  are  best  seen  with  high-power  objectives.  The 
major  portion  of  the  groundmass  consists  of  a  fine  felt  of  chlorite,  with  minute  grains  of  epidote.  It 
is  considered  to  have  resulted  from  the  alteration  of  a  vitreous  base.  The  amygdules  consist  of  calcite. 
(Described,  p.  99.) 

Fig.  B. 

(Sp.  No.  32541.     Without  analyzer,  x  35.) 

Portion  of  section  from  which  fig.  A  was  taken  viewed  with  a  higli  power.  In  this  the  sheaf-lilco 
aggregates  of  feldspar  can  be  seen.     (Described,  p.  99.) 

282 


U.S.   QeOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PART  I     PL.    XXVI 


M) 


(B) 


(/»    AMYCDALOIDAL   BASALT. 

(S)    SHEAF-LIKE    FELDSPAR   AGGREGATES. 


THE     MERIDEN    ORAVURE    CO. 


PLATE   XXYII. 


283 


PLATE    XXYII. 

Fig,  a. 

(Sp.  No.  32903.     Natural  size.) 

Reproduction  of  a  very  fine  grained,  possibly  vitreous,  .imydaloidal  basalt.  The  amygdaloidal 
cavities  show  very  little  contortion.  The  amygdules  cousist  of  white  quartz,  piuk  feldspar,  and 
dark-green  chlorite.     Compare  this  figure  with  photomicrograph  fig.  B,  PI.  XXV.     (Described,  p.  102.) 

Fig.  73. 

(Sp.  Xo.  32910.     Natural  size.) 

Colored  reproduction  of  the  pseudoainygdaloidal  phase  of  the  siderite-quartz  matrix  which 
occurs  iu  places  between  the  ellipsoids.  The  original  matrix  was  first  replaced  in  the  zone  of  weath- 
erinf  by  siderite.  Deep  burial  of  the  rock  resulted  iu  the  mashing  of  the  siderite  and  the  subsecjueut 
replacement  of  a  great  portion  of  it  by  silica,  leaving  a  few  oval  areas  uusilicifled.  Brought  into  the 
zone  of  weathering  again  by  erosion,  these  siderite  areas  are  removed  on  the  weathered  surface 
giving  a  scoriaceous  appearance  to  it,  as  may  be  seen  on  the  figure.     (Described,  p.  135.) 

Fig.  C. 

(Sp.  No.  33507.     Natural  size.) 

Fine-grained  volcanic  clastic.  The  water-dejiosited  character  of  this  specimen  is  unquestion- 
able. It  can  be  traced  in  the  field  down  into  a  coarse  bowlder  conglomerate.  Tlie  fragmental  nature 
can  only  be  seen  under  the  microscope  in  the  coarser-grained  portions.  The  finer-grained  material 
represents  apparently  the  excessively  fine-grained  mud  derived  from  the  trituration  of  the  coarser 
fragments.  The  eolian-deposited  sands  and  dust  have  essentially  the  same  appearance  as  the  specimen 
here  represented.  The  microscope,  however,  shows  the  very  angular  character  of  the  fragments  in 
the  coarser-grained  portions.  The  very  fine  eolian-deposited  dust  can  not  be  distinguished  from  that 
which  has  been  deposited  through  water.  It  is  sometimes  very  difficult  to  determine  the  nature  of 
rocks  of  this  fine-grained  character.     (Described,  p.  144.) 

284 


U  S  GEOLOGICAL  SURVEY 


MONOGRAPH    XXXVI  PL    XXVII 


S  BlENaCO  L1TM  N  V 


A.  AilYGDALOIDAL  BASATiT. 

B.  PSEUDO-AMYODiU,OmALi\L:VTRIX  OF  EUJPSOIDAI,  UAHALT 

C.  VOLCANIC  CLASTIC. 


PLATE   XXVIII. 


285 


PLATE    XXVIII. 

Fig.  a. 

(Sp.  No.  32909c.     Without  analyzer,  x  35.) 

Photomicrograph  of  section  of  flne-grained  basalt  with  well-developed  igneous  texture.     The 
feldspar  outlines  are  now  occupied  chiefly  by  flakes  of  muscovite  aud  grains  of  zoisite.     (Described, 

p.m.) 

Fig.  B. 

(Sp.  No.  32909c.     With  analyzer,  x  35.) 

Photomicrograph  of  the  same  section  when  viewed  between  crossed  nicols.  showing  the  obliter- 
ation of  the  igneous  texture.     The  lath-shaped  mineral  is  muscovite     (Described,  p.  127.) 

286 


U.    S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PART  I     PL     XXVIII 


(/»)    BASALT   SHOWING   CHARACTERISTIC   TEXTURE. 

(S)    BASALT   SHOWING   OBLITERATION   OF  TEXTURE   BETWEEN    CROSSED    NICOLS. 


HE     MEOlnEN     QDAVURE     CO. 


PLATE  XXIX. 


287 


PLATE    XXIX. 

Fig.  a. 

(Sp.  No.  32582.     Without  analyzer,  x  38.) 

Photomicrograph  showing  the  normal  igneous  texture  of  a  Ijasalt.  The  area  formerly  occuijied 
by  the  feldspar  substance  is  now  occupied  by  a  granular  aggregate  of  various  minerals.  (Described, 
p.  127.) 

Fig.  B. 

(Sp.  No.  32582.     With  analyzer,  x  38.) 

The  same  section  of  basal't  when  viewed  l)etweeu  crossed  nicols.  The  only  indication  of  au 
igneous  texture  is  shown  by  the  amygdnles  present.  The  igneous  texture  of  the  rock  is  completely 
concealed  as  .soon  as  the  aggregates  occupying  the  feldspar  areas  break  up  into  their  constituent  grains. 
(Described,  p.  127.) 

288 


U.   S.  GEOLOGICAL   SURVEY 


MONOGRAPH   XXXVI     PART  I     PL.    XXIX 


{B) 


(/»    BASALT   SHOWING   CHARACTERISTIC   TEXTURE. 

(SI    BASALT    SHOWING    OBLITERATION    OF   TEXTURE    BETWEEN    CROSSED    NICOLS. 


HE     MERIOEN     GRAVURE     CO. 


PLATE  XXX. 


MON   XXXVI 19 


PLATE   XXX. 

Fig.  a. 

(Sp.  No.  33313.     Without  analyzer,  x  38.) 

Photomicrograpli  of  a  section  of  basalt  from  a  pyroclastic  showing  in  ordinary  light  a  distinctly 
amygdaloidal  character.  The  alteration  of  the  hasalt  has,  however,  reached  such  a  stage  that  the 
groundmass  materials  have  for  the  most  part  been  completely  altered,  with  the  production  of  porphy- 
ritic  rhombobedra  of  calcite  and  plates  of  muscovite,  which  may  be  seen  in  abundance  in  the  lower 
right-haud  side  of  the  figure.     (Described,  pp.  129, 145.) 

Fig.  B. 

(Sp.  No.  33313.     AVith  analyzer,  x  38.) 

Photomicrograph  of  the  same  section  of  basalt,  showing  in  ordinary  light  a  distinctly  amygda- 
loidal character  when  viewed  between  crossed  nicols.  The  igneous  texture  is  almost  completely  oblit- 
erated by  secondary  products.  The  secondary  calcite  and  muscovite  stand  out  very  sharply  from 
the  very  tine  grained  groundmass.     (Described,  p.  129.) 

290 


U.  S.    GEOLOGICAL  SURVCY 


MONOGRAPH   XXXVI     PART  I     PL.    XXX 


C-A)    BASALT  FRAGMENT  IN  A  PYROCLASTIC  SHOWING  AMYCDALOIDAL  TEXTURE. 

CB)    BASALT  FRAGMENT  SHOWING  OBLITERATION  OF  THIS  TEXTURE  BETWEEN  CROSSED  NICOLS. 


THE    MfRiflEf*    QHAVURE    CO. 


PLATE  XXXI. 


291 


PLATE    XXX  I. 

Fir.  a. 

(Sp.  No.  32909c.     Without  aualyzer,  x  35.) 

Photomicrograpli  illustrating  the  igneous  texture  of  the  basalt  from  the  center  of  an  ellipsoiil. 
This  has  already  uutlergone  advanced  alteration,  and  the  feldspars  have  been  replaced  by  a  granular 
aggregate  of  calcite,  serlcite,  chlorite,  epidote,  quartz,  and  albite.  With  advancing  alteration  the 
igneous  texture  is  destroyed,  and  infiltrated  calcite  becomes  more  prominent.  The  photomicrograi)h 
shows  in  the  lower  left-hand  corner  a  portion  of  the  section  In  which  a  large  quantity  of  calcite  is 
present.     (Described,  p.  131.) 

Fig.  B. 

(Sp.  No.  32909c.     "With  analyzer,  x  35.) 

Photomicrograph  illustrating  the  igneous  texture  of  the  basalt  from  the  center  of  an  ellipsoid, 
when  viewed  between  crossed  nicols.  The  igneous  texture  is  almost  completely  destroyed  by  the 
breaking  up  of  the  secondary  aggregates.     (Described,  p.  131.) 

292 


U.S.   GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PART  I     PL.    XXXI 


w 


M) 


(B) 


CA)    BASALT 

CS)    BASALT   SEEN    BETWEEN    CROSSED    NICOLS. 


HE     MERIDEN    QRAVURE     CO 


PLATE  XXXIl. 


293 


PLATE   XXXIl. 

Fia.  A. 

(Sp.  No.  23645.     Without  analyzer,  x  35.) 

Perlitic  parting  In  a  fragment  from  a  basalt  tuff.  The  perlitic  cracks  are  marked  by  accumu- 
lations of  epidote  grains.  The  remainder  of  the  fragment  consists  of  an  exceedingly  fine  chlorite 
felt,  with  here  and  there  a  small  feldspar  microllte  embedded  in  it.  This  was  probably  a  fragment  of 
basalt  glass.     (Described,  p.  138.) 

Fig.  B. 

(Sp.  No.  23646.     Without  analyzer,  x  35.) 

Photomicrograph  of  a  section  of  basalt  tuff.  The  illustration  shows  the  sickle-shaped  bodies 
which  are  characteristic  for  the  fine  eolian-deposited  volcanic  ejectamenta.     (Described,  p.  142.) 

294 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PART  I     PL.   XXXIl 


CA)    PERLITIC   PARTING   IN    BASALT    (CLASS?). 
(B)    TUFF. 


THE     UEBIDEN     GBAVURE    CO. 


PLATE  XXXITI. 


295 


PLATE    XXXIII. 
Fig.  a. 

(Sp.  No.  32713.     Without  analyzer,  x  35.) 

Water-deposited  sand.  This  volcanic  sand  consists  chiefly  of  feldspar  and  liornblende.  The 
rounded  nature  of  the  feldspar  grains  is  readily  seen.  Under  the  microscope  some  of  these  show 
secondary  enlargements.  The  hornblende  is  in  irregular  areas,  and  is  presumed  to  be  secondary  after 
fragments  of  augite.     (Described,  p.  144.) 

Fig.  B. 

(Sp.  No.  32711.     Without  analyzer,  x  17.) 

Water-deposited  volcanic  sediment.  This  illustration  shows  very  clearly  the  griidation  from 
the  medium-grained  volcanic  sand  in  the  lower  portion  of  the  section,  through  the  finer-grained  mate- 
rial, to  the  very  fine  grained  dust.  The  very  fine  grained  material  was  deposited  following  the  con- 
tours of  a  large  pebble,  which  is  shown  In  the  upper  portion  of  the  figure.  The  fragments  of  this 
sand  consist  chiefly  of  basalt.     There  are  some  which  were  probably  feldspar.     (Described,  p.  144.) 

296 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PART  I     PL.    XXXIII 


(^) 


'm 


■^^f 


C/t)    WATER   DEPOSITED   SAND. 

(B1    GRADATION    IN   WATER    DEPOSITED   CLASTIC. 


THE    MERmEN     QRAVURE     CO. 


PLATE  XXXIV. 


297 


PLATE     XXXIV. 
Fig.  a. 

(Sp.  No.  23746.     Without  .analyzer,  x  17.) 

Contact  product  of  a  granite.  This  muscovite-biotite-gueiss  (  ?)  is  the  result  of  contiict  action 
of  a  granite  upon  a  graywacke.  Complete  recrystallization  of  the  sedimentary  rock  has  t.aken  place, 
with  the  production  of  a  porphyritio  schistose  structure.  The  large  porphyritic  muscovite  plates 
seen  as  white  areas  in  the  photomicrograph  evidently  represent  the  last  products  of  recrystallization, 
as  they  include  all  of  the  minerals  which  have  Iteen  previously  formed.  They  are  jjossihly  to  be 
looked  upon  as  the  product  of  mineralizers,  to  whose  action  may  also  be  referred  the  presence  of 
tourmaline,  which  occurs  in  the  section.  The  irregular  white  areas  represent  quartz  and  feldsp.ar. 
Dark  greenish-brown  biotite  is  included  in  the  muscovite,  and  with  iron  oxides  occupies  the  areas 
which  in  the  figure  are  dark.     (Described,  p.  197.) 

Fig.  B. 

(Sp.  No.  23226.     Without  analyzer,  xl7.) 

Photomicrograph  showing  the  brecciated  character  of  the  matrix  which  is  at  times  found  between 
the  ellipsoids  iu  the  ellipsoidal  bas.alts.  In  this  specimen  the  schistose  character  is  not  so  marked 
as  it  is  at  times.     'Described,  p.  117.) 

298 


U.S.   GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PL.    XXXIV 


(/»)    CONTACT   PRODUCT   OF   GRANITE. 

(B)    BRECCIATED    MATRIX   BETWEEN    ELLIPSOIDS. 


THE     MER10EN    QRAVURE     CO. 


PLATE   XXXV. 


299 


PLATE    XXXV. 
Pig.  a. 

(Sp.  No.  260.59.     Without  analyzer,  x  18.) 

Photomicrograph  showing  an  eruptive  contact  between  granite  and  metamorphosed  sedimentary 
rock.  Ab  a  result  of  this  contact  the  elements  of  the  granite  have  become  partly  automorphic.  The 
center  of  the  iigure  is  occupied  by  a  quartz  phenocryst  which  is  partly  surrounded  by  the  schistose 
metamorphic  product.  It  contains,  near  the  edge,  grains  of  feldspar  and  flakes  of  mica,  and  thus  an 
imperfect  poikilitic  zone  is  produced.  The  mineral  constituents  of  the  metamorphosed  sedimentary 
are  arranged  parallel  to  the  contours  of  the  phenocrysts.     (Described,  p.  198.) 

Fig.  B.' 

(Sp.  No.  26059.    With  analyzer,  x  18. ) 
The  same  section  as  the  above,  viewed  between  crossed  nicols.     (Described,  p.  198.) 
300 


U.S.   GEOLOGICAL  SURVEY 


MONOGHAPH   XXXVI     PART  I     PL.    XXXV 


A*, 


c*i^ 


^ 


m 


.i*"^ 


(fi) 


(>»)    CONTACT   OF   GRANITE   AND   SEDIMENTARY. 

(S)    CONTACT   OF   GRANITE   AND   SEDIMENTARY   SEEN   BETWEEN    CROSSED    NICOLS. 


THE     MFR   DEN    GRAVURE     CO- 


PLATE   XXXVI. 


301 


PLATE    XXXVI. 

Fig.  a. 

(Sp.  No.  32827.     Without  .analyzer,  x  38.) 

Photomicrograph  illustratiug  a  rather  exceptional  form  of  siiilositc  with  white  spots  lying  iu 
the  fine-grained  groundiiKiss.  Albite,  with  a  very  small  amount  of  chlorite  and  epidote,  forms  the 
spots.     (Described,  p.  206.) 

Fig.  B. 

(Sp.  No.  32827.     With  analyzer,  x  38.) 

This  is  the  same  section  as  above,  viewed  between  crossed  nicols,  showing  the  aggregate  char- 
acter of  the  spots  of  this  spilosite.     (Described,  p.  206.) 

302 


U.S.   GEOLOGICAL   SURVEY 


MONOGRAPH   XXXVI     PART  I     PL.    XXXVI 


'^l""'  wa 


r^^??^ 


■  »', 


(^) 


(-6) 


(>»)    SPILOSITE. 

(S)    SPILOSITE   SEEN    BETWEEN    CROSS   NICOLS. 


IFRIDEN    GHAVOKE    CO. 


PLATE  XXXVII. 


303 


PLATE     XXXVII. 
Fig.  a. 

(Sp.  No.  32958.     Without  analyzer,  x  18.) 

Normal  spilosite.  Oval  spots  of  xuacroscopical  size,  in  which  chlorite  is  the  predominant  con- 
stituent, with  some  quartz,  feldspar,  rutile,  and  muscovite,  are  sharply  defined  from  the  surrouuding 
fine-grained  grouudmass,  consisting  of  the  same  constituents,  but  with  the  muscovite  in  large  quantity 
and  the  chlorite  very  subordinate.     (Described,  p.  206.) 

Fig.  B. 

(Sp.  No.  32861.     Without  analyzer,  x  38.) 

Spilosite  in  which  the  spots  are  of  microscopical  size  and  consist  predominantly  of  chlorite 
aggregates  lying  in  tbe  fine-grained  quartz-albite  groundmass.     (Described,  p.  207.) 

304 


U.   S.  GEOLOGICAL   SURVeV 


MONOGRAPH   XXXVI     PART  I     PL     XXXVII 


■  .•^►>* 


'•^te*  ■  ^'■^'\''    ■'•if;'-''     ■ 


(^) 


(/I)    SPILOSITE. 
(B)    SPILOSITE. 


nEN     GSAVUHE     CO. 


PLATE  XXXVIII. 


MON    XXXVI 20  305 


PLATE    XXXVIII. 

Fig.  a. 

( Sp.  No.  32826.     Without  analyzer,  x  38. ) 

Photomicrograph  illu8trating  the  passage  of  spilosite  to  a  desmosite.  lu  the  upper  jiortiou, 
especially  iu  the  upper  left-hand  corner,  of  the  figure,  chlorite  .aggreg.ates  similar  to  those  illustrated 
in  lig.  i',  PI.  XXXVII,  are  seen.  These  hecome  united,  and  thus  there  is  a  passage  into  the  banded 
product.  This  banded  character  is  well  shown  in  the  lower  half  of  the  photomicrograph.  (Described, 
p.  207.) 

Fig.  B. 

(Sp.  No.  23755.     Without  analyzer,  x  90.) 

Occurrence  and  alteration  of  bronzite  in  bronzite-norite.  This  illustration  shows  the  way  in 
which  the  bronzite  occurs  in  the  bronzite-norite.  It  is  frequently  included  in  the  hornblende.  The 
bronzite  alters  around  the  edges  and  along  the  cracks  to  a  yellowish-green  fibrous  serpentine  mineral, 
which  is  represented  in  the  section  by  the  dark  fibrous  material  next  to  the  unaltered  bronzite.  This 
secondary  mineral  then  .alters  to  a  scaly  aggregate  of  talc.  These  two  secondary  products  can  be 
seen  bordering  tba  bronzite,  especially  well  where  it  is  traversed  by  a  crack.     (Described,  p.  238.) 

306 


U.S.   GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PL.    XXXVIII 


:»^' 


M) 


(5) 


(/»)    PASSAGE   OF   SPILOSITE    INTO    DEMSOSITE. 

(S)    ALTERATION   OF   BRONZITE    IN    BRONZITE    NORITE. 


THE     MtRIDEN    ORAVURE     CO. 


PLATE   XXXIX. 


307 


PLATE    XXXIX. 
Fig.  a. 

(Sp.  No.  23321.     With  analyzer,  x  35. ) 

Pliotomicrograph  nf  a  section  of  biotite-granite  from  the  center  of  a  dil^e  5  feet  Tvido.  On  the 
horders  of  the  dike  the  magma  has  crvstallized  as  a  normal  mica-diorite  -.vithout  qnartz  and  orthoclase. 
(Described,  p.  226.) 

Ficf.  B. 

(Sp.No.  26023.     With  analyzer,  x  3.5.) 

Mica-diorite  showing  tendency  toward  an  opliitic  texture.  Plagioclase  is  the  most  automorphic 
mineral-  Biotite  is  next,  but  it  is  poorly  developed.  Orthoclase  and  quartz  fill  irregular  areas 
between  the  plagioclase.     (Described,  p.  231.) 

308 


U.   S.  GEOLOGICAL  SURVEY 


MONOGRAPH  XXXVI     PL.    XXXIX 


(^) 


(B) 


(»)    BIOTITE   GRANITE. 
(B)    MICA    DIORITE. 


THE     MERinEN     GHAVURE    CO. 


PLATE   XL. 


309 


PLATE    XL. 
Fig.  a. 

(Sp.  No.  32643.     Without  analyzer,  x  35.) 

Qnartz-mioa-diorite-porphyry.  The  phenocrysts  of  feldspar  aud  quartz  stand  out  clearly  from 
the  fine  miciogranitic  grouudmass.  Mica  phenocrysts  are  not  seen  in  this  figure,  which  is  intended 
chiefly  to  illustrate  the  character  of  the  feldspar  phenocrysts.  Muscovite  has  resulted  from  theii 
alteration.  The  zone  which  surrounds  the  altered  center  is  very  fresh,  and  is  rendered  poikilitic  hy 
inclusions  of  minutt-  grains  ot  fpiartz.     (Described,  p.  229.) 

Fig.  B. 

(Sp.  No.  32643.     With  analyzer,  x  35.) 

Qnartzmioa-diorite-porphyry.  The  same  section,  viewed  lietweeu  crossed  nicols.  The  micro- 
granitic  texture  of  the  groundmass  is  well  shown.     (Described,  p.  229.) 

310 


U.  S.   GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PL.    XL 


-0^-^^. 


'^ 


■k: 


J"      w,' "' 


•y 


V^/'^ 


•^^r^; 


--*v-      i. 


M) 


(B) 


(A-)    MICA   DIORITE    PORPHYRITE. 

(S^    MICA   DIORITE   PORPHYRITE   SEEN    BETWEEN   CROSSED    NICOLS. 


TMt     MERlOEN    QRAVUBE     CO. 


PLATE  XLI. 


311 


PLATE    XL  I. 
Pm.  A. 

(Sp.  No.  23320.     Without  aualyzer,  x  18.) 

Pnrphyritic  poikilitic  hornblende-gabbro.  Tho  brown  honibk-ndo  occupying  tho  center  of  the 
phenocryst  grades  over  iuto  a  dull-green  hornblende.  Feldspar  and  pyroxene  are  included  in  the 
hornblende.     (Described,  p.  241.) 

Fig.  B. 

(Sp.  No.  23344.     With  analyzer,  x  18.) 

Hornblende-gabbro  showing  a  poikilitic  texture.  The  tendency  of  the  feldspars  toward  a  lath- 
shaped  development  is  very  evident.  Were  they  lath-shaped  tho  texture  would  agree  with  the  L^vy 
definition  of  tho  ophitio  texture.     (Described,  p. 233.) 

312 


U.S.   GEOLOGICAL  SURVEY 


MONOGRAPH   XXXVI     PL.    XLI 


(/»    PORPHYRITIC    POIKILITIC    HORNBLENDE   CABBRO. 
CS)    POIKILITIC    HORNBLENDE. 


HE     MERIOEN    QRAVURE     CO. 


PLATE    XLII. 


313- 


PLATE    XLII. 

Fig.  a. 

(Sp.  No.  23754.     Without  analyzer,  x  38.) 

A  moderately  fine  grained  hornbleude-gabbro  showing  parallel  texture.  This  liornblende- 
gabbro  occurs  in  dikes  cutting  the  coarse  forms  of  hornblende-gabbro.  The  sppcimen  shows  very 
clearly  the  parallel  texture  rather  commonly  found  in  sections  from  these  dike  rocks.  This  texture 
is  best  developed  nearest  the  contact,  and  i.s  presumed  to  be  a  flow  texture.  The  chief  mineral 
constituents — plagioclase,  hornblende,  and  mica — can  be  readily  distinguished  in  the  section. 
(Described,  p.  244.) 

Fig.  B. 

(Sp.  No.  23754.     With  analyzer,  x  38.) 

The  parallel  texture  in  the  hornblende-gabbro  is  brought  out  somewhat  better  when  viewed 
between  crossed  nicols.     (Described,  p.  244.) 

314 


U.   S.  GEOLOGICAL   SURVEY 


MONOGRAPH    XXXVI      PL.    XLII 


(^) 


(B) 


(/»    HORNBLENDE   CABBRO. 

(S)    HORNBLENDE   CABBRO   SEEN    BETWEEN    CROSSED    NICOLS. 


The    meriden    qravore  co. 


PLATE    XLIII 


315 


PLATE   XLIII. 
Fig.  a. 

(Sp.  No.  26070.     Without  analyzer,  x  17.) 

Normal  granular  hornblende-gabbro.  This  illustrates  the  normal  medium-grained  hornblende- 
gabbro  -which  occurs  in  this  district.  The  mineral  constituents  hornblende,  feldspar,  aud  iron  oxide 
can  be  readily  distinguished.     (Described,  p.  248.) 

Fig.  B. 

(Sp.  No.  26069.     With  analyzer,  x  18.) 

Schistose  hornblende-gabbro.  This  illustrates  a  crushe<l  specimen  of  a  normal  hornblende- 
gabbro  such  as  is  represented  above  in  fig.  A.  In  this  the  feldspar  has  been  partly  granulated,  and  ne^ 
feldspar  and  quartz  has  resulted  therefrom.  The  hornblende  has  also  to  a  considerable  extent  been 
recrystallized  as  light-green  secondary  hornblende.  The  mica  has  become  chloritized.  These  are  the 
important  secondary  products.  A  very  fiiir  schistose  structure  has  resulted  from  the  crushing. 
Carried  to  its  full  extent,  metamorphism  of  this  rock  would  result  in  producing  an  amphibolite  or  a, 
hornblende-gneiss.     (Described,  p.  248.) 

316 


U.    S.  GEOLOGICAL   SURVEY 


MONOGRAPH   XXXVI     PL.    XLIII 


^. 


[A) 


(B) 


(A)  COARSE    HORNBLENDE   CABBRO. 

(B)  SCHISTOSE    HORNBLENDE   CABBRO. 


THE     MERinE)4     GHAVURE    CO. 


PLATE    XLIV. 


317 


PLATE    XLIV. 
Pig.  .-1. 

(Sp.  No.  23319.     Without  analyzer,  x  35.) 

Motlerately  fine  grained  hornblende-gabbro.  The  constituents  horublende,  plagioclase,  and  iron 
oxide  can  be  readily  recognized.  Many  of  the  hornblende  individuals  contain  a  great  number  of 
minute  inclusions.     (Described,  p.  240.) 

Fig.  B. 

(Sp.  No.  23755.     Without  analyzer,  x  35.) 

Bronzite-noiite.  The  rock  consists  of  bronzite,  hornblende,  and  plagioclase.  The  bronzite 
individuals  show  a,  tinge  of  green  aud  ijinlc,  and  are  slightly  fibrous,  due  to  beginning  alteration. 
Cracks,  filled  with  alteration  products,  cross  them.  The  bronzite  is  frequently  surrounded  by  a  rim 
of  brown  hornblende.  In  some  parts  of  the  rock  it  is  partly  automorphic.  The  hornblende  can  be 
readily  recognized  by  its  strong  yellowish  and  reddish-browu  color.  It  is  in  auhedra.  The  plagioclase 
is  the  white  mineral  including  numerous  minute  dark  specks.  Jt,  like  the  bronzite,  is  xenomorphic. 
(Described,  p.  244. ) 

318 


USGEOLOGICAL    SURVEY 


MONOGRAPH   XXXVI    PL     XLIV 


.r:^r^^m^:' 


/^  /T  Denniston_  del 


JULIUS  BIEN  a  CO  LITH    N.V 


A.  HOBNBLKNDE-a\BBHO. 
B.  BRONZITE-NOIUTE. 


PLATE   XLV. 


319 


PLATE   XLV. 

Fig.  a. 

(Sp.  No.  23356.     Without  analyzer,  x  35.) 

Brouzite-norite-porphyry.  The  pUenocrysts  of  bronzite  lie  in  a  grouudmass  of  bronzite,  mouo- 
■olinlc  pyroxeue,  hornblende,  and  feldspar.  In  the  drawing  the  bronzite  and  monoclinic  pyroxene  of 
the  groundmass  can  not  be  distinguished.     (Described,  p.  247.) 

Fig.  B. 

(Sp.  No.  23353.     Without  analyzer,  x  18.) 

Fcldspathic  wehrlite.  One  can  readily  distinguish  the  constituents — augite,  hornblende,  olivine 
and  feldspar — in  the  illustration.  The  augite  is  automorphic  where  it  is  near  the  feldspar.  It  is 
surrounded  by  a  brown  hornblende  zone.  Between  the  olivine  and  the  feldspar  there  are  always  two 
zones.  One,  the  inner  one,  best  seen  between  crossed  nicols,  is  orthorhombic  pyroxene;  the  other, 
the  outer  one,  is  compact  green  hornblende.  This  green  hornblende  is  in  optical  continuity  with  the 
browu  hornblende  which  surrounds  the  augite.     (Described,  p.  255.) 

320 


US.GtOLOGICAL    SURVEY 


MONOGRAPH   XXXVI    PL.    X  LV 


F K.  Denniston,  del 


JULIUSBIEN6C0  LITH    N  ■- 


A,  BHONZITE-  NORITE  -PORPHYRY. 
B.  FELDSPATHIC  IVEREJLITE. 


PLATE   XL VI. 


MON  XXXVI 21  32X 


PLATE    XLVI. 

Fig.  a. 

(Sp.  No.  23353.     With  analyzer,  x  90.) 

Wehrlito.     There  are  shown  iu  this  illustration  the  zones  of  orthorhombic  pyroxene  and  horn- 
bliintle  which  lie  between  the  olivine  and  feldspar.     (Described,  p.  255.) 

Fig.  /}. 

(Sp.  No.  23353.     Without  analyzer,  x  90.) 

Wehrlite.     The  ramifying  feldspar  growths  in  the  hornblende  can  be  seen  better  when  the 
section  is  viewed  with  the  analyzer  in,  as  in  iig.  A.     (Described,  p. 255.) 

322 


U.S.   GEOLOGICAL   SURVEY 


MONOGRAPH   XXXVI     PL.    XLVI 


M) 


{H) 


(A;    WEHRLITE   VIEWED    BETWEEN    CROSSED    NICOLS. 
fS)    WEHRLITE. 


THE     WERIDEN    QRAVURE     CO- 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT  OF  MICHIGAN 


Part    II.— THE    EASTERN    PART    OF   THE    DISTRICT> 
INCLUDING  THE  FELCH  MOUNTAIN  RANGE 

By    HENKY    ULOYD    SMY'm 


A  CHAPTER  ON  THE  STURGEON  RIVER  TONGUE 

By  WILLIAM  SHIKLET  BAYLET 


323 


CONTENTS. 


Page. 

Chapter  I.— Geographical  omits  and  physiographv 329 

Intvoduction 329 

Preliminary  sketch  of  geology 331 

Character  of  surface 331 

Drainage 334 

Chapter  II. — Magnetic  observations  in  geological  mapping 336 

Section  I. — Introduction 336 

Section  II. — Description  of  the  magnetic  rocks 338 

Section  III. — Distribution  of  magnetism  in  the  magnetic  rocks 339 

Section  IV. — Instruments  and  methods  of  work 341 

Section  V. — Facts  of  observation  and  general  principles 344 

(1)  Observed  deflections  when  the  strike  is  north  and  south  and  the  dip  vertical 344 

(2)  Deflections  of  the  horizontal  needle 345 

(3)  Deflections  of  the  dip  needle 347 

(4)  Horizontal  and  vertical  components  when  the  magnetic  rock  dips  vertically 349 

(5)  Horizontal  and  vertical  components  when  the  magnetic  rock  dips  at  any  angle 350 

(6)  Determination  of  depth 354 

(7)  Summary 356 

Section  VI. — Applications  to  special  cases 356 

(1)  The  magnetic  rock  strikes  east  or  west  of  north  and  dips  vertically 357 

(2)  The  magnetic  rock  strikes  east  and  west 3.59 

(3)  Two  parallel  magnetic  formations 361 

Section  VII. — The  interpretation  of  more  complex  structures 366 

(1)  Pitching  synclines 367 

(2)  Pitching  anticlines 370 

(3)  Formations  split  by  intrusives 371 

(4)  Summary 372 

Chapter  III. — The  Felch  Mountain  range 374 

Section  I. — Position,  extent,  and  previous  work 374 

Section  II. — General  sketch  of  the  geology 383 

Section  III. — The  Archean 385 

Topography 386 

Petrographical  characters 387 

Section  IV. — The  Sturgeon  quartzite 398 

Distribution,  exposures,  and  topography 398 

Folding  and  thickness 399 

Petrographical  characters 401 

Section  V. — The  Randville  dolomite 406 

Distribution,  exposures,  and  topography 406 

Petrographical  characters 408 

Section  VI. — The  Mansfield  schists 411 

Distribution,  exposures,  and  topography 411 

Petrographical  characters 412 

325 


32(i  CONTENTS. 

Chapter  III. — The  Felch  Mountain  Range — Continued.  Page. 

Section  VII. — The  Groveland  formation 415 

Distribution,  exposures,  and  topography 415 

Petrographical  characters 417 

Section  VIII. — The  mica  schists  and  quartzites  of  the  Upper  Huronian  series 423 

Petrographical  characters 425 

Section  IX. — The  intrusives 426 

Chapter  IV. — The  Michigamme  Mountain  and  Fence  River  areas 427 

Section  I.— The  .\rchean 428 

Section  II. — The  Sturgeou  formation 430 

Section  III.— The  Randville  dolomite 431 

Distribution  and  exposures 431 

Folding  and  thickness 432 

Petrographical  characters 434 

Section  IV. — The  Mansfield  formation 437 

Distribution,  exposures,  and  topography 438 

Folding  and  thiclsness 438 

Petrographical  characters 439 

Section  V. — The  Hemlock  formation 440 

Distribution,  exposures,  and  topography 440 

Folding  and  thickness 441 

Petrographical  characters    442 

Section  VI. — The  Groveland  formation 446 

Distribution,  exposures,  and  topography 446 

Folding  and  thickness , 448 

Petrographical  characters 448 

Chapter  V. — The   northea.stern  area   and  the  relations  between  the  Lower   Mar- 
quette and  the  Lower  Menominee  series 451 

Chapter  VI.— The  Sturgeon  River  tongue,  by  William  Shirley  Bayley 458 

Description  and  boundary  of  area 458 

Literature 459 

Relations  between  the  sedimentary  rocks  and  the  granite-schist  complex 461 

The  Basement  Complex 463 

The  gneissoid  granites 463 

The  amphibole-schisls - 465 

Origin  of  the  amphibole-schists 466 

The  biotite-schists 467 

The  intrusive  rocks 469 

Comparison  of  the  Sturgeon  Eiver  and  the  Marquette  crystalline  series 470 

The  Algonkian  trough 471 

Relations  liet ween  the  conglomerate  and  the  dolomite  series 472 

Relations  between  the  dolomites  and  conglomerates  and  the  overlying  sandstones 473 

The  conglomerate  formation 473 

Important  exposures 474 

Petrographical  descriptions 477 

The  dolomite  formation 479 

Important  exposures 480 

Petrographical  description 481 

Slates  and  sandstones  on  the  Sturgeon  River 481 

The  igneous  rocks 482 

The  intrusive  greenstones 482 

Petrographical  description 482 

The  banded  greenstones 485 

Petrographical  description 486 


ILLUSTRATIONS 


Page. 

Plate  XLVII.  Relaiious  of  maguctic  beds  to  variation  auil  dip 352 

XL VIII.  Relations  of  magnetic  beds  to  variation  and  dii> 362 

XLIX.  Geological  map  of  the  FeUb  Mountain  range 374 

L.  Geological  map  of  a  portion  of  the  Crystal  Falls  district 450 

LI.  Geological  map  of  the  Sturgeon  River  tongue 458 

LIT.  Map  of  exposures  in  sec.  7  and  portions  of  sees.  8, 17,  and  18,  T.  42  N.,  R.  28  W 474 

LIII.  Schist  conglomerate  from  dam  of  Sturgeon  River 476 

Fig.  15.  Magnetic  cross  section  in  T.  45  N.,  R.  31  W 345 

16.  Circles  of  attraction 346 

17.  The  forces  acting  on  the  dip  needle 347 

18.  Curves  showing  the  relations  between  the  horizontal  components  at  the  points  of  maxi- 

mum deflection,  for  rocks  dijiping  at  various  angles  and  buried  to  various  depths 354 

19.  Truncated  anticlinal  fold  with  gontly  dipping  limbs 364 

20.  Truncated  anticlinal  fold  with  steeply  dipping  limbs 365 

21.  Plan  and  cross  sections  of  a  lutchiug  syncline 367 

22.  Magnetic  map  of  the  Groveland  Basin 370 

23.  Plan  and  cross  sections  of  a  pitching  anticline 371 

24.  Magnetic  map  of  a  single  formation  split  by  an  intruded  sheet 372 

327 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT 

OF  MICHIGAN. 


part  ii.  the  eastern  part  of  the   district, 
i:n^cludixg  the  felch  mountain  range. 


By  Henry  Lloyd  Smyth. 


With  a  Chaptek  on  the  Sturgeon  River  Tongue,  bv  William  Shirley  Bayley. 


CHAPTER!. 
GEOGRAPHICAL  LIMITS  AND  PHYSIOGRAPHY. 

INTRODUCTION". 

The  teiTitory  to  be  described  in  this  and  the  four  following  chapters  is 
situated  in  the  Upper  Peninsula  of  Michigan,  between  the  Marquette  and 
Menominee  iron  ranges,  and  is  all  embraced  within  T.  42  N.,  Rs.  28-30  W., 
and  Ts.  42-47  N.,  Rs.  30-31  W.  The  area  of  about  300  square  miles 
included  within  these  townships  had  for  the  most  part  been  covered  hastily 
by  previous  reconnaissances  of  the  Lake  Superior  Division  of  the  United 
States  Geological  Survey,  the  results  of  which  were  placed  at  my  disposal. 
Our  task  was  to  go  over  with  especial  care  those  portions  in  which  outcrops 
had  been  found  by  our  predecessors,  or  which  seemed  likely  to  contain  the 
iron-bearing  formations.  At  the  same  time  much  of  the  rest  was  examined 
more  hurriedly. 

The  tract  surveyed  in  detail  comprises  a  continuous  belt  about  30 
miles  in  length,  and  of  width  varying  from  2  to  5  miles,  lying  wholly 
within  the  drainage  basin  of  the  Michigamme  River  and  its  principal  upper 
tributary,  the  Fence  River,  and  extending  southward  from  the  northern  end 
of  the  Republic  tongue,  where  it  was  connected  with  rocks  of  well- 
determined  Marquette  types,  as  far  as  the  south  line  of  T.  43  N.,  R.  31  "W. 
From  this  line  we  passed  southeast  (leaving  a  gap  of  5  miles)  across  the  low 

329 


3B0  THE  CRYSTAL  FALLS  lEON-BEARING  DISTRICT. 

di^■ide  between  the  Michigamme  and  the  headwaters  of  the  Sturgeon,  to  the 
Felch  Mountain  range,  which  was  then  carefully  studied  for  a  distance 
extending  13  miles  to  the  east. 

Until  within  the  last  few  years  the  larger  part  of  this  area  had  been 
very  difficult  of  access,  and  much  of  it  is  dithcult  still.  The  rock  surface  is 
almost  wholly  concealed  by  a  cover  of  glacial  deposits  of  various  kinds; 
dense  forest  and  great  swamps  also  obscure  the  rocks,  and  make  traveling 
difficult  and  slow.  It  is  therefore  not  a  field  to  invite  geological  study. 
While  exploration  for  iron  ore  has  here  and  there  passed  the  frontiers  of  the 
productive  ranges  on  either  side,  the  general  ill-success  which  attended  the 
early  enterprises  has  discouraged  the  active  search  that  would  at  least  have 
resulted  in  important  additions  to  geological  knowledge.  For  these  reasons 
the  area  as  a  Avhole,  with  the  exception  of  the  Felch  Mountain  range,  has 
remained  almost  unknown  geologically,  until  our  work  in  1892.  The  i-efer- 
ences  to  it  in  geological  literature  are  consequently  but  few  In  number,  and 
are  for  the  most  part  merely  the  records  of  the  unrelated  observations  of 
casual  visitors. 

The  district,  nevertheless,  deserves  attention  from  both  the  economic 
and  the  geological  standpoint.  The  iron-bearing  formations  of  the  Mar- 
quette range  extend  into  it  from  the  north,  those  of  the  Menominee  range 
from  the  south.  On  the  west  the  ore  deposits  of  the  Crystal  Falls  area  are 
connected  geographically  at  least  with  the  western  extension  of  the  Menom- 
inee range.  Between  these  boundaries  the  area  stands  as  the  largest  one 
remaining  in  Michigan  in  which  iron-bearing  formations  are  known  to  occur, 
but  as  yet  not  yielding  important  bodies  of  ore.  Here,  too,  if  anywhere, 
the  questions  of  the  equivalence  or  nonequivalence  of  the  individual  for- 
mations of  the  Marquette  and  Menominee  iron-bearing  series  are  to  be 
answered. 

It  is  proper  to  state  that  the  field  study,  in  consequence  of  the  condi- 
tions under  Avhich  this  work  was  done,  was  almost  wholly  directed  to  eco- 
nomic questions,  and  that  it  was  not  originallj-  anticipated  that  the  results 
were  to  be  published  as  a  monograph  on  the  district.  This  will  explain  the 
very  brief  space  devoted  to  the  Archean  in  the  following  pages.  The  field 
work  was  begun  and  ended  in  1892.  Since  that  time  there  has  been  no 
opportunity  to  revisit  localities,  and  the  conclusions  now  stand  essentially 
as  they  were  reached  in  the  field.     Considering  both  the  obscurity  and  com- 


INTRODUCTION.  331 

plexitv  of  the  area,  it  is  very  probable*  that  further  study  of  important  local- 
ities would  clear  away  many  of  the  difficulties,  as  well  as  uiodify  certain  of 
the  opinions  now  held. 

The  writer  was  efficiently  aided  in  the  field  work  by  Messrs.  Samuel 
Sanford  and  Charles  N.  Fairchild  for  nearly  the  whole  period,  and  E.  B. 
Mathews  and  H.  F.  Phillips  for  part  of  it,  as  assistant  geologists,  and  by 
Messrs.  Lewis  and  Forbes  as  skilled  woodsmen. 

PRELiIMINARY   SKETCH   OF  THE   GEOL,OGY. 

The  rocks  of  the  Michigamme  and  Felch  Mountain  areas  range  in  age 
from  Archean  to  early  Paleozoic.  North  and  west  of  the  Michigamme 
River,  where  geological  boundaries  are  most  susceptible  of  determination, 
the  granites  and  gneisses  of  the  Archean  come  to  the  surface  in  three  oval 
areas  of  great  regularity  of  outline,  from  10  to  12  miles  long  by  2  to  6 
miles  wide,  while  the  intervals  between  the  Archean  ovals  are  occupied 
by  highly  tilted  sedimentary  and  igneous  rocks  of  Algonkian  age.  The 
lower  member  of  the  Algonkian  has  derived  its  materials  from  the  wasting 
of  rocks  lithologically  similar  to  the  underlying  granites  and  gneisses.  In 
the  southern  and  eastern  portions  of  the  district  the  edges  of  the  tilted 
older  rocks  are  partially  covered  by  a  blanket  of  gently  dipping  sandstones 
of  Cambrian  age,  very  soft  and  easil}"  disintegrating.  These  rocks  first 
appear  near  the  ]\Iichigamme  River  as  detached  outliers.  In  going  south 
and  east  from  that  river  the  separated  patches  become  larger  and  more 
a,bundaut,  until  finally  a  few  miles  beyond  the  eastern  limit  of  our  work  in 
the  Felch  Mountain  range  they  unite  and  entirely  cover  the  pre-Cambrian 
formations. 

CHARACTER  OF  THE  SURFACE. 

In  its  most  general  aspect  the  surface  throughout  this  area  is  a  plam, 
somewhat  rolling  indeed,  which  slopes  gently  upward  from  the  southeast 
toward  the  northwest.  The  surface  is  formed  partly  by  the  soft  and  gently 
inclined  Upper  Cambrian  sandstones  and  partly  by  the  much  harder  and 
highly  tilted  pre-Cambrian  rocks  of  diverse  physical  and  mineralogical 
characters,  and  yet  over  all  it  maintains  a  very  uniform  slope.  On  the 
southeast,  in  the  Felch  Mountain  range,  the  plain  has  an  average  elevation 
above  the  sea  of  1,200  to   1,300  feet.     In  the  northwest,  in  the  southern 


332 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 


sections  of  T.  47  N.,  R.  31  W.,  the  average  elevation  is  1,800  to  1,900  feet. 
Since  the  intervening'  distance  is  somewhat  more  than  30  miles,  the  gen- 
eral slope  is  therefore  less  than  20  feet  to  the  mile. 

The  minor  topographical  features  based  upon  this  plain  are  multitudi- 
nous in  variety  and  detail,  but  generally  quite  insignificant  in  relief.  The 
maxiinum  difference  of  elevation  between  the  top  of  the  highest  hill  and 
the  bottom  of  the  neighboring  valley  is  less  than  300  feet,  and  this  is 
reached  in  Ijut  two  cases.  The  country  possesses  no  commanding  emi- 
nences, and  in  the  widest  panoramas  now  and  then  obtainable  from  the 
summits  of  glaciated  knobs  the  background  is  restricted  to  a  radius  of  a 
few  miles.  In  these  the  general  evenness  of  the  sky  line  is  usually  broken 
only  by  the  remnants  of  the  old  forest,  which  have  not  yet  succumbed  to 
fire  and  the  lumberman. 

These  lesser  features  have  been  shaped  mainly  by  the  work  of  the  conti- 
nental ice-sheet,  both  through  the  materials  which  it  brought  in  and  through 
those  which  it  earned  away.  In  the  areas  underlain  by  relatively  massive 
rocks,  particularly  the  Archean  crystallines,  the  surface  has  been  left  mam- 
millated  with  rocky  knobs,'  which  doulitless  were  the  unattacked  cores  rising 
into  the  pre-Grlacial  zone  of  disintegration.  These  are  separated  by  the 
similar  inverse  forms,  now  for  the  most  part  occupied  by  swamps.  In  the 
Archean  borders  of  the  Felch  Mountain  area,  where  the  glacial  cover  was 
originally  thin,  the  periodical  fires  that  have  followed  lumbering  operations 
have  burned  out  the  organic  matter  from  the  soil  and  so  loosened  it  that, 
on  the  steeper  slopes,  it  has  been  entirely  washed  away  and  the  rock 
surface  laid  bare.  The  hummocks  and  bowls  are  generally  elongated  east 
and  west,  which  is  the  direction  both  of  the  gneissic  foliation  and  of  the  ice 
movement.  The  elevations  rise,  often  with  steep,  smooth  walls,  for  5,  10, 
20,  or  even  in  some  cases  60  feet,  above  the  intervening  depressions.  The 
latter  hold  muskeg  to  the  rims.  In  the  wet  season  they  fill  with  water, 
which  ovei-flows  to  the  next  bowl  below,  but  permanent  lines  of  minor 
di-ainage,  here  as  elsewhere  in  the  Archean  areas,  are  very  infrequent. 

Over  most  of  the  area,  however,  the  ice  has  spread  a  sheet  of  till,  and  has 
here  and  there  deposited  the  materials  swept  along  in  the  subglacial  streams 
in  characteristic  complexity  of  form  and  grouping.  The  more  prominent 
elevations  are,  in  fact,   deposits  of   modified   drift,   although  occasionally 


TOPOC.KAPHY.  333 

small  rock  masses  like  Michig'amine  Mountain,  which  is  composed  of  mate- 
rial that  offers  a  most  stubborn  resistance  to  all  degrading-  agents,  reach  an 
elevation  of  100  to  200  feet  above  the  general  level  of  the  surrounding 
country.  The  fact  that  the  name  "mountain"  has  been  applied  to  hillocks 
ot  this  order  by  the  surveyors  and  woodsmen,  who  have  the  widest  knowl- 
edge of  the  Upper  Peninsula,  conveys  perhaps  the  clearest  idea  of  the 
generally  level  character  of  the  surface. 

While  the  details  of  the  topography  are  thus  mainly  glacial  in  origin, 
the  broader  features  of  the  next  order  of  importance  have  often  clearly 
been  determined  by  the  presence  of  the  more  resistant  rocks.  The  large 
structural  domes  of  the  Archean,  which  are  such  characteristic  geoloo-ical 
features,  are  also  indicated  by  a  general  upward  swell  of  the  surface  of  the 
areas  which  they  occupy.  The  topographical  transitions  at  the  margins  of 
these  swells  are  frequently  abrupt,  and  sometimes  for  considerable  distances 
are  marked  by  scarp-like  slopes  in  the  granites,  caused  by  the  almost  ver- 
tical contacts  with  the  softer  Algonkian  formations.  Considerable  portions 
of  all  three  of  the  Archean  ovals  in  the  northern  part  of  the  district  display 
this  slight  topographical  prominence.  Marginal  scarps  are  particulai-ly  well 
shown  in  the  oval  west  of  Republic,  in  sees.  19  and  30,  T.  47  N.,  R.  30  W., 
and  along  the  south  side  of  the  oval  which  lies  between  the  Fence  and 
Deer  rivers,  near  their  junctions  with  the  Michigamme.  The  more  impor- 
tant bodies  of  greenstone  also  are  generally  expressed  by  a  noticeable  degree 
of  elevation.  Thus  the  great  intruded  sheets  folded  in  with  the  Lower  Mar- 
quette series  in  sees.  24,  25,  and  36,  T.  47  N.,  R.  31  W.,  give  rise  to  long 
broad  ridges  that  closely  follow  the  changes  in  the  strike.  But  in  all  these 
cases  the  topographical  emphasis  is  very  slight,  and  the  plain  as  a  whole 
may  truly  be  said  to  maintain  its  general  slope  with  practical  indifference 
to  the  weather-resisting  differences  in  the  underlying  rocks. 

These  broader  swells  of  the  harder  rocks  are  separated  by  broad, 
slightly  lower-lying  plains,  in  many  of  which  a  valley  character  is  still  dis- 
tinctly recognizable  in  spite  of  the  fact  that  they  especially  have  been 
favored  with  deposits  of  modified  drift.  The  present  drainage,  in  its  main 
lines,  largely  follows  these  older  valleys,  although  much  confusion,  which 
is  especially  noticeable  in  the  details,  has  of  course  resulted  from  their 
partial  choking  by  the  drift. 


334  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

DRAINAGE. 

Nearly  all  the  surface  water  ot"  this  district  finds  its  way  to  Lake 
Michisran  tlu'ouo'h  the  Michiijamme  and  the  Sturcreon  riA'ors,  which  are  inde- 
pendent  bi-anches  of  the  Menominee — the  largest  river  flowing-  into  Lake 
Michigan  from  the  west.  A  few  square  miles  along  the  eastern  boundary, 
however,  are  tributary  to  the  Ford,  which  flows  into  Green  Bay  north  of 
the  Menominee.  Of  these  the  Michigamme  drains  by  far  the  largest  j^art 
of  the  district.  This  river  heads  in  Lake  Michigamme,  which  it  leaves  in 
sec.  9,  T.  47  N.,  R.  30  W.,  near  the  northeast  corner  of  the  area  shown 
on  the  general  map  (PI.  II),  at  an  elevation  above  the  sea  of  1,580  feet. 
Thence  it  flows  for  8  miles  southeast  to  Republic,  in  a  synclinal  valley  cut 
out  of  the  soft  schists  of  the  Michigamme  formation.  This  valley,  which 
is  nearly  a  mile  wide  at  the  northern  end  and  less  than  .half  as  wide  at  the 
southern,  is  bordered  on  both  sides  by  the  harder  Archean  granites,  which 
rise  with  rather  steep  slopes  to  the  general  level  of  the  plain.  Tlu-oughout 
the  length  of  the  valley  the  river  flows  over  glacial  drift,  but  at  Republic, 
where  the  soft  rocks  come  to  an  end,  it  breaks  across  rocky  barriers  in  a 
succession  of  rapids,  and  continues  first  nearly  due  south  (leaving  the  dis- 
trict covered  b}'  our  map),  and  then  flows  southwest  over  glacial  deposits, 
which  completely  mask  the  bed  rock  for  10  miles.  South  of  the  Archean 
oval,  which  occupies  the  western  part  of  T.  44  N.,  R.  31  W.,  and  the  east- 

« 

ern  part  of  T.  44  N.,  R.  32  W.,  the  limestones  and  slates  of  the  pre- 
Cambrian  are  again  exposed,  and  over  these  the  Michigamme  flows  in  close 
conformity  to  the  general  strike  as  far  as  the  range  line. 

In  the  southern  sections  of  T.  44  N.,  R.  31  W.,  the  Michigamme  receives 
two  tributaries  from  the  north — the  Fence  River,  which  comes  from  the 
eastern  side  of  the  Archean  mass  just  mentioned,  and  the  Deer  River,  which 
comes  from  its  Avestern  side.  The  headwaters  of  the  Deer  and  of  the  west- 
ern branch  of  the  Fence  flow  through  the  same  section  (21)  in  T.  46  N., 
R.  32  W.,  north  of  the  Archean  oval,  but  farther  south  they  diverge  to  an 
extreme  distance  of  10  miles,  and  afterwards  converge  so  that  their  points 
of  junction  with  the  Michigamme  are  but  4  miles  apart.  The  area  thus 
inclosed  is  broadly  concentric  with  the  Archean  oval.  In  the  case  of  the 
Fence,  at  least,  the  river  is  placed  within  a  wide  depression  coincident  with 
the  softer  stratified  rocks  of  the  Algoukian,  and  follows  very  faithfully  their 


DRAINAGE.  335 

general  .strike.  Depo.sits  of  glacial  sand  and  gravels  are  very  abundant 
witliin  this  valley,  and  for  these  the  river  often  swings  aside  across  the  strike 
for  a  mile  or  more.  In  sees.  21  and  29,  T.  45  N.,  R.  31  W.,  and  in  sec.  10, 
T.  44  N.,  R.  31  W.,  excellent  rock  sections  are  afforded  by  such  digressions. 

Tlie  old  valley  between  the  two  Archean  ovals  west  of  the  Republic 
tongue  (see  PI.  Ill)  is  on  the  south  entirely  filled  with  glacial  gravels  to 
the  level  of  the  old  divides,  and  the  large  brook  known  as  the  east  branch 
of  the  Fence  is  diverted  to  the  till-covered  western  of  the  two  Archean 
ovals.  The  valley  is  clearly  indicated,  however,  by  an  interesting  series  of 
lakes,  of  which  Squaw,  Trout,  and  Sundog,  each  about  1  mile  in  length, 
are  the  most  considerable. 

The  area  drained  by  the  Sturgeon  lies  in  the  extreme  southeastern  part 
of  the  district,  wholly  within  the  marginal  fringe  of  sandstone.  The  rela- 
tion of  its  course  to  the  geology  is  known  in  detail  only  within  portions  of 
the  Felch  Mountain  range.  This  it  first  enters  in  Ihe  northern  portions  of 
sees.  35  and  36,  T.  42  N.,  R.  30  W.,  in  a  loop  into  the  Algonkian,  from  the 
northern  Archean  margin,  to  which  it  again  returns.  Five  miles  farther 
east  it  crosses  the  trough  from  north  to  south,  transverse  to  the  strike  of  the 
Algonkian  formations,  to  the  contact  with  the  southern  Archean  mass.  It 
follows  this  contact  eastward  for  2  miles,  and  then  strikes  southward  across 
the  Archean  to  the  Menominee  River,  not  again  returning  to  the  Felch 
Mountain  range.  The  river  valley  in  the  Felch  Mountain  range  is  very 
distinct,  and  where  bordered  by  Potsdam  outliers  is  rather  deep,  with  pre- 
cipitous banks.  It  is  but  slightly  affected  by  drift  deposits.  Its  course 
shows  an  almost  complete  disregard  of  the  structure  of  the  Algonkian  and 
Archean  rocks,  and  so  has  the  usual  characters  of  a  superimposed  stream. 

The  Michigamme  River,  as  was  early  noted  by  Pumpelly,  has  practically 
no  eastern  branches  within  this  district.  The  Escanaba  and  Ford  rivers, 
which  reach  Lake  Michigan  directly,  and  the  Sturgeon,  which  joins  the 
Menominee  below  the  mouth  of  the  Michigamme,  all  head  within  2  or  3 
miles  of  the  latter,  the  course  of  which  is  transverse  to  their  general  direc- 
tion. The  Michigamme  thus  flows  along  the  eastern  edge  of  its  drainage 
basin.  This  fact — the  most  striking  in  the  general  distribution  of  the  streams 
of  the  district — is  the  result  of  causes  which,  in  part  at  least,  go  back  to  very 
remote  geological  periods. 


CHAPTER    II.i 
MAGNETIC  OBSERVATIONS  IN  GEOLOGICAL  MAPPING. 

SECTION  I.   INTRODUCTION. 

As  has  been  said  already,  the  area  in  which  our  work  was  done  is 
largely  drift  covered,  to  somewhat  varying  but  usually  considerable  depths; 
the  mantle  on  the  whole  is  so  evenly  spread  that  outcrops  of  any  rocks 
except  those  belonging  to  the  Archean  are  in  many  sections  few  and  scat- 
tered and  sometimes  are  almost  entirely  lacking  over  whole  townships. 

Under  these  circumstances,  and  since  also  the  pre-Cambrian  rock 
structure  is  complex,  even  a  general  outlining  of  the  old  formations  would 
be  impossible  by  the  usual  geological  methods,  and  if  we  were  restricted  to 
these  there  would  be  no  alternative  but  to  map  most  of  the  territory  as 
Pleistocene.  It  happens,  however,  that  the  Algonkian  rocks  of  Michigan 
contain  a  large  amount  of  magnetite,  which  is  known  from  observation  in 
the  developed  ranges  to  be  characteristic  of  certain  geological  formations. 
It  undoubtedly  occurs  in  more  or  less  amount  in  all  the  sedimentary  rocks 
and  is  also  present,  sometimes  in  considerable  quantities,  in  rocks  that  are 
not  sedimentary,  as,  for  example,  around  the  margins  of  the  old  intrusive 
diorite  bosses.  But  generally  speaking,  its  occurrence  in  large  quantities  is 
confined  so  closely  to  definite  geological  formations,  in  which  it  is  found  in 
characteristic  association  with  certain  other  minerals,  or  to  horizons  within 
those  formations,  that  it  can  be  guardedly  used  in  identifymg  them,  and  in 
tracing  them  from  localities  where  they  outcrop  through  areas  in  which  they 
are  buried.  This  use  is  not  only  justified,  from  an  empirical  standpoint,  by 
the  presumption  in  favor  of  analogies  to  which  no  exceptions  are  known,  but 
it  has  a  rational  basis,  in  the  view  of  the  late  Professor  Irving,^  which  is 

'  This  chapter  is  abridged  from  a  paper  of  the  same  title  presented  at  the  Colorado  meeting  of 
the  American  Institute  of  Mining  Engineers  in  September,  1896. 

^  Classificatior  of  early  Cambrian  and  pre-Cambrian  formations,  byR.  D.  Irving:  Seventh  Ann. 
Kept.  U.  S.  Geol.  Survey,  1888,  pp.  451-452. 
336 


MAGNETIC  OBSEUVATIONS.  337 

steadily  j^aiiiinji'  j^rouiul,  tlint  at  least  luucli  of  the  iron  of  this  iiuvgnetite  was 
originally  Iniricd  in  thesanu'  t\tnnations  in  which  it  now  occurs,  through  the 
ageiic}'  of  organic  life.  From  this  point  of  view  the  magnetite  is  in  a  cer- 
tain sense  a  fossil,  but  with  the  important  practical  advantage  over  other 
organic  remains,  that  it  need  not  be  dug  up  in  order  to  prove  its  existence. 

These  magnetite-bearing  rocks  always  produce  disturbances  in  the 
compass-needles  held  in  their  neighborhood.  By  a  systematic  location  and 
comparison  of  these  disturbances  the  position  of  the  rocks  which  produce 
them  can  be  determined  with  a  considerable  degree  of  precision,  even  when 
they  are  deeply  buried.  Besides  their  position  on  the  map,  the  magnetic 
observations  may,  and  often  do,  indicate  certain  other  geologically  impor- 
tant tacts,  such  as  whether  the  rocks  are  flat  lying  or  highly  tilted,  the 
direction  of  strike  and  dip,  and,  in  some  cases,  the  depth  to  which  they  are 
buried.  The  methods  employed  in  the  field  work  were  based  on  those 
described  by  Maj.  T.  B.  Brooks,^  who  perfected  the  dial  compass  and  pre- 
dicted the  importance  of  magnetic  methods  in  geological  mapping;  but  the 
results  reached  in  interpretation  were  gradually  developed  in  the  progress 
of  this  work,  as  we  were  daily  brought  face  to  face  with  phenomena  which 
called  for  explanation. 

It  must  be  clearly  understood  at  the  outset  that  in  the  iron  ranges  of 
the  south  shore  of  Lake  Superior  magnetite  is  i-arely  concentrated  in  large 
bodies,  and  that,  in  fact,  its  known  occurrence  as  such  is  restricted  to  a 
small  part  of  the  western  Marquette  district,  where  in  one  producing  mine 
it  now  forms  practically  the  whole  product  and  in  another  a  variable  but 
usually  important  part  of  the  whole.  It  is  therefore  well  understood  in  the 
Upper  Peninsula  that  disturbances  of  the  magnetic  needle,  however  great, 
do  not  mean  workable  deposits  of  magnetite.  Whatever  significance  such 
disturbances  possess  is  stratigraphical,  and  properly  interpreted  may  lead  to 
discoveries  of  rich  ore,  other  than  magnetite,  in  formations  to  the  position 
and  attitude  of  which  the  attractions  may  furnish  a  clew.  But  it  may  be 
asserted  as  a  general  proposition,  the  essential  truth  of  which  has  been 
established  by  the  experience  of  many  years,  that  in  the  region  referred  to 
magnetic  disturbances  usually  mean  that  magnetic  iron  ore  in  a  workable 
deposit  does  not  exist  in  the  area  of  disturbance. 


'  Geological  Survey  of  Michigan,  Vol.  I,  Part  I,  1873,  Chapter  VII. 
MON   XXXVl 22 


338  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

SECTION  II.   DESCRIPTION"  OF   THE  MAGNETIC   ROCKS. 

The  magnetic  rock  of  the  Lower  Huronian  series  of  the  western  por- 
tion of  the  Marquette  area,  which  is  of  special  importance  to  notice,  since 
it  forms  one  of  the  chief  horizons  of  reference  to  which  our  work  is  tied, 
is  the  Negaunee  iron  formation.  It  is  finely  exposed  at  the  south  end  of 
the  Republic  trough;  but  farther  north  has  been  greatly  reduced  in  thick- 
ness, or  locally  cut  out  altogether,'  by  the  Upper  Marquette  denudation, 
and,  where  present  at  all,  is  usually  drift  covered. 

This  rock  often  possesses  a  very  distinct  banding,  caused  by  the  alter- 
nation of  layers,  in  which  one  of  the  constituent  minerals  predominates 
over  the  others,  sometimes,  indeed,  to  their  total  exclusion.  In  the  lower 
part  of  the  formation,  quartz  and  griinerite  constitute  the  bi;lk  of  the 
rock,  with  magnetite  scattered  somewhat  indiscriminately  through  them. 
Higher  up,  the  magnetite  and  quartz  relatively  increase,  until  near  the  top, 
but  below  the  jasper,  the  griinerite  goes  out  almost  entirely,  and  the  rock 
consists  of  quartz  bands,  heavily  charged  with  magnetite,  in  alternation  with 
bands  of  nearly  pure  magnetite.  In  the  Negaunee  formation,  as  exposed 
at  Republic,  the  magnetite  therefore  occurs  concentrated  in  some  of  the 
parallel  bands  and  disseminated  through  the  others. 

In  the  same  district  there  is  another  much  less  prominent  locus  of  mag- 
netite at  and  near  the  base  of  the  Upper  Marquette  or  "hanging-wall" 
quartzite.  Along  the  strike  of  this  zone,  which  is  of  small  thickness,  the 
distribution  of  magnetite  is  very  irregular;  and  for  this  and  the  additional 
reason  that  when  the  magnetic  portion  of  the  Negaunee  formation  comes 
up  to  it  the  disturbances  which  it  produces  can  not  be  discriminated  from 
those  produced  by  the  latter,  the  position  of  the  plane  usually  can  not  be 
inferred. 

In  the  Menominee  district  and  its  extensions  there  are  two  horizons  in 
the  lower  series,  characterized  by  the  presence  of  magnetite.  The  lower 
of  these  is  not  known  to  outcrop,  but  it  occurs  somewhere  near  the  juiac- 
tion  of  the  dolomite  and  the  underlying  quartzite.  The  magnetic  disturb- 
ances due  to  this  formation  ai'e  feeble,  but  they  are  quite  persistent  in  the 
Felch  Mountain  area,  and  have  thrown  some  light  on  the  geological  structure. 

'  The  Marquette  iron-bearing  district  of  Micliigan,  by  C.  R.  Van  Hise  and  W.  S.  Bayley,  with  a 
chapter  on  the  Republic  trough,  by  H.  L.  Smyth :  Mou.  U.  S.  Geol.  Survey,  No.  XXVIII,  1897,  pp.  531, 537. 


MAGNETIC  OBSERVATIONS.  339 

The  other  formation  which  produces  disturbances  is  that  wliicli  I  liave 
correhited  witli  the  Neg-aunee  formation  and  named  in  a  former  paper^  the 
Michiganmio  jasper,  but  which  is  here  renamed  the  Groveland  formation. 
This  rock,  while  varying  a  great  deal  in  character,  is  generally  much  like 
that  magnetic  phase  of  the  Negaunee  formation  in  which  the  griinerite  is 
rare  or  absent.  From  the  fact  that  it  now  survives  for  the  most  part  only 
in  shallow  and  shattered  synclines,  it  often  lacks  the  regular  banding;  and 
hematite  is  always  present  in  greater  or  less  amount.  The  relative  propor- 
tions of  the  two  iron  minerals  vary  along  the  strike  also.  The  rock  as  a 
whole,  however,  is  very  magnetic,  but  not  so  strongly  so  as  the  Negaunee 
formation  in  the  Republic  trough. 

In  the  Felch  IMountain  rang-e  there  is  still  a  third  magnetic  formation, 
which  seems  to  overlie  unconformably  the  lower  series,  and  is  therefore 
provisionally  assigned  to  the  Upper  Huronian.  This  formation  consists  of 
ferruginous  schists,  interstratified  with  layers  of  ferruginous  fragmental 
quartzite.  It  is  generally  much  less  highly  inclined  than  the  magnetic 
rocks  of  the  lower  series  as  well  as  less  rich  in  iron,  and  the  disturbances 
produced  by  it  are  correspondingly  small. 

Besides  these  rocks  of  sedimentary  origin,  with  which  this  paper  pro})- 
erly  deals,  it  may  be  mentioned  that  along  the  Fence  River  there  is  a 
considerable  area  of  metamorphic  eruptives,  which  are  often  exceedingly 
magnetic.  These  are  restricted  to  a  definite  geological  horizon,  within 
which  the  magnetic  disturbances  are  remarkable  for  their  complexity  and 
irregularity,  no  doubt  as  the  result  of  a  very  irregular  distribution  of 
magnetite  and  of  the  formations  which  chiefly  contain  it.  The  rocks  in 
portions  of  this  belt  outcrop  freely,  and  the  disturbances  can  therefore 
easily  be  assigned  to  the  proper  causes. 

SECTION  III.   THE   DISTKIBUTION   OF  INIAGNETISM  IN  THE  IMAGNETIC 

ROCKS. 

Magnetite  occurs,  therefore,  in  these  Algonkian  rocks  in  different 
ways.  In  some  instances  it  is  mainly  concentrated  in  nearly  pure  parallel 
layers ;  in  othei-s,  it  is  more  or  less  evenly  disseminated  through  non- 
magnetic material;  and  still  again  it  is  present  in  both  forms.     Moreover, 


'Relations  of  the  Lower  Menominee  and  Lower  Marquette  Beries  of  Michigan  (Preliminary), 
by  H.  L.  Smyth:  Am.  .Jour.  Sci.,  Vol.  XLVII,  1894,  pp.  217,  218,  223. 


340  THE  CRYSTAL  PALLS  IRON-BEARING  DISTRICT. 

these  rocks  have  all  been  folded,  more  or  less  strongly,  at  more  than  one 
period ;  and  wherever  they  are  exposed,  they  are  seen  to  be  inclined  to  the 
horizon,  often  at  high  angles,  and  to  be  traversed  by  intersecting  sets  of 
joint-planes  and  cleavage-planes,  some  of  which  always  cut  the  bedding, 
and  often  have  been  the  seat  of  the  development  of  secondary  minerals. 
By  the  crossing  of  these  various  surfaces,  the  rocks  are  divided  into  small 
unit  masses,  at  the  boundaries  of  which  there  is  either  an  actual  physical 
parting  or  a  break  in  the  continuity  of  the  magnetite. 

It  is  well  known  that  when  a  bar  magnet  is  broken  and  the  severed 
ends  are  again  joined,  the  two  pieces  do  not  unite  to  form  one  magnet,  but 
remain  as  two.  It  may  be  conceived,  therefore,  from  the  manner  of  distribu- 
tion of  the  magnetite,  and  the  secondary  partings  existing  in  these  rocks, 
that  their  magnetism  is  seated  in  an  enormous  number  of  small  separate 
magnets,  at  least  one  for  each  of  the  physicallj^  distinct  unit  volumes. 

It  is  a  fact  of  observation,  as  will  appear  hereafter,  that  the  upper 
surfaces  of  these  magnetic  rocks  invariably  attract  the  north  end  of  the 
compass  needles  and,  of  course,  repel  the  south  end.  From  this  it  must  be 
inferred  that  the  small  magnets  are  generally  similarly  oriented,  and  have 
their  north  ends,  which  would  repel  the  north  end  of  the  compass  needle, 
pointing  downward,  and  their  south  ends,  which  attract  it,  pointing  upward. 
As  this  is  the  arrangement  that  would  result  from  induction  from  the  earth's 
magnetism,  it  can  be  (joncluded  further  (as,  of  course,  might  be  assumed) 
that  these  rocks  are  magnetic  from  the  earth's  induction. 

It  is  also  well  known  that  when  sevei'al  bar  magnets  are  joined  in  line 
at  opposite  poles,  the  effect  upon  a  compass  needle  within  the  range  of  influ- 
ence is  nearly  the  same  as  if  the  joined  magnets  were  replaced  by  a  single 
magnet  of  the  combined  length.  For  each  member  of  the  pairs  of  inter- 
mediate poles,  one  attracting  and  the  other  repelling,  is  about  the  same 
distance  away,  and  their  effects  so  balance  each  other.  The  result,  there- 
fore, is  to  leave  one  pole  unchanged  in  position  and  to  remove  the  other  to 
the  end  of  the  last  magnet  added.  If  enough  magnets  are  added,  the  iinal 
result  is  to  carry  the  moving  pole  so  far  away  that  it  has  no  appreciable 
influence  upon  the  needle.  This  is  a  condition  which,  from  the  distribution 
of  the  magnetite  and  the  parting  surfaces  which  run  through  the  magnetic 
rocks,  must  always  be  realized  more  or  less  completely.  It  is  a  necessary 
consequence  of  such  an  ai-rangement  of  the  small  magnets  that,  in  the  case 


MAGNETIC  OBSERVATIONS.  341 

of  a  tliiu  sheet  of  mao-netic  rock  lying  at  a  low  angle  itf  dip,  the  l)urie(l 
north  poles  would  not  be  much  farther  removed  than  the  upper  south  poles, 
and  consequently  the  compass  needle  should  be  relatively  only  slightly  dis- 
turbed.    This  is  j)recisely  what  is  found  to  be  the  ease. 

Thus  there  is  firm  ground  for  the  conception  of  the  magnetic  rocks  as 
made  up  of  sheaves  of  small  magnets,  all  similarly  oriented  in  a  general 
way  and  all  having  their  south  poles  upward  at  or  near  the  rock  surface, 
while  the  effective  north  poles,  by  the  continual  addition  of  similarly  ori- 
ented sheaves  below,  are  carried  down,  when  the  rocks  are  vertical  or  nearly 
so,  to  such  depths  that  their  influence  is  greatly  diminished  or  altogether 
imperceptible.  In  equal  small  areas  the  individual  magnets  are  no  doubt 
of  -very  unequal  number  or  strength.  This  can  be  proved  by  holding  a 
swinging  needle  close  to  the  surface  of  a  magnetic  rock,  shifting  its  position 
without  moving  it  out  of  the  parallel  plane  and  observing  the  changes  in 
the  pointing.  These  are  almost  always  large  and  are  undoubtedly  due  to 
the  variations  in  strength  of  small  areas  of  the  upper  poles.  In  consequence 
of  the  law  of  magnetism,  by  which  the  attraction  (or  repulsion)  varies 
inversely  as  the  square  of  the  distance,  the  areas  immediately  surroiuiding 
tlie  needle  are  very  much  more  important  factors  in  the  resultant  than  those 
farther  removed.  When  the  needle  is  held  higher  up,  or,  what  is  the  same 
thing,  the  magnetic  rock  is  buried,  the  effects  are  much  more  regular,  since 
a  larger  number  of  the  unit  areas  enter  into  the  resultant  with  equal  weight 
due  t<^  equal  distance,  and  the  extremes  of  indi^'idual  ^•ariation  are  lost  in 
the  general  mean.  Since  successive  magnetic  cross  sections  over  bui'ied 
rocks  showr  on  the  whole  a  great  degree  of  regularity,  we  can  finally  con- 
clude that  the  magnetic  force  of  these  rocks  is  seated  in  an  immense,  prac- 
tically an  infinite,  number  of  small  magnets,  which  furnish  free  magnetism 
at  the  upper  and  lower  bounding  surfaces  of  the  magnetic  formation,  and 
that  on  the  average  there  is  about  the  same  number,  of  about  the  same 
aggregate  strength,  or,  in  other  words,  equal  intensity  in  equal  areas  of 
these  surfaces,  if  the  areas  are  taken  large  enough. 

SECTIOIf  IV.  THE  INSTRUMENTS  AND  METHODS  OF  WORK. 

The  instruments  used  in  this  work  are  simple  and  well  known.  The 
dial  compass  is  an  ordinary  compass,  carrying  a  2^-inch  needle  swinging 
inside  a  cii'cle  graduated  to  degrees,  which  is  further  supplied  with  a  grad- 


342  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

uated  hour  circle.  It  is  therefore  a  portable  sundial.  Tlie  gnomon  is  a 
thread,  which  is  attached  at  one  end  to  the  center  of  graduation  of  the  hour 
circle  near  the  rear  sight  and  at  the  other  to  a  point  in  the  forward  sight  so 
taken  that  the  angle  made  by  the  thread  with  the  plane  of  the  hour  circle 
is  equal  to  the  latitude  of  the  place.  When  this  instrument  is  leveled  and 
set  up  in  the  meridian  on  a  sunny  day,  the  thread  will  cast  a  shadow  on  the 
hour  circle  at  the  correct  apparent  solar  time,  from  which  mean  time  may 
be  determined  by  applying  the  equation  of  time.  Conversely,  if  it  is  so  set 
up  that  the  shadow  of  the  thread  falls  on  the  correct  apparent  time,  the 
sights  of  the  instrument  are  in  the  true  meridian.  In  this  position  the 
declination  of  the  horizontal  needle  maj"  be  read  off  from  the  graduated 
circle.  At  work,  this  instrument  is  mounted  on  a  light  Jacob's  staif,  or  it 
may  be  held  in  the  hand.  The  Jacob's  staff,  although  often  inconvenient 
to  carry,  is  preferable,  as  with  it  the  readings  are  all  taken  at  the  same 
height  above  the  ground  and  the  leveling  is  more  exact  and  steady.  In 
a  correctly  constructed  instrument,  Avith  good  time,  the  readings  may  be 
made  to  half  a  degree.  Coirectness  in  the  time,  however,  is  indispensable 
to  good  work,  and  this  is  best  secured  by  keeping  a  standard  watch  in  camp 
and  referring  the  working  watches  to  it  daily. 

The  dip  needle  needs  no  description.  In  geological  work  that  form 
known  as  the  Norwegian,  in  which  the  needle  is  pivoted  on  a  universal 
joint,  is  not  so  useful  as  the  type  in  which  the  needle  is  rigidly  confined  to 
one  plane.  In  taking  the  readings,  this  plane  in  which  the  needle  is  free 
to  swing  is  made  to  coincide  with  the  vertical  plane  determined  by  the 
pointing  of  the  horizontal  needle.  The  circle  is  graduated  to  single 
degrees,  and  with  skillful  work  the  readings  ai'e  reliable  to  one  or  two 
degrees.  It  ma}-  be  added  that  the  south  end  is  weighted,  in  order,  either 
partly  or  completely,  to  balance  the  vertical  component  of  the  earth's  force. 
It  was  found  better  not  to  balance  it  completely,  but  only  to  such  an  extent 
that  the  north  end  of  the  needle  would  dip  about  10°  (the  graduation  zero 
being  horizontal)  in  an  area  removed  from  local  disturbances.  It  is  no 
doubt  desirable  that  all  the  dip  needles  used  in  the  same  work  should  be 
brought  to  approximately  the  same  index  error,  in  order  that  the  readings 
may  be  more  directly  comparable.  In  practice,  however,  it  was  found  quite 
impossible  to  keep  our  three  needles  in  unison,  on  account  of  the  rough 
usage  to  which  they  were  unavoidably  subjected.     As,  however,  the  form 


MA(}NET1C  015SEKVATI0NS.  343 

of  the  (lip  curves  is  veiil.y  the  subjeet  souyht,  and  since  these,  in  the  pres- 
ence of  consideraljle  disturbances,  are  sensiljly  indejjendent  of  .small  differ- 
ences in  the  index  error,  it  is  not  indispensable  that  the  needles  should  be 
exactly  together. 

These  instruments  are  simple,  and,  of  course,  do  not  give  precise  results. 
But  the  observations  are  rapidl}'  and  chea])ly  made,  and  to  a  sufficient 
degree  of  acctn-acy  for  the  end  in  view.  It  may  be  stated  again  that  the 
object  is  to  detect  and  compare  relative  magnetic  disturbances,  and  to  find 
out  the  bearing  of  these  disturbances  on  the  distribution  and  attitude  of  the 
rocks  which  produce  them.  For  this  purpose  the  instrmnents  are  exceed- 
ingly well  adapted. 

The  field  work  was  canned  out  by  parties  of  two  men  each,  one  of 
whom,  a  skilled  woodsman,  carried  along  the  line  and  observed  the  hori- 
zontal needle,  while  the  other  read  the  dip  needle,  kept  the  notes,  and 
atteiided  to  the  geology.  According  to  the  general  plan  of  the  field  work, 
a  series  of  parallel  lines  was  run  either  north  and  south  or  east  and  west 
across  each  section.  The  direction  of  the  lines  of  travel  was  chosen  so  as 
to  cut  the  strike  of  the  rocks  at  the  largest  angle.  The  jn'obable  direction 
of  strike  for  each  day's  work  could  be  inferred  in  advance  from  what  had 
ji'one  before.  If  it  were  more  nearly  north  and  south  than  east  and  west, 
the  traverse  lines  were  run  east  and  west,  and  vice  versa.  These  directions 
were  in  many  cases  not  the  most  desirable  for  the  magnetic  work  alone,  but 
the  choice  was  practically  limited  by  the  lines  of  the  United  States  Land 
Survey,  which  give  for  each  square  mile  eight  points  of  departure  (at  the 
four  corners  and  four  quarter  posts  of  each  section),  which  are  generally 
identifiable  on  the  ground.  On  these  lines  of  travel  the  instruments  were 
read  at  various  intervals,  from  6  to  10  or  100  paces,  depending  upon  the 
local  complications.  The  intervals  Ijetween  the  lines  varied  from  one- 
sixteenth  to  one-fourth  of  a  mile,  and  were  determined  not  onl)'  by  the 
magnetic  complications,  but  by  the  character  of  the  surface,  it  being 
especially  desirable  that  the  ground  should  be  so  closely  covered  that  no 
outcrop  could  escape  detection.  The  distances  along  and  off  the  lines  of 
travel  were  measured  by  pacing.  The  general  accuracy  of  the  pacing  is 
remarkable,  and  is  essentially  within  the  platting  error  of  the  scale  of  the 
maps.  The  aA^erage  closing  error  for  August,  1892,  during  which  about 
100  miles'  of  traverse  lines  were  run,  was  20  paces  per  mile,  or  1  per  cent. 


344  THE  CRYSTAL  PALLS  IRON-BEARING  DISTRICT. 

Two-thirds  of  tlie  errors  averaged  10  paces  per  mile,  or  1  in  200,  while  the 
maximum  was  1  in  30.  But  this  was  better  thau  the  average  for  the  season. 
The  observations  at  each  station  consisted  in  a  reading  of  the  horizon- 
tal and  dip  needles.  When  there  was  no  local  magnetic  disturbance,  the 
horizontal  needle  would  come  to  rest  in  the  magnetic  meridian,  which  in 
this  region  is  about  N.  2°  to  3°  E.,  or  almost  coinciding  with  the  true  merid 
ian.  The  dip  needle,  when  held  in  the  same  meridian,  would  indicate  the 
index  error.  When,  however,  disturbing  mateiial  was  present,  the  horizon- 
tal needle  would  point  to  the  east  or  west  of  the  magnetic  meridian,  at  an 
angle  determined  by  the  direction  of  the  resultant  of  the  horizontal  com- 
ponents of  the  earth's  and  the  local  forces.  The  dip  needle  would  come  to 
rest  in  the  same  vertical  plane,  at  an  angle  with  the  horizon  determined  by 
the  amount  and  direction  of  the  three  forces,  the  whole  pull  of  the  earth's 
force,  the  whole  pull  of  the  local  forces,  and  the  balancing  weight,  and  in 
general  would  show  a  downward  deflection.  After  making  and  recording 
the  set  of  observations  at  a  station,  the  party  proceeded  to  the  next,  and  so 
on  to  the  end  of  the  day.  At  the  end  of  each  day,  or  as  soon  as  possible 
afterwards,  the  day's  work  was  jjlatted  on  a  large-scale  map,  on  which  the 
readings  of  the  horizontal  needle  were  represented  by  short  arrows  drawn 
through  the  stations,  turned  east  or  west  of  the  true  meridian,  as  the  case 
might  be,  and  carrying  the  amount  of  declination  written  at  the  arrow 
point.  The  dij)  observations  were  laid  off  to  scale  immediately  below  the 
stations,  measuring  all  from  the  same  horizontal  line,  and  the  })oints  thus 
established  were  connected  by  a  free-hand  curve. 

8ECTIOX  V.     FACTS  OF  OBSERVATION  AXD  GET^ERAIj  PRINCIPI.ES. 

I.    OBSERVED    DEFLECTIONS  WHEN    THE    STRIKE    IS    NORTH   AND    SOUTH  AND 

THE  DIP  VERTICAL. 

If  a  magnetic  rock,  striking  north  and  south  and  dipping  vertically,  is 
crossed  by  an  east-and-west  traverse,  it  is  found,  as  the  disturbing  belt  is 
approached,  say  from  the  western  side,  that  the  horizontal  needle  points 
toward  the  east  of  north,  and  that  this  easterly  pointing  gradually 
increases  to  a  maximum.  Continuing  east  from  the  maximum  point  the 
eastward  declination  decreases,  and  soon  a  station  is  reached  at  which  the 
needle  points  due  north.  Still  farther  east  the  declination  changes  to  west- 
ward, and   soon  thereafter  reaches   a  westward  maximum,  beyond  which 


MAGNETIC  OHSERVATIOTSrS.  345 

again  tlio  westward  pointing  in  its  turn  gradually  decreases,  until  finally 
the  needle  reaches  its  normal  eastward  declination,  after  passing  through  a 
second  zero.  The  dip-needle  readings  at  the  same  stations  generally 
mcrease  slowly  at  first,  and  then  rapidly,  and  soon  reach  a  maximum  at  the 
first  zero  point  between  the  converging  arrows ;  beyond  this  to  tlie  east  they 
decrease  corresj)ondingly,  so  that  the  dip  curve  is  symmetrical  east  and  west 
of  the  maximum  These  statements  will  l)e  made  clear  bv  a  reference  to 
fig.  15,  which  represents  an  actual  traverse  in  T.  45  N.,  R.  31  W. 

2.  DEFLECTIONS  OF  THE  HORIZONTAL  NEEDLE. 

It  is  evident  that  in  crossing-  a  rock  belt  which  stretches  away  indefi- 
nitely in  both  directions,  only  a  limited  part  of  it  will  affect  the  readings  on 

»  10       10!^      II        IS         15         14        II       8K        0  13         7        93<        II      13%     ISii      13     11>J     ■0>i 


_b:p  Cui  V 


.-^ 


Fig.  15.— Magnetic  cross  .sectiiin  in  T.  45  N.,  R.  :il  W. 

a  given  cross  section.  Since  the  })ull  of  the  poles  of  a  magnet  on  a  com- 
pass needle  diminishes  with  the  square  of  the  distance  of  separation,  it 
follows  that  the  limits  to  the  material  that  would  noticeabl}'  disturb  com- 
paratively insensitive  instruments  would  soon  be  reached.  If  we  consider 
for  the  moment  only  the  horizontal  components,  and  call  the  distance  a 
(fig.  16)  at  which  the  needle  would  respond  to  the  attraction  of  material 
possessing  the  magnetic  force  of  that  with  which  we  are  dealing,  then  at 
any  station,  P,  the  material  inside  a  circle  di'awn  with  P  as  a  center  and 
radius  a  (shaded  in  the  figure)  would  exert  force  on  P,  the  material  outside 
would  not.  If  the  circle  drawn  from  a  station,  P',  does  not  reach  tii  the 
magnetic  belt,  the  needles  at  P'  will  not  be  disturbed.^ 

For  reasons  of  symmetry,  it  is  seen  that  the  attraction  of  the  magnetic 

'  The  actual  distances  at  which  disturl)<atues  of  the  Deedles  can  be  detected  are  exceedingly 
variable,  since  they  depend  (as  will  be  shown  hereuftei')  not  only  upon  the  lithological  character  of 
the  magnetic  formation,  but  also  upon  its  strike,  dip,  thickness,  extent,  and  nearness  to  the  .surface. 
One  formation  in  which  all  the  conditions  are  exceptionally  favorable  distinctly  deflects  the  dial- 
compass  needle  at  a  distance  of  3^  miles. 


346 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 


belt  would  act  along  the  line  P  N,  drawn  through  P  perpendicular  to  the 
strike  of  the  rock.  Since  there  is  as  much  material  on  one  side  of  this 
line  as  on  the  other,  the  components  perpendicular  to  it  will  balance  each 
other,  and  the  instruments  at  P  will  be  affected  exactly  as  if  all  the  attract- 
ing material  were  concentrated  along  this  line.  The  horizontal  needle  will 
take  a  position  in  the  line  of  the  resultant  of  the  two  forces  which  act  upon 
it,  namely,  the  horizontal  component  of  the  earth's  magnetism  (which  acts 
in  the  line  of  the  magnetic  meridian)  and  the  horizontal  component  of  the 
material  within  ths  circle  of  attraction  (which  acts  along  the  line  P  N). 

The  force  which  de- 
flects the  needle  from 
its  general  local  direc- 
tion is  the  component 
along  P  N,  and  it  is 
evident  that  the  great- 
er this  component  the 
greater  will  be  the  de- 
flection of  the  needle, 
since  the  direction  in 
which  it  acts  always 
remains  the  same  at 
all  stations  for  any 
given  direction  of 
strike  of  the  rock. 
For  if  ft  is  the  strike 
of  the  rock  measured 
from  the  north,  and  H  and  H'  are  the  horizontal  components  of  the  earth's 
and  the  rock  force  respectively,  it  is  readily  shown  that  5,  the  angle  of  deflec- 
tion of  the  horizontal  needle  at  any  station,  P,  is  given  by  the  equation: 


Fig.  10.— Circli-s  of  attraction. 


tan  d  rr 


COS  /? 


H 


(1) 


E 


^  ±  sin  /? 


From  this  equation  it  is  easily  seen  that,  no  matter  how  great  may  be 
the  horizontal  component  of  the  force  of  the  magnetic  rock,  the  horizontal 
needle  can  not  be  deflected  past  the  normal. 


MAGNETIC  OHSKRVATIONS.  347 

.  As  1'  moves  towM-iI  the  iiiiiyiietic  belt  the  liorizoiital  coinponent  ut  first 
increases,  nnd  with  it  the  westward  deflection  of  the  needle.  Finally,  the 
niaximuin  westward  deflection  is  reached,  beyond  which  tlie  needle  begins  to 
return;  it  is  evident,  therefore,  that  at  this  point  the  horizontal  component 
has  reached  a  maximum  value. 

3.  DEFLECTIONS  OF   THE  DIP  NEEDLE. 

The  balanced  dip  needle  (i.  e.,  without  index  error),  in  an  area  of 
no  local  disturbance,  is  in  equilibrium  under  the  action  of  two  couples, 
namely,  the  vertical  component  of  the 
earth's  magnetism  and  the  added  weight. 
When  displaced  from  the  position  of  __  »*-^ — 
equilibrium,  the  horizontal  couple  re- 
stores it. 

In  fig.  17  let  PP  be  a  balanced  Fig.  n.-The  rcees acing o„  ti,e  .n,, «ee<iie. 

dip  needle  which  has   been  displaced 

through  the  angle  a.  At  the  two  poles  the  attraction  and  repulsion  of  the 
earth's  magnetism  may  be  resolved  into  horizontal  and  vertical  components, 
H  and  V. 

Taking  moments  about  C,  we  have,  if  «=zO,  the  needle  in  equilibrium 
under  the  couples, 

V.  2h  —  m(/.  a  =  0, 

where  2&rrPP,  mg^the  added  weight,  and  a  its  distance  from  the  center. 

If  this  needle,  so  balanced,  is  carried  to  a  station  within  the  influence 
of  a  magnetic  rock,  its  dip  will  be  determined  by  the  composition  of  the 
new  forces  with  the  old.  The  vertical  plane  will  be  that  in  which  the  hori- 
zontal needle  points  at  the  same  station.  The  equations  above  give  us  a 
ready  means  of  determining-  the  angle  of  dip  in  terms  of  all  the  forces. 

Suppose  the  needle  finally  comes  to  rest  at  the  angle  a  with  the  hori- 
zontal (the  north  pole  being  depressed).     Then 

V,..  2h  .  cos  a  —  mga  cos  a  — H^ .  26  .  sin  az=0,       .      .     .      (2) 

where  H^  and  V^  signify  the  resultants  of  the  horizontal,  and  vertical  com- 
ponents of  the  earth's  and  the  local  force. 


348  THE  CRYSTAL  FALLS  IRON-BEAEING  DISTRICT. 

Equation  (2)  is  easily  reduced  to 

—'-^h!^''      ■■....-     (3) 

If,  however,  the  dip  needle  is  not  balanced,  but  has,  where  there  is  no 
local  disturbance,  a  constant  index  error  0  (measured  from  the  horizontal), 
it  is  readily  seen  that 

In  an  area  of  local  disturbance  the  angle  of  dip  a  is  given  by  equation 
(3).     Since  V  and  V  always  act  in  the  same  line. 

Substituting  this  and  the  value  of  mga  fi-om  equation  (4),  equation  (3) 
becomes 

±V'  +  Htan0,  .,. 

tan  a  =.-^ U== '- {p) 

If  the  index  error  is  0' ,  and  the  corresponding  deflection  «',  we  have 

tan  a  _±  V  +  H  tan  0 
tana'"~±V'  +  H  tan  & 

Therefore  at  the  same  station,  the  greater  the  index  error  the  greater 
is  the  angle  of  dip  in  the  same  or  two  similar  instruments.  It  is  also  evi- 
dent that  the  greater  the  vertical  component  of  the  pull  of  the  rock,  the 
less  will  be  the  difference  between  the  deflections  in  the  two  cases. 

From  an  inspection  of  equation  (5)  it  is  seen  tiiat  tan  a  — qo,  or 
a:zi90°  only  when  H,.— 0.     H^  is,  in  general,  given  by  the  equation 


H,=  VH^+H''±2  H  H'  sin  ^, 
where  /?  is  the  strike  of  the  rock  measured  from  tlie  north.      H,.  can  there- 
fore equal  zero  only  when  /?=^,  or  the  rock  strikes  east  and  west,  and  at 

the  same  time  H'.  is  numerically  equal  to  H,  and  acting  in  the  opposite 
direction.     Dips  of  90°  can  not  occur  in  other  cases,  no  matter  how  strong 


MAGNETIO  OBSERVATIONS.  319 

the  magiu'tic  force  ot  tlie  rock  iiiay  ho.  It  is  also  evident  that  iu  <>eiieral 
Hr  has  its  iniiiimum  vahie  when  H':=  —  H  sin  /?.  When  the  rock  strikes 
north  and  south  or  /?rr(),  11^  is  a  minimum  when  H':rrO. 

4.    HORIZONTAL    AND    VERTICAL    COMPONENTS    WHEN    THE    MAGNETIC    ROCK 

DIPS   VERTICALLY. 

If  we  assume  that  the  magnetic  rock  has  a  uniform  strike  in  any  direc- 
tion, a  vertical  dip  and  a  surface  width  or  thickness  equal  to  2a,  it  is  easy  to 
show  that  the  horizontal  and  vertical  components  of  the  rock  force  are  given 
by  the  following  equations,  where  x  is  the  horizontal  distance  of  the  station 
of  observation  from  the  middle  plane  of  the  formation,  h  is  the  depth  of 
surface  covering,  assumed  to  be  uniform,  and  a?  is  a  constant. 

GO  -'^^li^J^(x-af ^^ 

^'-'>  ^tan->^±^-tan-i'-^^' J (7) 


GO 


TT/ 

In  equation  (6 )  —  rr  0  when  ;r  —  0 ;    therefore  a  point  of  no  deflec- 

GD 

tion  of  the  horizontal  needle  is  found  vertically  over  the  middle  jjoiiit  of  the 
magnetic  rock.  It  is  also  evident  that  at  corresponding  stations  on  opposite 
sides  of  the  middle  point,  the  horizontal  components  are  equal,  but  act  in 
opposite  directions. 

To  obtain  the  points  of  maximum  or  minimum  values  of  the  horizontal 
component,  we  differentiate  the  right-hand  side  of  equation  (6)  with  respect 
to  X,  and  place  the  result  equal  to  zero.     This  gives 

irrr±V'F+^2 (8) 

which  determines  two  points,  at  equal  distances  from  0  on  opposite  sides  of 
the  rock,  at  which  the  horizontal  component  has  maximum  values.  Writing 
for  X  the  measurable  distance  d,  and  squaring,  we  have 

d''=}i'+a^ (9) 

The  thickness  of  the  magnetic  formation  is  therefore  always  less  than 
the  distance  between  the  points  of  maximum  horizontal  deflection,  except 
when  7irzO,  or  the  rock  is  uncovered,  in  which  case  the  thickness  and  sepa- 
ration of  the  maxima  are  the  same. 


350  THE  CEYSTAL  FALLS  IRON  BEARING  DISTRICT. 

By  differentiating  tlie  right-hand  side  of  (7),  with  respect  to  x,  it  is 
easy  to  show  that  V  has  a  maximum  value  when  .czrO.  When  the  rock 
strikes  north  and  south,  this  also  corresponds  to  a  minimum  value  of  H„  as 
has  already  been  shown;  and,  therefore,  by  a  reference  to  equation  (5)  it 
is  readily  seen  that  a  point  of  maximum  dip  coinciding  with  a  point  of  no 
horizontal  deflection  is  in  that  case  found  over  the  middle  plane  of  the 
bui'ied  magnetic  rock. 

Where  the  strike  is  inclined  to  the  meridian,  the  points  of  maximum 
dip  and  zero  deflection  will  not  coincide,  since  the  maximum  value  of  V 
does  not  occur  at  the  same  station  as  the  minimum  value  of  H^.  As  has 
already  been  shown,  H^  is  a  minimum  when  H'=H  sin  fi  (/?  being  the  ang-le 
of  the  strike),  and  this  is  in  general  on  the  side  of  the  rock  on  which  the 
angle  made  with  an  east  and  west  traverse  is  obtuse.  The  point  of  maximum 
dip  will  be  situated  on  the  same  side  of  the  rock  between  this  station  and 
the  point  of  no  horizontal  deflection,  and  will  approach  the  latter  as  the 
strike  approached  the  meridian,  and  also  as  V  increases  relatively  to  H'. 
With  strongly  magnetic  rocks  the  points  of  no  deflection  and  maximum  dip 
practically  coincide  on  maps  platted  to  the  scale  of  4  inches  to  the  mile, 
except  where  the  strike  is  nearly  east  and  west. 

5.    HORIZONTAL    AND    VERTICAL    COMPONENTS    WHEN    THE    MAGNETIC    ROCK 

DIPS  AT  AN  ANGLE. 

Under  the  last  heading  it  was  assumed  that  the  magnetic  rock  dips 
vertically,  and  that  it  continues  indefinitely  downward  at  this  angle;  In 
consequence  of  this  assumption,  and  also  of  the  conception  of  the  manner 
in  which  magnetism  is  distributed  through  magnetic  rocks,  it  has  been  con- 
cluded that  the  north  poles  of  the  rock,  which  repel  the  north  end  of  the 
compass  needle,  are  situated  so  far  below  the  surface  that  their  effect  may 
be  neglected.  Therefore  we  have  taken  into  account  only  the  south  poles, 
which  are  situated  at  the  rock  surface. 

In  the  case  of  rocks  which  do  not  dip  at  high  angles  this  assumption 
can  not  safely  be  made,  and  the  influence  of  the  bottom  poles  must  be 
taken  into  account.  Since  the  force  of  these  poles  acts  in  opposite  direc- 
tions from  that  of  the  upper  poles,  and  since  they  are  more  deeply  buried, 
it  would  seem  that  their  influence  in  general  must  be  to  diminish  the  total 
force  which  acts  upon  the  needles  at  any  station,  and  therefore  that  the 


MAGNETIC  OBSERVATIONS,  351 

deflections  botli  of  tlie  horizontal  and  dip  needles  caused  by  the  same  rock 
should  be  less  in  amount,  ceteris  paribus,  where  that  rock  dips  at  a  low 
ang-le  than  wliere  it  dips  at  a  high  angle. 

In  tlie  course  of  the  field  work  certain  peculiar  deflections  of  the 
needles  were  encountered  in  traverses  across  rocks  dipping  at  moderate  or 
low  angles.  These  were  not  thoroughly  understood  at  the  time,  but  the 
cause  was  believed  to  be  connected  with  the  angle  of  dip  of  the  rock.  For 
example,  it  was  found  along  traverses  crossing  certain  north-and-south- 
striking  rocks,  which  were  known  to  have  a  westward  dip  that  may  have 
been  either  high  or  low,  that  the  two  points  of  maximum  deflection  of  the 
horizontal  needle  were  not  situated  at  equal  distances  from  the  point  of  no 
deflection  between  them,  but  that  the  distance  of  the  western  maximum 
was  much  the  shorter.  It  happens  in  this  region  that  no  east-dipping  rocks 
occur  which  are  so  far  removed  from  other  magnetic  formations  as  to  be  out 
of  range  of  their  possible  influence,  but,  so  far  as  they  go,  traverses  across 
these  showed  that  the  nearer  maximum  was  situated  on  the  eastern  side  of  the 
point  of  no  deflection.  It  therefore  seemed  probable  that  the  cause  of  the 
inequality  in  the  distances  from  the  zero  point  to  the  maxima  was  the  dip 
of  the  rock,  and  that  the  dip  was  in  the  direction  of  the  nearer  maximum. 

If  the  magnetic  formation  has  a  surface  width  nz  b,  is  uniformly  buried 
to  the  depth  h,  and  dips  at  the  angle  ^,  then,  if  A  —  tan  z/,  it  may  be 
shown  that  the  horizontal  and  vertical  components  at  any  point  P,  the  hori- 
zontal distance  of  which  from  the  lower  edge  of  the  formation  is  x,  are 
given  by  the  following  equations: 

H' _    \'     ,  h'+x-  2  A 

S  4.     -1  x—h  —  Xh       ,        ,x  —  Xh 
<  tan  '- — — =-  —  tan^ 


h  > 
Xx—Xl)  +  h  Xx+li] ^^^^ 

-1 


— -  =2  j  tan-i^-tan-i^ 
w  (  h  h 

V+x""       .       2 


+  i-nr2lo&i 


tan-Kli^tML±^)_tan-  i+ii^  [.     .     .  (11) 
A-(l+u;)  Ji—XJjj^x  x) 


352  THE  CRYSTAL  FALLS  IRON -BEARING  DISTRICT. 

If  A:iicc> ,  and  the  coordinates  are  referred  to  axes  in  the  middle  of  the 
rock,  these  equations  reduce  to  equations  (6)  and  (7). 

By  differentiating  the  right-hand  side  of  equation  (10),  placing  the 
result  equal  to  zero,  and  solving  for  x,  the  positions  of  the  stations  at  which 
H'  is  a  maximum  may  be  determined.     This  gives: 

■^-"^^=^ 2l ^  ^^ 

Calling  the  difference  of  the  roots,  or  the  measurable  distance  between 

2a 
the  maxima,  2d,  and  substituting  for  h  its  value  -■ — -7-  2a  being  the  true 

thickness  of  the  rock,  we  have: 

d2_^jL+^' ■.     .     .     .     (13) 

sm  ~/l 

For  rocks  of  high  dip,  therefore,  the  distance  between  the  maximum 
points  is  but  little  greater  than  it  would  be  were  the  dip  vertical,  and  it 
increases  inversely  as  the  angle  of  dip. 

A  general  algebraic  determination  of  the  points  at  which  H'  is  0  and 
V  is  a  maximum  is  impossible,  since  it  involves  the  solution  of  equations  of 
a  degree  higher  tlian  the  fifth.     However,  hj  assuming  numerical  values 

XT'  Y' 

for  A,  h,  and  a  (or  li)  curves  expressing  the  relations  between  —  and  —  and 

GO  GO 

X  can  be  plotted,  from  which  the  maximum  and  zero  points  can  be  deter- 
mined in  any  desired  number  of  special  cases. 

Let  us  first  assume  that  A  zr  3  (or  that  the  rock  dips  at  an  angle  of  about 
70°  34'),  I1  —  2,  and  a  — 6.    The  ordinates  to  the  curves  of  fig.  1,  PI.  XLVII, 

TI/  Y' 

give  the  values  of  —  and  —  corresponding  to  different  values  of  x.     The 

GO  GO 

ordinates  to  —  do  not  reijresent  the  deflections  S  of  the  horizontal  needle 

GO 

from  the  meridian,  but  quantities  that  are  connected  with  those  deflections 
by  equation  (1).  The  deflections,  however,  vary  as  H'  varies,  and  will  have 
maximum  and  minimum  values  at  the  same  points. 

From  this  figure  it  appears,  first,  that  the  nearer  maximum  is  situated 
on  the  dip  side  of  the  rock;  secondly,  that  the  point  of  no  deflection  is  not 
over  the  middle  plane,  but  is  nearer  the  upper  edge;  thirdly,  that  the  hori- 
zontal force  of  the  rock  is  numerically  less  at  the  nearer  than  at  the  more 


U.    6.    OeOLOGlCAL   SUXVE^ 


MONOGRAPH  XXXVI      PL.    XLVII 


(^) 


(^) 


\, 


RELATIONS   OF   MAGNETIC    BEDS   TO   VARIATION    AND    DIP. 


MAGNETIC  OBSERVATIONS.  353 

distant  m;ixinium,  and,  fourthly,  that  tlie  distance  between  the  maximum 
points  is  nearly  the  same  for  the  inclined  I'ock  as  for  the  vertical. 

In  I'l.  XLVII,  fi<>-.  2,  the  constants  have  the  same  numerical  values  as 
before,  e.\ce]it  //,  which  now  zr4  instead  of  2.  The  rock  is  thus  bm-ied  to 
twice  the  depth  of  the  former  case.  The  same  conclusions  are  true  for  this 
case  as  for  the  first.  The  zero  point  is  still  nearer  the  upper  edge  of  the 
rock,  and  the  maxima  are  farther  apart. 

Let  it  next  be  assumed  that  A  =  0.5  (or  that  the  rock  dips  at  an  angle 
of  al)out  26°  34'),  /(rr2,  and  (i-^6.  These  data  lead  to  the  curves  of 
PI.  XLVII,  fig.  3,  in  which,  as  in  the  case  of  the  rock  ot  higher  dip,  the 
maximum  })oints  are  unsymmetrical  to  the  }joint  of  no  deflection,  the  nearer 
lying  on  the  dip  side. 

In  PI.  XLVII,  fig.  4,  we  have  a  rock  of  the  same  thickness  and  dip  at 
a  depth  /;^4;  and  the  same  conclusions  hold  true. 

From  these  four  curves,  which  represent  formations  dipping  at  high 
and  moderately  low  angles,  and  buried  to  depths  which  are  in  the  one  case 
small  and  in  tlie  other  great,  relative  to  the  thickness,  it  is  probably  safe 
to  draw  the  following  general  conclusions: 

(«)  The  direction  of  dip  of  a  magnetic  formation  is  toward  the  nearer 
and  (for  north-and-south-striking  rocks)  the  numerically  smaller  maximum. 

(li)  The  point  of  no  deflection  between  the  converging  maxima  is  not 
situated  over  the  middle  plane  of  the  formation,  but  is  nearer  the  upper 
edge.  But  with  increasing  depth  and  diminishing  angles  of  dip,  this  point 
may  pass  beyond  the  upper  edge. 

(c)  With  slightly  inclined  rocks,  for  moderate  deptlis  of  siu-face  cover- 
ing, the  disturbances  are  spread  out  over  a  much  wider  zone  on  each  side, 
and  the  maxima  are  less  sharp,  particularly  the  maximum  on  the  dip  side. 
Under  these  'circumstances  irregular  and  anomalous  deflections  would  be 
expected  in  practice,  as  will  be  seen  in  the  following  sections. 

(d)  The  curves  of  the  vertical  component  show  maximum  values  near 
the  zero  value  of  the  horizontal  component  only  in  the  case  of  the  rock  of 
high  dip.  In  the  case  of  the  rock  of  lower  dip,  the  vertical  component  has 
a  negative  value,  or  is  directed  upward  over  a  wide  zone  on  the  side  of  the 
rock  opposite  to  the  dip  side.  Over  this  zone  the  readings  of  the  dip  needle 
will  be  less  than  normal,  or  even  negative  if  V  >  H  tan  9.  This  is  in 
accordance  with  the  facts  of  observation. 

MON  XXXVI 23 


354 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 


It  is  also  interesting-  to  determine  the  relative  values  of  the  horizontal 
components  at  the  two  maximum  points,  for  different  angles  of  dip,  and  for 
different  ratios  of  /;  to  a — that  is,  for  different  depths  of  burial.  Fig.  18 
shows  in  graphical  form  these  relations  for  all  angles  of  dip  between  90° 

and  0°  for  seven  different  ratios  between  h  ^  ^7.  and  h  =z  4«.     The  ordinates 

50 

to  the  dotted  curves  (concave  upward)  give  the  relative  values  of  the  hori- 
zontal component  at  the  maximum  point  on  the  dip  side  (B  maximum); 
those  to  the  full  cur^'es  (convex  upward)  the  values  at  the  maximum  point 


90^  80"  70J  60°  50°  40°  30°  20o  IQo  0° 

Flo.  18.— Curves  showing  the  relations  between  the  horizontal  components  at  the  points  of  luaximum  deliectioo,  lor  rocks 

dipping  at  various  angles  and  buried  to  various  depths. 

on  the  other  side  of  the  rock  (A  maximum).  These  curves  show  that  the 
horizontal  component  at  the  B  maximum  is  always  numericallv  much  less 
than  at  the  A  maximum,  and  that  for  moderate  depths  of  burial  it  diminishes 
very  rapidly  at  both  points  with  small  ang-les  of  di]). 

6.   DETERMINATION  OF  DEPTH. 

The  relations  between  the  dip  and  thickness  of  a  magnetic  rock,  the 
distance  between  the  horizontal  maxima,  and  the  depth  of  covering  are 
given  in  equation  (13).  By  assuming  numerical  values  for  ^,  and  either 
for  a,  the  half  thickness  of  the  rock,  or  for  r/,  the  half  distance  between  the 
maximum  points,  it  is  easy  to  plot  the  curve  which  expresses  the  relations 


ma(ini<:tic  observations.  355 

between  the  other  two  quantities.  It'  d  is  taken  as  tlie  constant,  the  equa- 
tion re})reseuts  a  circle;   it'  (i,  it  represents  a  hyperbola. 

This  e(iuati(in  wvax  have  a  useful  application  in  making  it  possible  to 
judge,  in  advance  of  actual  test  pitting,  of  the  probable  depth  of  surface 
covering  over  a  magnetic  rock,  for  which  the  original  assumptions  are  ful- 
filled, and  the  numerical  values  of  ^,  «,  and  d  are  determinable.  It  will  be 
remembered  that  the  assumptions  upon  which  equation  (13)  rests  are  the 
fundamental  ones  of  Section  III,  and  also  that  the  rock  has  a  uniform 
strike.  In  practice,  the  uniformity  of  the  strike  can  be  established  Ijy  other 
traverses  on  each  side  of  the  one  in  question,  and  J,  the  angle  of  dip,  may 
usually  be  determined  by  the  outcrop  of  other  formations  in  the  same 
series. 

For  any  jjractical  application  it  is  also  necessary  that  a  (half  the 
thickness  of  the  rock)  and  d  (half  the  distance  between  the  stations  at 
which  the  horizontal  component  is  a  maximum)  should  be  known.  From 
an  inspection  of  the  equation  it  is  evident  that  any  close  determination  of 
h,  except  for  great  depths  of  covering,  depends  upon  very  precise  knowl- 
edge of  the  ratio  between  a  and  d.  The  practical  difficulties  in  the  way  of 
the  measurement  of  a  and  the  ever-present  probability  that  the  rock  may 
vary  from  point  to  point,  not  only  in  actual  but  in  effective  magnetic  thick- 
ness (which  is  what  a  actually  signifies),  make  it  clear  that  for  the  most 
part  h  can  only  be  found  approximately.  Also,  in  the  case  of  a  rock 
striking  due  east  and  Avest  the  methods  fail,  from  the  fact  that  2d  can  not  be 
determined  on  the  ground. 

The  determination  of  h  is  therefore  hedged  in  with  important  limita- 
tions; yet  in  many  cases  the  information  supplied  by  the  equation  may  be 
very  useful.  The  difficulties  in  the  way  of  measuring  a  are  disposed  of  in 
the  event  that  along  one  traverse  on  the  strike  of  the  rock  li  is  known,  as  it 
may  be,  by  the  sinking  of  a  test  pit.  This  value  of  li  at  once  gives  a  value 
of  a,  which  may  be  used  on  other  traverses  across  the  same  rock  with  much 
more  accurate  results,  in  the  lack  of  disturbing  factors,  than  if  a  were 
known  only  by  measurement  It  should  be  added  that  when  the  traverse 
crosses  the  strike  of  the  magnetic  rock  at  the  angle  ,)',  the  distance  d  meas- 
m.*ed  on  the  line  of  the  traverse  must  be  multiplied  by  sin  y  in  order  to  get 
the  value  of  d  to  be  ixsed  in  the  determination  of  /;,  and  also  that  h  must  be 
.  corrected  for  the  height  of  the  instrument. 


356  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

More  general  inforinatiou  as  ti)  relative  depths  of  Ijurial  is  also  given 
by  the  dip  curves.  It  is  easily  seen  that  where  the  superficial  covering  is 
small  the  vertical  component  of  the  rock  force  must  remain  small,  except 
immediatel}'  over  the  rock.  This  condition  is,  therefore,  indicated  bv  steep 
slopes  in  the  di^)  curves.  On  the  other  hand,  where  the  depth  of  covering 
is  considerable,  the  vertical  component  increases  slowly  and  steadily,  Ijegin- 
ning  at  stations  at  a  distance  from  the  rock,  and  the  resulting  dip  curve 
approaches  the  maximum  with  gentle  slopes. 

7.   SUMMARY. 

(1 )  The  strike  of  a  magnetic  rock  is  given  by  the  line  joining  the  points 
on  successive  traverses,  at  which  the  horizontal  needle  is  not  deflected  from 
the  local  meridian  between  the  converging  aiTOws,  or  at  which  the  dip 
angles  are  a  maximum.  When  the  rock  is  vertical,  this  line  lies  in  the  middle 
plane  of  the  rock  and  fixes  its  position.  It  may  be  called  a  line  of  mag- 
netic attraction. 

(2)  The  dip  of.  a  magnetic  rock  is  toward  the  nearer  horizontal 
maxinuini. 

(3)  The  thickness  of  the  magnetic  formation  must,  if  buried,  always 
be  less  than  the  distance  between  the  maximum  points. 

(4)  Where  the  superficial  cover  is  not  very  great,  a  change  in  the  dip 
of  a  magnetic  rock  from  moderate  or  high  angles  to  low  angles  is  attended 
with  a  rapid  decrease  in  the  values  of  the  horizontal  component,  with  a 
corresponding  decrease  in  the  deflections  of  the  horizontal  needle. 

SECTION  VI.   APPLICATIOT«rS   TO   SPECIAIL,  CASES. 

In  the  preceding  section  certain  general  conclusions  have  been  estab- 
lished with  regard  to  the  relative  positions  of  the  stations  at  which  the 
horizontal  and  vertical  components  of  the  force  of  a  magnetic  rock  have 
maximum  and  zero  vakies.  The  deflections  produced  by  these  comjjonents 
from  the  ^Jositions  which  the  magnetic  needles  assume  under  the  action  of 
the  earth's  force  have  maximum  and  zero  values  at  the  same  stations  at 
which  the  components  have  maximum  and  zero  values,  and  therefore  the 
conclusions  as  to  the  relative  positions  of  these  points  are  true  for  any 
anffle  of  strike.  But  certain  numerical  relations  between  the  deflections 
depend  upon  the  orientation  or  strike  of  the  magnetic  formation  and  ujion 
the  direction  of  dip,  and  these  will  now  l)e  considered. 


MAGNETIC  OBSERVATIONS.  357 

1.  THE    MAGNETIC    ROCK    STRIKES    EAST    OR    WEST    OF    NORTH    AND    DIPS 

VERTICALLY. 

Let  US  first  take  the  case  of  a  rock  strikino'  east  of  north.  At  the 
stations  within  range  of  the  h)cal  influence  on  the  east  side  of  such  a  rock 
belt  the  liorizontal  needle  is  pulled  west  of  the  meridian,  reaches  a  west- 
ward niaxiiuum,  tlieii  points  north,  then  on  the  west  side  of  the  belt,  east 
of  the  meridian,  and  reaches  an  eastward  maximum.  It  is  observed,  how- 
ever, that  the  westward  deflections  on  the  east  side  of  the  belt  are  generally 
not  so  great  as  the  corresponding  eastward  deflections  on  the  west  side  of 
the  belt.  The  reason  for  this  is  easily  seen.  At  each  station  east  of  the 
belt  the  local  })ull  acts  along  the  normal  to  the  belt  di-awn  through  the 
station.  This  normal  makes  with  the  local  magnetic  meridian  an  acute 
angle.  The  needle  will  come  to  rest  within  this  acute  ang-le  alonar  the  line 
of  the  resultant  of  the  horizontal  components  of  the  two  forces,  the  earth's 
and  the  local  force,  which  determine  its  position.  However  strong  the 
local  pull  may  be,  the  horizontal  needle  can  not  be  deflected  past  the 
normal. 

At  the  corresponding  stations  on  the  west  side  of  the  disturbing  belt 
the  local  pull  also  acts  along  the  normal  from  the  station  to  the  belt,  and 
has  the  same  numerical  value.  But  in  this  case  the  normal  makes  an 
obtuse  angle  with  the  magnetic  meridian.  For  two  points  equall}'  distant 
from  the  magnetic  belt,  one  on  the  east  and  the  other  on  the  west,  the 
resultant  for  the  western  point  will,  therefore,  make  a  larger  angle  with  the 
meridian  than  that  for  the  eastern. 

On  the  other  hand,  when  the  rock  strikes  west  of  north,  it  is  observed 
that  the  horizontal  deflections  are  greater  on  the  east  side  than  on  the  west, 
and  the  explanation  is  entirely  similar  to  that  given  above. 

The  dip-needle  observations  at  the  same  stations  show  general  })he- 
nomena  quite  like  those  in  the  case  in  which  the  strike  of  the  rock  coincided 
with  the  meridian.  They  gradually  increase  to  a  maximum  near  the  sta- 
tion, where  the  horizontal  needle  stands  at  zero  between  the  converging 
arrows,  and  gradually  decrease  from  this  maximum  on  the  other  side.  It . 
is  noted,  however,  that  the  readings  are  not  equal  at  corresponding  stations 
on  opposite  sides  of  the  maximum.  When  the  strike  is  east  of  north,  the 
western  station  shows  a  higher  dip  than  the  eastern;  when  the  strike  is 


358  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

west  of  uorth,  the  eastern  station  shows  a  higher  dip  than  the  corresponding' 
station  on  the  west.  Generally  stated,  then,  the  stations  on  that  side  of  the 
magnetic  rock  on  which  the  angle  between  the  strike  of  the  rock  and  the 
line  of  traverse  is  obtuse  show  greater  dip  angles  than  the  corresponding 
stations  on  the  side  on  which  this  angle  is  acute.  As  the  angles  of  dip  are 
represented  graphically  by  a  continuous  curve,  this  is  the  same  thing  as 
saying  that  the  dip  curve  is  steeper  on  the  side  of  the  acute  than  on  that  of 
the  obtuse  angle. 

These  facts  are  easily  explained  by  the  following  considerations.  The 
vertical  components  tend  to  lower  the  needle,  and  would  cany  it  to  a  ver- 
tical position  except  for  the  action  of  the  horizontal  forces,  which  tend  to 
keep  it  horizontal.  At  any  station  on  the  acute-angle  side  of  the  magnetic 
belt  the  resultant  of  the  two  horizontal  components  is  larger  than  at  the 
corresponding  station  on  the  obtuse-angle  side,  the  two  being  represented 
by  the  longer  and  shorter  diagonals  of  a  parallelogram.  Since  the  vertical 
forces  are  the  same  at  the  two  stations,  it  follows  that  on  the  obtuse-angle 
side  the  angle  of  dip  must  be  larger  than  on  the  acute-angle  side.  Or, 
expressed  algebraically,  since  H,.  is  the  only  variable  on  the  right-hand  side 
of  equation  (5)  it  is  evident  that  tan  a,  and  therefore  a,  the  angle  of  dip, 
increases  with  a  decrease  in  H,. 

If  the  rock  dips  at  an  angle  less  than  90°,  these  results  are  either 
intensified  or  gi-eatly  modified,  depending  upon  the  direction  of  dip.  It 
4  was  shown  in  the  last  section  that  the  horizontal  component  of  the  rock 
force  is  smaller  on  the  dip  side.  If  the  strike  and  dip  are  both  toward  the 
same  side  of  the  meridian  (e.  g.,  if  the  strike  is  northwest  and  the  dip  south- 
west), it  is  evident  that  the  numerical  difference  between  the  deflections  of 
the  horizontal  needle  on  the  two  sides  of  the  rock  will  be  still  greater  than 
if  the  rock  were  vertical.  On  the  other  hand,  if  the  strike  and  dip  are 
toward  opposite  sides  of  the  meridian  (e.  g.,  if  the  strike  is  northeast  and 
the  dip  northwest),  the  diff"erence  between  the  deflections  on  the  two  sides 
is  less  than  for  a  vertical  dip,  or  may  even  be  reversed. 

The  deflections  of  the  dip  needle  in  the  case  of  rocks  dipping  at  angles 
less  than  90°  are  also  greatly  influenced  by  the  direction  of  dip.  If  strike 
and  di})  are  toward  the  same  side  of  the  meridian,  the  diff'erence  noted 
above  in  angle  of  dip  on  the  two  sides  of  the  rock  is  neutralized,  and  the 
dip  curve  tends  to  become  symmetrical;   while  if  thev  are  toward  opposite 


MAGNETIC  OBSERVATIONS.  359 

sides  of  tlu'  meridian,  the  difference  is  increased.  It  is  to  be  noted  that  the 
intiuencc  on  hotli  instnnnents  of  tlio  direction  and  angle  of  dip  of  the  rock 
becoini's  weakened  with  an  increase  in  surface  covering. 

2.  THE  MAGNETIC  ROCK  STRIKES   EAST  AND  WEST. 

When  a  vertically  dipping  magnetic  rock  strikes  east  and  west,  or 
nearly  so,  the  traverse  lines  must  be  run  north  and  south  so  as  to  cross  it 
as  nearly  as  possible  at  right  angles.  In  approaching  such  a  belt  from  the 
south  the  instruments  give  little  warning.  The  readings  of  the  horizontal 
needle  .show  either  no  deflections,  or  else  very  slight  deflections,  from  the 
mairnetic  meridian.  Past  the  middle  of  the  foi'mation  the  horizontal  needle 
is  strongly  deflected,  often  through  an  angle  of  180°,  so  that  it  may  point 
due  south.  But  as  the  magnetic  rocks  having  this  strike  which  were 
encountered  in  our  work  were  not  deeply  buried,  and  had  also  quite  irregular 
upper  surfaces,  generally  the  needle  pointed  either  east  or  west  of  south  on 
account  of  the  weight  which  the  nearness  to  the  surface  gave  to  the  adja- 
cent material,  either  from  the  irregular  distribution  of  magnetite  or  from 
the  protrusion  of  small  masses  above  the  general  level.  Continuing  noi-th, 
the  horizontal  deflections  gradually  diminish  and  eventually  disappear. 

The  behavior  of  the  horizontal  needle  is  explained  in  the  same  way 
as  in  the  preceding  cases.  The  position  of  the  needle  at  any  station  is  deter- 
mined by  the  resultant  of  the  horizontal  components  of  the  two  forces — the 
eai-th's  force  and  the  rock  force — that  act  upon  it.  South  of  the  magnetic 
rock  these  two  components  act  in  the  same  direction  and  essentially  in  the 
same  line,  since  the  magnetic  meridian  practically  coincides  with  the  true 
meridian.  The  resultant,  therefore,  is  equal  to  their  sum  and  coincides  with 
them  in  direction,  and  consequently  there  is  no  deflection.  North  of  the 
magnetic  rock  the  two  horizontal  components  act  in  opposite  directions,  and 
when  they  are  in  the  same  line  the  needle  takes  up  its  position  in  the  direc- 
tion of  the  greater,  which  determines  that  of  the  resultant;  when  H'  is 
greater  than  H  (which  often  happens  near  and  north  of  the  rock)  this  direc- 
tion is  due  south.  When  the  two  components  do  not  act  in  exactly  the 
same  line,  the  needle  will  point  east  or  west  of  south  at  an  angle  which 
depends  on  the  angle  between  the  two  forces  and  their  ratio. 

Still  farther  north  the  horizontal  component  of  the  rock  force  dimin- 
ishes rapidly  and  we  consequently  first  pass  tln-ough  a  zone  of  large  and 


360  THE  CRYSTAL  FALLS  lEOX-BEAEING  DISTRICT. 

diminishiug  deflections  to  the  east  or  Avest,  depending  on  the  side  of  the 
meridian  on  which  this  component  falls ;  and  finally,  when  it  becomes 
insensible,  the  needle  rests  again  in  the  meridian. 

In  the  case  of  a  rock  striking  east  and  west,  the  points  at  which  the 
horizontal  component  of  the  magnetism  of  the  rock  has  maximum  values 
become  indeterminate  by  the  methods  hitherto  described,  from  the  fact  that 
throughout  the  traverse  the  two  components  act  in  or  nearly  in  the  same  line, 
and  the  deflections  from  the  local  magnetic  meridian,  therefore,  do  not 
indicate  the  relative  strengths  at  difi"erent  stations  of  the  horizontal  com- 
ponent of  the  rock  force. 

The  dip-needle  readings  for  an  east-and-west-striking  rock  are  as  fol- 
lows: At  some  distance  south  of  the  rock  the  angles  are  constant  at  the 
index  error.  As  the  rock  is  approached,  the  angles  of  dip  depend  upon  the 
depth  of  burial.  If  the  surface  covering  is  considerable,  an  increase  in  the 
dip  angles  begins  at  a  considerable  distance  away,  and  progresses  continu- 
ously as  the  magnetic  belt  is  approached.  If  the  rock  is  near  the  surface, 
the  dip  needle  shows  either  the  constant  index  error  or  else  angles  of  dip 
less  than  the  index  error  for  all  stations  south  except  those  very  near  the 
southern  margin  of  the  rock.  The  maximum  reading  is  attained  north  of 
the  middle  plane  of  the  rock,  at  a  distance  from  it  which  also  depends  upon 
the  depth  of  covering.  Farther  north  the  dip  angles  decrease  slowly  and 
are  in  general  greater  than  at  the  coiTesponding  stations  south.  The  form 
of  the  dip-curve,  therefore,  shows  a  steeper  slope  south  of  the  magnetic 
rock  than  north  of  it.  The  reasons  for  these  differences  will  be  evident 
from  the  following  considerations. 

Let  it  be  supposed,  for  the  sake  of  simjjlicity,  that  throughout  the 
north-and-south  traverse  the  two  horizontal  components  act  in  the  same  line 
in  the  meridian.  At  any  station  south  of  the  magnetic  rock  they  act  in  the 
same  direction,  and  their  resultant  will  be  their  numerical  sum.  At  the 
cori'esponding  station  north  they  act  in  opposite  directions,  and  their  result- 
ant will  be   their  numerical  difl^erence.     The  angle   of  dip  is  giveii  by 

equation  (5): 

V'+  H  tan  & 
tan  a  rz !— yt 

For  the  two  corresponding  stations,  V  will  be  the  same.  The  other 
quantities  are  all  constants  except  H^.     For  the  south  station  H^rz  H'  -f  H; 


MAGNETIC  OBSERVATIONS.  361 

for  t\\v  north  station,  11^  =  11  —  11',  where  11  ;iufl  H'  are,  respectively,  the 
horizontal  components  of  the  magnetism  of  tlie  earth  and  of  the  rock,  as 
before.  The  numerator  of  the  right-hand  side  of  the  etjuation  will  be  the 
same  for  both  stations,  while  the  numerical  value  of  the  denominator  will 
be  less  for  the  north  station  than  for  the  south.  Consequently  tan  a,  and 
therefore  «,  will  l)e  greater  for  the  north  station. 

For  great  depths '  of  superficial  covering,  however,  these  differences 
become  almost  imperceptible,  owing  to  the  fact  that  H'  is  so  small  that 
Hr  is  essentially  tKe  same  at  the  two  con-esponding  stations.  The  tendency, 
therefore,  as  h  increases  is  for  the  dip  curve  to  become  symmetrical. 

In  the  special  case  in  which  H'r=  — H,  H,.=:  0,  and  the  dip  needle  stands 
at  90°.  This  can  only  take  place  north  of  tile  rock,  and  may,  depending 
on  the  strength  of  H',  be  found  at  two  stations,  one  on  either  side  of  the 
station  at  which  H'  is  a  maximum.  At  the  same  stations  the  horizontal 
needle  is  not  acted  on  by  any  unbalanced  force,  and  rests  indifferently  in 
any  position. 

The  dips  less  than  normal  which  are  often  observed  at  stations  south 
of  a  magnetic  rock  which  lies  A^er}^  near  the  suiface  are  also  easily  under- 
stood by  a  reference  to  equation  (5).  At  these  stations  the  resultant  pull 
of  the  rock  is  so  nearly  horizontal  that  the  vertical  component  V  is  very 
small  in  comparison  witli  the  horizontal  component  H'.  If  V  is  a  negligible 
quantity,  equation  (5)  becomes 

TT 

tan  a  z=  jj — Yn  ■  *^^^  ^ 
rl-|-xl 

In  sucli  cases  the  angle  of  dip  is  therefore  less  than  the  index  error.  With 
north  or  south  dipping  rocks,  where  V  is  negative,  tan  a  becomes  negative 
when  V'>H  tan  0. 

3.   TWO  PARALLEL  MAGNETIC  FORMATIONS. 

The  cases  so  far  considered  have  involved  only  one  belt  of  magnetic 
rock,  which  has  been  assumed  to  ha^•e  a  uniform  dip  in  one  direction,  or,  in 
other  words,  to  be  a  monocline.  In  practice,  however,  owing  to  complexi- 
ties of  structure  and  other  causes,  which  will  be  considered  hereafter,  it 
frequently  happens  that  two  or  more  approximately  parallel  belts  are  Ibund 
within  the  range  of  one  another's  influence.     Under  these   circumstances 


362  THE  CRYSTAL  FALLS  IRON-BEARIKG  DISTRICT. 

the  effects  produced  upon  the  magnetic  needles  are  corresponding!}^  com- 

pHcated. 

For  the  purposes  of  ilhistration  it  is  sufficient  to  consider  a  few  extreme 

H'  V 

cases,  and  to  represent  the  vahies  of  —  and  —  for  these  graphically.     Let 

it  tirst  be  assumed  that  the  two  parallel  belts  are  vertical,  that  the  distance 
between  them  is  8,  and  that  a^3,  and  /?zr2.  This  represents  the  conditions 
when  the  distance  of  separation  is  large  compared  Avith  /;.     The  ordinates  to 

XT' 

the  curve  of  PI.  XL VIII,  fig.  1,  give  the  values  of  —  which  correspond  to 

the  different  stations  of  observation.  Those  parts  of  the  curve  which  are 
above  the  horizontal  axis  of  •coordinates  represent  the  portions  of  the  trav- 
erse in  which  H'  is  directed  toward  the  west;  the  parts  below,  those  in 
which  it  is  directed  toward  the  east.  It  is  seen  that  besides  the  middle  point 
there  are  two  other  points  of  no  horizontal  deflection,  which  do  not  exactly 
correspond  with  the  points  A'ertically  over  the  middle  of  the  magnetic  forma- 
tions, but  are  somewhat  nearer  the  adjoining  edges;  and  also  four  points  of 
maximum  deflection,  one  on  each  side  of  each  rock.  The  maximum  points 
inside  of  the  two  formations  have  smaller  deflections  than  those  outside,  as 
shown  by  the  relative  lengths  of  the  ordinates  to  the  curve,  and  also 
between  the  inside  maximum  points  the  horizontal  components  are  directed 
away  from  the  middle  point. 

V 
The  curve  of  — ,  represented  by  the  dotted   line,  has  two  maximum 

valvies,  which  fall  nearly  over  the  two  rocks. 

If  next  we  assume  that  the  distance  between  the  rocks  is  8,  and  that 
fflrz3,  and  h-=8,  we  obtain  the  curve  of  PI.  XLVIII,  fig.  2,  which  represents 

TT/ 

the  value  ol  —  when  h  is  large  compared  with  the  distance  of  separation. 

This  case  shows  but  one  point  of  no  horizontal  deflection  between  the  two 
rocks  and  but  two  points  of  maximum  deflection,  one  on  the  outside  of 

each. 

V 
The  cvirve  of  — ,  represented  by  the  dotted  line,  shows  the  interesting 

feature  of  three  maximum  points,  one  at  the  center  and  one  over  each 
rock.  If  h  were  relatively  a  little  greater,  these  would  evidently  coincide 
at  the  center. 


U.  8.  GEOLOGICAL  8URVEV 


MONOGRAPH  XXXVt      PL.   XLVIII 


RELATIONS   OF   MAGNETIC   BEDS  TO   VARIATION    AND    DIP. 


MAGNETIC  OBSERVATIONS.  363 

It"  the  two  parallel  toriuatious  are  not  vertical,  l)ut  dip  in  tlie  same 
direction  at  the  same  angle,  the  resulting  curves  are  somewhat  ditlerent. 
I'l.  LXVIII,  tigs.  3  and  4,  show  two  cases  in  which  the  elements  are  the 
same,  excejit  the  depth  of  covering  and  the  thickness  of"  the  intervening  non- 
magnetic material.  Here  the  rocks  dip  at  an  angle  of  71  ^  34'.  and  the  width 
at  the  rock  surface  is  6.3  for  each. 

In  fig.  3,  PL  XLVIII,  where  Ji-=2 and  the  width  of  the  nonmagnetic 
bed  is  10.7,  and  the  covering  is,  therefore,  relatively  small,  the  presence  of 
two  rocks  is  distinctly  shown  by  the  curves  of  both  components,  and  the 
chief  result  of  their  interaction  is  to  introduce  an  additional  point  of  no  hori- 
zontal deflection  between  them,  on  each  side  of  which  the  horizontal  arrows 
diverg-e.  The  positions  of  the  maximum  and  of  the  other  zero  points  are 
hardlv  disturbed,  and  consequently  the  direction  of  dip  is  very  clearly 
indicated. 

In  fig.  4,  PI.  XLVIII,  where  /?rr4  and  the  formations  are  separated  by 
nonmagnetic  material  4.7  wide,  there  is  but  one  zero  point,  nearly  over  the 
middle  of  the  upper  formation,  toward  which  the  pointings  of  the  hori- 
zontal needle  converge.  West  of  this  are  two  points  of  maximum  eastern 
deflection,  between  which  a  faint  minimum  represents  the  backward  pull  of 
the  lower  formation. 

If  the  two  magnetic  formations  are  parallel  in  strike,  but  dip  toward  each 
other  at  equal  angles,  the  resulting  curves  of  the  two  components  are  shown 
in  PI.  XLVIII,  figs.  5  and  6.  Fig.  5  illustrates  the  eff'ects  on  a  s^Tichne 
with  steeply  dipping  sides,  the  supei-ficial  covering  being  relatively  shallow. 
These  conditions  result  in  a  point  of  no  horizontal  deflection  over  the  mid- 
dle of  the  trough  with  diverging  arrows  on  each  side,  and  besides  a  point 
of  no  horizontal  deflection  over  each  rock,  toward  which  the  aiTows  con- 
verge. The  positions  of  the  two  maximum  points  for  each  rock,  and  of  the 
zero  between  them,  is  nearly  the  same  as  if  the  other  rock  were  absent,  and 
conseqiiently  the  fact  that  the  rocks  dip  toward  each  other  is  clearly 
indicated  by  the  unsymmetrical  distances. 

In  fig.  6,  PI.  XLVIII,  the  depth  of  the  rock  surface  is  much  greater 
relatively  to  the  inside  distance  between  the  legs  of  the  syncline,  and  the  dip 
is  flatter.  In  this  case  there  are  but  two  points  of  maximum  deflection,  one 
on  each  side  of  the  syncline,  and  but  one  point  of  no  deflection,  over  the 
middle  of  the  trough.     The  maximum  points  represent  the  outside  maxima 


364 


THE  CEYSTAL  FALLS  IRON-BEARING  DISTRICT. 


of  the  former  case,  and  the  result  of  the  interaction  of  the  two  legs  is  to 
increase  the  numerical  values  of  these,  as"  well  as  to  bring  them  nearer 
too-ether.  It  is  evident  that  the  deflections  of  the  horizontal  needle  in  this 
case  could  hardly  be  distinguished  from  those  that  would  be  produced  by  a 
single  vertical  fonnation  buried  to  a  considerable  depth. 

Let  it  next  be  supposed  that  the  two  rocks  dip  toward  each  other  at 
different  angles,  the  strikes  remaining  parallel,  and  also  that  the  rock  of 
lower  dip  is  buried  to  the  greater  depth.  This,  then,  is  a  case  in  which  the 
magnetic  effect  of  one  limb  of  the  syncline  is  much  stronger  than  that  of 
the  other. 

In  PI.  XLVIII,  fig.  7,  the  curves  of  the  two  components  are  given  for 
the  special  case  in  which  the  right-hand  limb  of  the  synclinal  dips  at  an 
angle  of  90°,  and  has  a  surface  covering  /i=r2,  while  the  left-hand  limb 
dips  at  an  angle  of  26°  34',  and  has  a  surface  covering  h=i4:.     It  is  interest- 


Fig.  19 — Truucated  anticlinal  fold  with  gently  dipping  limbs. 


ing  to  compare  the  theoretical  results  of  this  figure  with  the  curves  of  PI. 
XLVIII,  fig.  8,  which  represent  deflections  actually  observed,  and  not 
components. 

In  the  latter  figure  the  strike  of  the  two  rocks  is  represented  by  the 
heavy  lines.  The  two  rocks  are  the  same  formation,  brought  up  by  folding 
on  opposite  sides  of  a  synclinal  trough.  The  synclinal  is  slightly  pushed 
over,  so  that  the  eastern  rock  dips  nearly  vertical,  while  the  western  has  a 
much  lower  dip  toward  the  east,  and  is  also  more  deeply  buried.  These 
facts  rest  on  independent  evidence,  yet  they  might  all  be  inferred  from  the 
observations  recorded  in  this  figure. 

The  dip  curve  in  this  case  shows  two  distinct  maxima,  a  smaller  under 


MAGNETIC!  OKSERVATIOXS. 


865 


the  zone  of  retardation  and  a  lar«iCT  over  tlie  ))oint  of  no  liorizontal  deflec- 
tion, uliicii  correspond  i-espeotively  to  the  two  niaji'netic  i-ocks. 

If  the  two  formations  are  parallel  in  strike,  l)nt  diii  away  from  each 
other,  the  cnrves  of  the  horizontal  and  vertical  comjjonents  for  different 
anjrles  of  dij)  and  different  relations  of  thickness  and  depth  of  coverino-  are 
shown  in  figs.  19  and  20.  In  fig-.  19  the  formations  are  widely  separated, 
k  is  relatively  small,  and  the  angles  of  dip  are  equal  and  low;  the  inter- 
action of  the  two  rocks  therefore  extends  over  a  narrow  zone  only,  and  the 
curves  of  the  components  clearly  indicate  the  presence  of  two  formations 
and  the  direction  of  dip  of  each.  In  fig.  20  the  anticlinal  is  so  truncated 
that  magnetic  material  occupies  the  whole  space  on  the  rock  surface 
between  the  outer  boundaries 
of  the  two  formations.  The 
angles  of  dip  are  equal,  and  are 
hig-her  than  in  the  preceding 
case,  while  the  depth  of  cover- 
ing is  relatively  much  greater. 
The  horizontal  component  is 
zero  in  the  axial  plane  of  the 
anticlinal,  and  has  maximum 
values  at  two  points,  one  on 
each  side  of  the  zero.  The  ver- 
tical component  is  a  maximum 
at  one  point,  also  in  the  axial 
plane.  The  deflections  pro- 
duced by  these  conditions  could 

not  be  distinguished  in  practice  from  those  produced  by  a  single  vertically 
dipping  fomiation. 

In  general,  therefore,  when  two  magnetic  formations  lie  within  range 
of  each  other's  influence,  the  deflections  are  determined  by  the  relative 
magnetic  strengths  of  the  .two  rocks,  hj  their  distance  apart,  by  their  strike 
and  dip,  and  by  their  depth  of  burial.  It  is  evident  that  for  certain  given 
relations  among  these  factors  the  special  cases  above  described  will  occiu-, 
and  it  is  found  that  they  really  do  occur  in  practice.  For  other  relations  it 
is  not  possible  to  make  a  general  statement  either  as  to  the  number  or  the 
position  of  the  maximum  and  mininuini  points. 


rig,  20.— Truncated  anticlinal  fold  with  steepl.y  dipping  limbs. 


366  THE  CRYSTAL  FALLS  lEON-BEARING  DISTRICT. 

SECTION  VII.  THE  INTERPRETATION  OF  MORE  COMPIjEX  STRUCTURES. 

The  existence  of  two  parallel  belts  of  magnetic  rocks  may  be  accounted 
for  geologically  in  more  than  one  way.  They  may  represent  two  distinct 
formations  occurring  at  different  horizons  in  the  same  series,  or  they  may 
represent  the  same  formation  either  duplicated  by  folding  or  faulting  or 
separated  into  two  parts  by  the  intrusion  of  a  sheet  of  igneous  rock  parallel 
to  its  bedding.  Since,  then,  two  magnetic  lines,  the  existence  of  which  has 
been  established  b)'  observation,  ma}'  have  more  than  one  interpretation, 
the  discrimination  of  these  cases,  when  possible,  is  of  special  importance. 

The  question  whether  any  given  case  belongs  to  the  first  of  these  cate- 
gories can  generally  be  settled  only  by  following  the  lines  of  attraction  into 
a  district  which  affords  a  geological  section  across  the  formations  involved, 
or  by  the  occasional  outcrop  of  the  rocks  which  give  rise  to  the  disturl^ances, 
in  which  case  lithological  resemblances  or  differences,  the  relations  to  other 
formations,  and  the  observed  structure  will  decide  the  matter  one  way  or 
the  other.  In  the  special  case  in  which  either  or  both  lines  can  be  followed 
completely  round  an  anticlinal  dome  or  a  synclinal  basin,  which  of  course 
can  only  rarely  happen,  the  question  would  be  settled  affirmatively,  e^^en  if 
outcrops  were  entirely  lacking. 

In  the  other  instances  the  magnetic  observations  themselves  often  give 
means  of  discrimination,  even  when  the  outcrops  are  so  few  or  so  obscure 
as  to  be  in  themselves  indecisive.  It  is  characteristic  of  the  folds  in  the  pre- 
Cambrian  rocks  of  this  region  that  the  axes  are  not  usually  parallel  with 
the  horizon  for  long  distances,  but  are  often  inclined  to  it;  in  other  words, 
when  followed  for  greater  or  less  distances  they  pitch.  The  outcropping 
edges  of  any  formation  involved  in  an  anticlinal  or  synclinal  fold  which  has 
been  cut  by  a  plane  of  denudation  will  be  parallel  to  each  other  wherever 
the  axis  of  the  fold  is  horizontal,  but  will  approach  each  other  where  the 
axis  is  inclined.  In  an  anticlinal  fold  they  converge  in  the  direction  in 
which  the  axis  sinks,  while  in  a  synclinal  the}"  converge  in  the  direction 
in  which  the  axis  rises.  If  the  formation  is  a  magnetic  one,  conformably 
placed  between  beds  of  nonmagnetic  character,  the  magnetic  lines  to  which 
the  outcropping  edges  give  rise  will  therefore  run  parallel  to  each  other 
when  the  axis  is  horizontal  and  will  converge  or  diverge  when  the  axis 
pitches.  The  convergence  or  divergence  takes  place  gradually,  since  the 
angles  of  pitch  usually  are  not  large. 


MAGNETIC  OBSERVATIONS. 


367 


111  the  case  also  of  a  «iu<>-le  tbrinatiou  which  stands  on  edge  and  has 
been  spUt  b}-  the  intrusion  of  a  sheet  of  eruptive  rock  parallel  to  the  bed- 
ding planes,  the  magnetic  observations  will  often  show  two  parallel  lines, 
which,  at  the  extremities  of  the  eruptive  rock,  where  it  wedges  out,  merge 
into  one. 

In  general,  therefore,  two  parallel  magnetic  lines  which  represent  two 
distinct  formations  preserve  their  identity,  and  do  not  pass  into  each  other; 
when,  however,  they  represent  the  same  formation,  they  will  often  come 
together  if  followed  far  enough.  The  principles  which  have  already  been 
applied  to  the  analysis  of  simpler  cases  are  useful  in  discriminating  among 
the  three  cases  of  convergence. 

I.  PITCHING  SYNCLINES. 

Let  us  first  consider  a  pitching  synclinal  fold,  which  is  repi*esented  in 
plan  and  by  successive  cross  sections  in  fig.  21.  It  is  evident  that  on  the 
lines  of  traverse  along  Sections 
I  and  II  the  deflections  of  the 
needle  will  observe  the  usual 
sequence  for  two  parallel  belts, 
the  details  depending  upon 
separation  and  depth  of  cover- 
ing, while  on  lines  along  Sec- 
tions III,  IV,  and  V  the  phe- 
nomena will  be  those  caused 
by  a  single  belt  of  magnetic 
rock.  Also,  on  account  of  the 
rise  in  the  axis,  the  south  poles 
of  the  rock  are  brought  continually  nearer  the  surface  on  these  successive 
cross  sections,  and  therefore  the  two  components  of  the  rock  force  will  be 
smaller  for  each  traverse  than  for  the  one  preceding.  Since  the  magnetic 
material  comes  to  an  end  at  A,  it  is  no  longer  true  that  there  is  as  much 
magnetic  material  on  one  side  of  these  sections  as  on  the  other.  Conse- 
quently the  horizontal  component  due  to  the  pull  of  the  rock  does  not 
become  zero  at  any  point  along  these  sections,  but  for  every  station  has  a 
positive  numerical  value  and  acts  in  the  general  direction  in  which  the  syn- 
clinal pitches.     At  the  station  in  the  plane  of  symmetry  of  the  fold  this 


V^ 


CRCSS   CCCTIONS 


Fig.  21 .  —Plan  and  cross  sections  of  a  pitching  syncline. 


368  THE  CRYSTAL  FALLS  IRON-BEARIKG  DISTRICT. 

component  acts  parallel  to  the  axis.     The  direction  and  amount  of  the  deflec- 
tion depend  upon  the  direction  of  strike  and  pitch  of  the  synclinal. 

Let  us  suppose,  first,  that  the  axis  of  the  synclinal  strikes  north  and 
pitches  north.  In  this  case  Section  I,  in  fig.  17,  is  the  most  northern, 
Section  V  the  most  southern. 

The  traverses  along  Sections  I  and  II  display  the  usual  phenomena 
for  two  parallel  belts.  East  of  the  eastern  limb  and  west  of  the  western 
the  horizontal  needle  will  be  deflected  toward  the  syncline.  Between  the 
two  limbs  there  will  be  at  least  one  point  of  no  deflection,  and  "frequently, 
depending  upon  the  relations  between  the  depth  of  burial  and  the  thickness 
of  the  intervening  nonmagnetic  material,  either  two  other  points  of  no 
deflection  or  two  zones  of  retardation,  one  on  each  side  of  this  middle  zero. 

Along  Sections  III,  IV,  and  V  there  will  be  but  one  point  of  no 
deflection  of  the  horizontal  needle,  which  will  correspond  with  the  axial 
line  of  the  fold.  Since  this  axis  is  north  and  south,  and  so  coincides  with  the 
magnetic  meridian,  the  horizontal  component  of  the  rock  force  coincides  in 
direction  with  the  horizontal  component  of  the  earth's  pull,  and  consequently 
there  is  no  deflection  of  the  horizontal  needle.  For  other  stations  east  and 
west  of  the  central  station  the  deflections  are  toward  the  west  and  east, 
with  the  usual  maximum  points. 

The  deflections  on  successive  sections  south  grow  smaller,  since  the 
angle  between  the  two  horizontal  components  progressively  diminishes. 
The  relative  value  of  the  horizontal  component  of  the  rock  force  also 
progressively  diminishes,  since  the  thinning  of  the  magnetic  material  due 
to  the  rise  in  the  axis  of  the  fold  brings  the  buried  north  poles  into  promi- 
nence. Therefore  the  deflections  of  the  horizontal  needle  after  the  mag- 
netic rock  has  been  left  behind  very  soon  become  imperceptible. 

The  dip  needle  deflections  for  the  northern  sections,  I  and  II,  reach 
their  maximum  values  at  the  usual  points,  over  the  central  zero  and  near 
the  outside  zeros  or  points  of  retardation.  For  the  southern  sections  the 
dips  grow  less,  since  the  horizontal  restoring  couple  due  to  the  rock  has 
always  a  positive  immericai  value,  and  also  because  the  vertical  component 
of  the  rock  force  diminishes,  owing  to  the  nearness  of  the  south  poles.  As 
the  section  approaches  the  limits  of  the  magnetic  material  the  points  of 
maximum  dip  become  less  and  less  clearly  defined,  and  the  dip  curve  [)asses 
into  an  irregular  line,  slightly  depressed  below  the  line  of  no  deflection. 


MAGNETIC  OBSERVATIONS.  369 

Tlie  reasons  for  this  iire,   of  course,   obvious  from   what  has  been  said 
above. 

In  the  case  of  a  synchnal  fohl  pitcliing  soutli,  Section  I  (fig.  17) 
becomes  the  most  southern,  Section  X  the  most  northern,  hue  of  traverse. 
Sections  I  and  II  present  the  same  general  phenomena  as  before  for  both 
needles.  In  Sections  III,  IV,  and  V  the  horizontal  component  due  to  the 
rock  has  a  positive  value  for  all  stations  as  before,  but  in  this  case  acts  in  a 
generally  opposite  direction  to  that  of  the  horizontal  component  of  the 
earth's  force.  Therefore,  on  these  sections  we  should  expect  at  first  greater 
deflections  of  the  horizontal  needle,  which  would  diminish  rapidly  as  the 
sections  approached  and  passed  beyond  the  northern  limit  of  the  magnetic 
material,  but  whicli,  for  corresponding  sections,  would  be  greater  than  for 
the  northerly  pitching  fold.  The  deflections  of  the  dip  needle  would  also ' 
be  gi'eater  for  the  same  reasons. 

For  a  synclinal  pitching  west,  Section  I  is  the  most  western,  Section  V 
the  most  eastern,  traverse.  In  this  case,  along  I  and  II,  the  deflections  of 
the  horizontal  and  dip  needles  are  dependent  for  their  details  upon  the  ratio 
of  de^^th  to  distance  of  separation,  but  if  far  enough  to  the  west  will  show 
clearly  two  belts  of  magnetic  material,  aj^proximately  parallel,  and  striking 
approximately  east  and  west.  For  Sections  III,  IV,  and  V,  in  which  the 
distance  of  separation  is  either  nothing  or  relatively  small,  the  phenomena 
will  indicate  but  one  belt.  On  these  sections,  owing  to  the  fact  that  the 
horizontal  component  of  the  rock  pull  is  nowhere  zero, and  has  everywhere 
a  general  westerly  direction,  the  deflection  of  the  horizontal  needle  will  be 
westerly  throughout,  and  will  reach  a  maximum  north  of  the  east-and-west 
axial  plane  of  the  material,  where  the  ang-le  which  it  makes  with  the  magnetic 
meridian  is  more  than  90°. 

In  accordance  with  the  general  principles  stated  in  the  discussion  of  a 
single  belt  with  the  same  strike,  the  angles  of  dip  are  in  general  smaller 
south  of  the  sjaicline  than  north,  and  the  maximum  dip  is  reached  at  a 
point  north  of  the  axial  plane.  On  sections  farther  to  the  east,  near  the 
limits  of  the  rock  and  beyond  them,  the  dip-needle  deflections,  like  those 
of  the  horizontal  needle,  rapidly  diminish  and  soon  become  imperceptible. 
These  facts  are  well  shown  in  fig.  22,  which  represents  a  series  of  north- 
and-south  traverses  across  the  Groveland  basin,  the  limits  of  which  are 
defined  by  outcrops  on  the  eastern  side. 
MON  xxxvi 24 


370 


THE  CRYSTAL  FALLS  IRON  BEARING  DISTRICT. 


In  this  figure  it  is  instructive  to  notice  tlie  small  dip  angles  in  the 
sections  east  of  the  end  of  the  syncline.  In  the  first  of  these  the  dip 
curve  shows  a  hollow  near  the  axis  of  the  fold  or  angles  of  depression  less 
than  the  normal.  This  is  easily  understood  upon  considering  that,  since 
the  surface  covering  is  here  small,  the  vertical  comjjonent  of  the  I'ock 
force  becomes  very  small  at  these  stations  compared  with  the  horizontal 
component. 

For  an  eastward-pitching  syncline  it  is  obvious  that  the  facts  will  be 
entirely  similar  to  those  stated  above,  except  that  tlie  deflections  of  the 
horizontal  needle  will  be  toward  the  east  instead  of  toward  the  west.     This 


; 
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■IS6 


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y" 


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lis. 

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Fig.  22. — Magnetic  map  of  the  Grovelantl  Basin. 


is  also  well  shown  at  the  western  end  of  the  Groveland  basin  in  fig.  22. 
This  basin  does  not  show  the  jihenomena  of  two  lines,  however,  from  the 
fact  that  it  is  so  narrow  and  shallow  that  it  does  not  include  in  its  interior 
any  overlying  nonmagnetic  material,  and  there  is  accordingly  no  sej^aration 
of  its  rims. 

2.  PITCHING  ANTICLINES. 

In  the  cases  of  pitching  anticlines  (fig.  23)  the  sequence  of  observa- 
tions in  the  area  of  separation  of  the  rims  is  very  similar  to  that  of  pitching 
synclines.  In  the  zone  of  coincidence  the  structural  diff'erence  in  the  two 
cases  is  that  the  material  does  not  come  to  an  end,  but  continues  as  one 
band,  which,  as  the  axis  sinks,  is  ^progressively  buried  to  a  gi'eater  depth. 


MAGNETIC  OBSERVATIONS. 


371 


Thereftiro,  in  yoneral,  tliL-  l)uried  north  poles  of  the  magnetic  formation  are 
not  brought  nearer  the  surface;  and  this,  together  with  the  fact  that  the 
material  continues  on  in  the  line  of  the  axis,  jiroduces  characteristic  phe- 
nomena in  the  magnetic  sections. 

These  phenomena,  "the  details  of  which  can  be  easily  followed  out  for 
any  given  direction  of  pitch,  and  need  not  here  be  described,  show  in  gen- 
eral two  lines  of  attraction  luerging  into  one,  which  continues  in  the  same 
direction  as  a  strong  line,  showing,  as  it  is  followed,  the  peculiarities  due  to 
an  increasing  deptli  of  burial.  The  points  of  maximum  deflection  of  the 
liorizontal  needle  continue  to 
separate  from  each  other  on  suc- 
cessive sections.  The  dip  curve 
shows  a  definite  maximum 
closely  corresponding,  except  for 
due  east-and-west  strikes,  to  the 
point  of  no  horizontal  deflection. 
Where  the  axis  of  tlie  fold  is  so 
oriented  that  these  points  can 
be  establi.shed,  they  indicate  the 
nature  of  the  fold.  If  the  strike 
is  east  and  west,  in  which  case 
they  become  indeterminate,  the 
continuity  of  the  line  and  its 
A'ery  gradual  decrease  in  power  may  give  an  excellent  basis  for  inference 
as  to  the  nature  of  the  fold. 


r^7n\ 


A 


Fig.  23. 


CROSS  SECTIONS 

-Pliiii  aud  cruss  sectioBs  of  a  pitching  anticline. 


3.  FORMATIONS   SPLIT  BY   INTRUSIVES. 

When  a  single  formation  has  been  split  into  two  by  the  intrusion  of  a 
nonmagnetic  igneous  rock,  there  are  in  the  area  in  which  the  igneous  rock 
occm-s  two  pai-allel  magnetic  formations,  which  give  rise  on  cross  traverses 
to  phenomena  the  precise  features  of  which  depend  upon  the  strike  and  dip 
of  the  formation  and  upon  the  relation  which  the  width  of  the  intruded 
mass  bears  to  the  depth  of  burial.  To  describe  these  would  involve  a  mere 
repetition  of  what  has  been  said  before.  Such  intruded  masses  always  have 
a  definite  limit  in  length,  which  is  usually  not  very  great.  When  the  limits 
are  reached,  the  two  parallel  lines  pass  into  a  single  line  which  continues  on 


372  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

in  undimiilislied  vigor.  Also,  such  intruded  masses  are  seldom  confined  to 
definite  horizons  for  great  distances  and  seldom  split  the  formation  into 
symmetrical  halves.  Two  nearly  parallel  lines  of  unequal  strength,  which, 
as  they,  are  followed,  become  equal,  and  then  again  become  unequal,  with 
the  stronger  on  the  opj^osite  side,  are  often,  therefore,  characteristic  phenom- 
ena of  this  case.  A  good  illustration  of  the  unequal  division  of  a  magnetic 
rock  at  diff'erent  points  along  its  strike  by  an  intruded  sheet  which  wedges 

SS    84    3S    39    40    40    40    CO    61    IS  59   IS    69    64    60    C3 
f    >     ^    >■     f     f     r     f     f    r   \    \     \     \    \     1- 

.^i  ,^ 

Jl  34  36  40  44  45  48  ST  II.  46  VA'^i "  08  M  68 

V.  ,  f  t  f  *  f  'Y WX"  '  '  * 


31    81    34    31    41    47    49    69    16  -^i   3«4  66    86    M    " 
t     r      t     t      '      r     f      f     >    ^V  \    *      '      * 

36     34     40    43    46    46    ST^S  «*  68  «    19  63  6J 

N  '\vC>v 


3?    35    4.  .}   4;    48    6^66    ^9>;jVl    »   «T 

39     41    43    41  •jjg  eiSji  >  84>«1  «  •} 

'   »  J.  /  »  «  y  '^y^^ — s — i- 


^ 


Fig.  24. — Magnetic  map  of  a  single  formation  split  by  an  intruded  sheet. 

out  at  both  ends  is  given  in  fig.  24.  Between  the  second  and  third  trav- 
erses from  the  north  end  the  existence  of  this  sheet  has  been  proved  by 
drilling. 

4.   SUMMARY. 

The  means  of  discrimination  among  these  cases  of  convergence  are 
therefore  founded  on  the  deflections  in  the  critical  areas,  where  the  separated 
bands  of  magnetic  material  merge  into  one.  Strong  deflections  toward  the 
point  where  they  run  together,  with  a  rapid  disappearance  of  all  disturb- 
ances within  a  short  distance  of  this  point,   indicate  a  pitching  synclinal 


MAGNETIC  OBSERVATIOXS.  373 

fold.  A  louy  (.•(iiitiiuiaiu-e  of  the  disturbances,  with  the  cliaracteristic  phe- 
nomena attending  deeper  burial,  beyond  the  point  of  coincidence,  indicates 
an  anticlinal  fold.  A  coincidence  in  both  directions  and  the  continuation  of 
the  disturbances  without  diminution  indicate  an  intrusive  sheet.  To  these 
may  be  added  the  delicate  criterion  which  the  unsymmetrical  distances  of 
the  horizontal  maxima  from  the  central  zero  may  afford.  If  in  the  area 
of  separation  the  two  belts  depart  from  each  so  far  as  to  be  out  of  range  of 
each  other's  influence,  and  it  is  found  on  successive  cross  sections  that  the 
nearest  maxima  are  inside  the  lines  of  no  deflection  which  directly  indicates 
the  position  of  the  rock,  it  can  be  concluded  that  the  rocks  dip  toward  each 
other,  and,  on  the  other  hand,  if  the  nearer  maxima  are  outside  these  lines, 
that  they  dip  away  from  each  other.  In  the  one  case  a  syncline  and  in  the 
other  an  anticline  would  be  indicated,  and,  of  course,  in  either  case  it  would 
be  certain  that  tlie  phenomena  could  not  be  due  to  an  intruded  mass. 


OHAPTEE   III. 

THE  FELCH  MOUNTAIN  RANGE. 
SECTioisr  I.  posiTio:^,  extent,  and  previous  work. 

Our  map  (PL  XLIX)  of  the  Felch  Mountain  range  includes  12  sections 
in  the  southern  tier  of  T.  42  N.,  Rs.  28,  29,  and  30  W.,  beginning-  with  sec. 
33,  T.  42  N.,  R.  28  W.,  on  the  east,  and  ending  with  sec.  34,  T.  42  N., 
R.  30  W.,  on  the  west.  The  range  is  known  to  extend  beyond  these  limits 
both  to  the  east  and  to  the  west.  Rominger  states^  that  it  has  been  traced 
4  miles  east  of  our  eastern  boundary,  and  also  west  of  our  western  boundary 
to  the  Menominee  River  north  of  Badwater  Village.  From  a  hasty  recon- 
naissance of  the  country  to  the  east  it  seemed  probable  that  but  few  addi- 
tional facts  could  be  determined,  because  of  the  swamps  and  the  extensive 
cover  of  the  Paleozoic  sandstone,  and  these  sections  were  therefore  not 
studied  in  detail.  We  were  not  able  to  continue  the  work  to  the  west,  on 
account  of  the  lateness  of  the  season,  but  it  is  desirable  that  this  should  l)e 
done  at  some  future  time.  The  sections  surveyed  include,  however,  that 
portion  of  the  range  in  which  outcrops  are  most  abundant  and  which  has 
been  the  principal  seat  of  exploration  for  iron  ore. 

The  strong  magnetic  attractions  in  several  of  these  sections  and  the 
prominent  outcrops  of  ferruginous  jaspers  at  Felch  Mountain  in  sec.  32, 
T.  42  N.,  R.  28  W.,  and  in  sec.  31,  T.  42  N.,  R.  29  W.,  Avere  early  noticed 
by  the  United  States  land  surveyors  and  indicated  on  the  township  plats. 
With  the  rapid  development  of  the  Marquette  range  after  the  close  of  the 
civil  war  the  attention  of  miners  was  quickly  drawn  to  these  as  to  other 
outlying  prospects,  with  the  result  that  vigorous  exploration  was  begun  on 
this  range  even  earlier  than  on  the  Menominee  range  proper. 

'Geological  report  on  the  Tipper  Peninsula  of  Michigan,  by  C  Koniinger:  Geol.  Survey,  Mich., 
Vol.  V,  1895,  p.  35. 
374 


49 


us   GEOLOGICAL  SURVEY 


MONOGRAPH    XXXVIPLXLIX. 


I      R  28  W 


/R^r 


US  BIEN  SCO  L 


.\RCHEAN 
Granite 


GEOLOGICAI.  MAP  OF  THE  FELCH  MOUNTAIN  RANGE 

TOPOGRAPiri'.\XU  GEOLOGY 
BY  H.L.SMYTH 

SCALE    2  INCHES  - 1  MILE  ^  "^'TOIIR  INTERV'AL    20  FEET 

HORIZONTAL   SCALE  OF  SECTIONS   ::  INCHES  -1  MILE         \'ERTICAL  SCALE   1   INCH  -  1320  FEET 
ELEVATION    OF  BASl.  LINES   600  FEET 
W  Oulcrops  wilhoi'  t-bscH-ed  Bli-jlu.  or  rtiu 

;  Trel  pa«  botlomed  mrock 
o  Drill  holes 

ALGONKIAN 


CAMBRIAN 


LOWER  HUHONIAN 


StiirqoonQiinrtzilr 
I      Als     I 


Rand^■ille  Dolomito  Mansfield  Schist 


AlrH 


Aim 


Grovelaiul  Formntion 


UPPER  HURONIAN 
Ulldmded 


INTRUSIVE 


Diabase 


Granite 


Au 


X 


D 


PREVIOUS  WORK  ON  FELCH  MOUNTAIN  RANGE.  375 

Tlio  following  abstracts  of  the  published  literature  upon  the  Felch 
jrouiitMin  range  are  given  as  far  as  possible  in  the  author's  words: 

1850. 

Bi'BT,  Wm.  a.  Geological  report  of  the  survey,  "with  reference  to  mines  and 
minerals,"  of  a  district  of  townsbip  lines  in  the  State  of  Michigan,  in  the  year  184^0, 
and  tabular  statement  of  specimens  collected.  Dated  March  20,  18-1:7.  Thirty-flrst 
Congress,  lirst  session,  1849-50.  Senate  Documents,  Vol.  Ill,  No.  1,  pages  8J:2-875. 
With  maps. 

The  earliest  mention  of  the  part  of  the  Upper  Peninsula  included 
within  the  Felch  Mountain  range  was  made  by  Burt  in  describing  the  dis- 
tribution of  the  talcose  and  argillaceous  slates  of  the  area  covered  in  the 
coui-se  of  his  land  survey  in  1846.  He  states  that  the  argillaceous  slates 
"are  developed  in  ])arts  of  township  42,  ranges  29  to  30  west"  (p.  846). 

The  existence  of  the  iron  ore  in  this  area  was  discovered  l)y  this 
explorer. 

The  flrst  bed  discovered  of  this  ore  was  found  while  traveling  from  the  Pesha- 
kumme  Falls,  near  the  Meuomonee  River,  east  to  Fort  River,  before  it  was  surveyed 
in  May  last,  but  was  not  discovered  again  during  the  suivey.  It  is  believed,  however, 
that  this  bed  of  iron  ore  is  not  far  distant  from  the  corner  of  townships  41  and  43  N., 
between  ranges  29  and  30  W.  It  was  found  in  a  low  ridge  about  3  chains  wide, 
course  WNW.  This  ridge  appeared  to  be  nearly  one  mass  of  iron  ore,  stratified  and 
jointed ;  consequently  it  may  be  quarried  with  ease.  This  ore  has  generally  a  granular 
or  micaceous  structure,  but  specular  varieties  sometimes  occur;  color,  iron  black,  pass 
ing  into  a  steel  gray;  luster  when  fresh  broken,  metallic,  but  soon  oxidizes  when 
exposed  to  the  atmosphere.  This  is  supposed  to  be  an  extensive  and  rich  bed  of  iron 
ore.  The  variation  of  the  needle  was  taken  on  the  east  side  of  the  ridge  at  the  cross- 
ing of  a  hunter's  trail,  and  its  north  end  stood  S.  82°  E.  Three  or  4  miles  west  of 
this,  on  the  north  side  of  a  ridge,  near  a  cedar  swamp,  the  variation  was  N.  45°  30' 
W.    Probably  iu  this  vicinity  may  be  found  another  extensive  bed  of  similar  iron 

ore  (p.  849). 

1851. 

Foster,  J.  W.,  and  Whitney,  J.  D.  Report  on  the  geology  and  topography  of 
the  Lake  Superior  land  district.  Part  II.  The  iron  region,  together  with  the  general 
geology.  Dated  November  12,  1851.  Thirty-second  Congress,  special  session,  1851. 
Senate  Documents,  Vol.  Ill,  No.  4;  406  pages;  with  maps  and  plates. 

Foster  and  Whitney,  in  sketching  the  distribution  of  the  rocks  of  their 
Azoic  system,  which  comprises  "for  the  most  part  gneiss,  hornblende, 
chlorite,  talcose  and  argillaceous  slates,  interstratified  with  beds  of  quartz, 
saccharoidal  marble,  and  immense  deposits  of  specular  and  magnetic  oxide 
of  iron"  (p.  8),  after  describing  the  main  area  of  these  rocks  to  the  north- 
west and  north  of  the  Felch  Mountain  range,  say:  "Another  arm  about 


376  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

18  miles  in  length  and  10  in  breadth  extends  easterly  into  T.  42  and  43, 
R.  28"  (p.  14).  This  east-and-west  trending  area  corresponds  in  part  with 
the  Felch  Mountain  range,  but  also  includes  two  .adjacent  troughs,  one  to 
the  north  of  it,  and  the  other  to  the  south. 

Subsequent  to  the  above  reports,  nothing  is  known  to  have  been  pub- 
lished containing  any  matter  concerning  this  iron-bearing  area  until  1869. 

1869. 
Ceednbr,  Hermann.    Die  vorsiluiiachen  Gebilde  der  "oberen  Halbiiisel  von 
Michigan"  in  Nord-Auierika.     Zeits.  der  deutschen  geol.  Gesell.,  Vol.  XXI,  1869,  pp. 
516-568.     With  map  and  three  plates  of  sections. 

In  Credner's  article  we  find  a  considerable  advance  in  knowledge 
concerning  the  relations  of  the  rocks  of  the  Felch  ]\Iountain  area.  This 
advance  is  indicated  by  his  general  map  (PL  IX)  and  by  his  two  profiles 
(fig.  3,  PI.  IX,  and  fig.  1,  PI.  X). 

In  profile  No.  3,  going  from  south  to  north,  the  granite  is  represented 
as  overlain  by  quartzite,  separated  by  a  narrow  interval  from  the  marble, 
which  in  its  turn  is  overlain  by  a  great  thickness  of  the  ore  formation.  The 
dip  is  steep  to  the  noi'th.  Unconformably  upon  the  last  two  formations — the 
marble  and  the  iron  formation — there  are  caps  of  Potsdam  sandstone,  with 
the  beds  dipping  flat  to  the  north. 

In  fig.  1,  PI.  X,  the  gneiss  overlain  by  the  quartzite,  dipping  steep  to 
the  north,  are  the  only  pre-Paleozoic  rocks  shown.  The  same  profile  shows 
the  unconformable  Potsdam,  with  Silurian  dolomite  resting  conformably 
upon  it. 

In  the  text  there  is  mention,  with  an  illustration  (fig.  5,  PI.  IX),  of  the 
granite  dike  cutting  across'  the  iron  formation  in  the  upper  course  of  the 
Sturgeon  River.  Beyond  this  no  reference  to  the  area  under  discussion 
occurs  in  the  text. 

1873. 
Brooks,  T.  B.    Iron-bearing  rocks  (economic).     Geol.  Surv.  of  Michigan,  Vol. 
1, 1869-1873,  New  York,  1873,  Part  I.    319  pages.    With  maps. 

In  the   year  1873   Maj.  T.  B.  Brooks's  very  important  study  of  the 

Michigan  iron  ranges  appeared,  and  in  it  the  Felch  Mountain  area  for  the 

first  time  is  distinctly  separated  as  "the  North  Iron  Range"  from  the  "South 

Iron  Range"  of  the  Menominee  River  iron  region,  both  together,  however, 

constituting  the  Menominee. 

The  north  iron  range,  about  12  miles  from  the  other  in  the  south  i>art  of  T.  42, 
Rs.  28,  29,  and  30,  is  in  places  a  prominent  topographical  feature.    The  capping  of 


PREVIOUS  WORK  OJf  FELCH  MOUNTAIN  RANGE.  377 

horizontal  sandstones,  wliicli  -has  already  been  mentioned  as  characterizing-  the 
Menominee  liills,  gives  a  somewhat  more  even  character  to  the  crest  lines,  and  in 
places  produces  a  strikingly  ditterent  prottle  (p.  72). 

As  stilted  by  Major  Brooks  in  a  footnote  on  p.  157,  Hie  facts  contained 
in  the  chapter  on  the  Menominee  iron  reg'ion  were  derived  largely  from  the 
sm-veys  and  explorations  of  Prof.  R  Purapelly  and  his  assistant,  Dr.  H. 
Credner.  The  following  passage  is  the  most  important  statement  conceni- 
iug  the  Felch  Mountain  area,  or  the  "North  Range:" 

The  north  iron  belt  or  range  has  a  coarse  nearly  due  east  and  west,  and  is  all 
embraced,  so  far  as  known,  in  the  south  tier  of  sections  of  T.  42,  Rs.  28,  29.  and  30. 
The  most  easterly  discovered  exposure  of  ore,  known  as  the  Felch  Mountain,  is  in  the 
N.  J  of  sees.  32  and  33,  T.  42,  E.  28.  Traveling  due  west,  fragments  of  iron  ore  are 
found  in  NE.  ^  of  sec.  31,  T.  42,  R.  28;  after  which  no  absolute  proof  of  the  presence 
of  iron  is  found  (although  it  is  probably  continuous)  until  we  reach  sec.  31,  T.  42,  R.  29, 
where,  in  the  center  of  the  section,  is  an  immense  exposure  of  iron  ore  in  an  east- west 
ridge,  which  can  be  traced  westerly  halfway  across  section  36  of  the  next  township. 
The  natural  exposure  of  ore  on  section  31  is  larger  than  at  any  other  point  in  the 
Menominee  region,  and  the  quality  is  as  good,  if  not  better,  so  far  as  can  be  judged  by 
surface  indications.  Magnetic  attractions  and  iron  bowlders  found  farther  west  and 
southwest  on  this  range  prove  its  extension  in  that  direction.  Whether  the  westerly 
course  continues,  or  whether  it  curves  to  the  southwest,  as  seems  probable  from  the 
position  of  the  lower  quartzite  and  local  magnetic  attractions  in  the  northwest  part  of 
T.  41,  R.  30,  has  not  been  determined.  The  latter  hypothesis  is  most  in  accordance 
with  the  known  facts,  although  the  southeast  dip  of  the  quartzite  on  sections  17  and  18, 
observed  by  Dr.  Credner,  is  not  explained.  If  this  hypothesis  is  true,  the  iron  range 
should  cross  the  Menominee  somewhere  in  sees.  24  or  25,  T.  41,  K.  31,  into  Wisconsin. 
There  can  be  little  doubt  but  that  the  north  and  south  belts  belong  to  one  geological 
horizon,  hence  somewhere  come  together  (pp.  159-160). 

A  geological  section  through  the  north  range  on  the  line  between  Rs. 
29  and  30,  T.  42,  is  given,  and  is  also  represented  as  section  CC  on  Atlas 
Plate  IV.     The  succession  from  south  to  north  is  as  follows: 

Granite. 

Quartzite. 

Interval. 

Marble. 

Iron-ore  formation. 

Interval. 

Marble. 

Interval. 

Granite-gneiss  and  hornblende  and  mica  schist. 

As  represented  in  the  section,  the   beds  all  dip  toward  the  north  at 


378  THE  CRYSTAL  FALLS  IRONBEAKIJ^G  DISTRICT. 

high  angles.  In  attempting  to  correlate  these  vhrions  beds  with  those  of  the 
sonth  iron  belt,  Brooks  expei-iences  difficulty  with  the  uppermost  formation 
of  granite-gneiss  and  schist.     He  says: 

The  gueiss  and  granite  outcrop  above  described  may  be  almost  regarded  as  a 
typical  Laurentian  rock  in  its  appearance.  If  future  investigations  prove  them  to  be 
Laurentian,  a  very  trouble.some  structural  problem  would  be  presented  here,  as  we 
would  have  Laurentian  roclss  conformably  overlying  beds  unmistakably  Huronian 
(p.  175). 

As  will  be  seen,  this  objection  which  Brooks  had  anticipated  was  raised 
bv  later  workers  in  the  area  and  was  explained  by  Eominger. 

The  main  points  of  Brooks's  conclusions  may  briefly  be  summarized : 

(1)  The  iron-bearing  rocks  of  the .  Menominee  region  occur  in  two 
approximately  parallel  east-and-west  belts  (the  north  belt  being  the  Felch 
Mountain  range  and  the  south  belt  the  Menominee  range),  separated  by  a 
broad  granite  area  which  narrows  toward  the  west  by  the  convergence  of 
the  iron  belts.  The  north-and-south  belts  were  not  traced  into  each  other, 
but  their  probable  connection  was  inferred  from  their  bending  toward  each 
other  and  from  the  occurrence  of  rocks  of  the  iron-bearing  series  west  of 
the  granite  area.  The  equivalence  in  age  of  the  two  belts  was  inferred  from 
the  lithological  and  stratigraphical  similarity  exhibited  by  the  great  quartz- 
ite  and  marble  formations,  by  the  probable  continuity  above  referred  to, 
and  by  the  similar  relations  of  these  formations  to  the  basement  granites. 

(2)  The  iron-bearing  formations  of  the  Felch  Mountain  range  were 
believed  to  occur  at  two  horizons.  That  of  Felch  Mountain  itself  in  sec. 
32,  T.  42  N.,  R.  28  W.,  was  held  to  be  a  ferruginous  phase  of  the  lower 
quartzite.  On  the  other  hand,  the  exposures  of  sec.  31,  T.  42  N.,  R.  29  W., 
were  rea'ai'ded  as  belong'ing'  to  a  horizon  above  tlie  lower  marble,  and  as 
the  close  equivalent  of  unimportant  lean  ores  of  the  Menominee  range. 

(3)  In  geological  structure  the  Felch  Mountain  area  was  held  to  be  a 
northward-dipping  monocline. 

(4)  As  a  consequence  of  this  conception  of  the  structure,  Major  Brooks 
supposed  that  there  were  two  marble  formations. 

1880. 

Brooks,  T.  B.  The  geology  of  the  Menominee  iron  regioii,  east  of  the  center  of 
Range  17  E.,  Oconto  County,  Wisconsin.  Geology  of  Wisconsin,  1873-1879,  Vol.  Ill, 
published  in  1880,  Part  VII,  pages  429-599. 


PKEVIOUS  WORK  OX  FELOU  MOUNTAIN  RANGE.  379 

In  Vol.  Ill  of  tlie  Geoloy-y  of  Wisconsin,  published  in  this  year,  Brooks 
reiterates  the  views  previously  published  in  the  Michigan  reports.  The 
only  new  material  atlded  is  a  table  (p.  447)  giving  the  estimated  minimum 
thickness  for  the  north  belt  as  5,200  feet. 

1881. 

RoMTNGBR,  C.  Geol.  Survey  of  Michigan,  Vol.  IV,  Part  II,  Menominee  iron 
region.    New  York,  1881,  pp.  155-241.     With  map.   . 

This,  the  first  of  the  reports  of  the  geological  survey  of  Michigan  pub- 
lished while  Dr.  Rominger  was  in  charge,  contains  a  great  number  of  details 
concerning  explorations  in  the  Felcli  Mountain  range,  as  it  is  thus  called 
(p.  194)  for  the  first  time  in  the  chapter  on  the  Menominee  iron  region. 
The  iron-bearing  belt  along  the  Menominee  River  is  referred  to  as  the 
Quinnesee  range. 

Rominger  criticises  Brooks's  position  with  reference  to  the  age  of  the 
granite  and  gneiss  along  the  northern  border  of  the  Felch  Mountain  range 
as  follows: 

Major  Brooks  declares  the  granites  and  the  gneisses  north  of  the  Felch  Moun- 
tain ore  range  as  younger  than  the  ore  formation,  which  like  them  dips  northward; 
but  their  superposition  upon  the  ore  formation  is  nowhere  observable;  on  the  con- 
trary, the  south  side  of  the  ore  range  exhibits  in  several  places  the  direct  superposi- 
tion of  the  ore  formation  on  the  granite.  This  fact  is  known  to  Major  Brooks,  but 
he  solves  the  dilemma  by  identifying  the  granites  on  the  south  side  of  the  ore  forma- 
tion with  the  Laurentian;  those  on  the  north  side,  he  claims,  represent  the  youngest 
Hurouian  rocks.  How  he  can  do  so  I  can  not  conceive,  as  the  concerned  granitic  and 
gneissoid  rocks  north  and  south  of  the  ore  formation  are  so  absolutely  identical  that 
no  one  who  ever  sees  them  can  doubt  for  a  moment  the  quality  and  age  of  these  rocks. 
Moreover,  this  identification  of  the  northern  granite  with  the  Upper  Huronian,  and 
of  the  southern  with  the  Laurentian,  implies  another  abnormity;  groups  of  rocks, 
usually  separated  from  each  other  by  thousands  of  feet  of  intervening  strata,  are  in 
this  case  thought  to  be  in  immediate  superposition,  which  does  sometimes  occur,  but 
not  in  coincidence  with  another  improbability  like  the  one  stated  in  this  instance 

(p.  207). 

^  1887. 

Irving,  R.  D.  Is  there  a  Huronian  group?  Am.  Jour.  Sci.,  Vol.  XXXIV,  1887, 
pp.  204-263,  365-374.    Read  before  the  National  Academy  of  Sciences  April  22,  1887. 

In  discussing  this  question.  Professor  Irving  takes  occasion  to  refer  to 

the  structure  of  the  Felch  Mountain  range: 

In  the  case  of  the  Felch  Mountain  belt,  which  does  not  exceed  a  mile  in  width, 
all  of  the  strata  are  described  by  Dr.  Rominger  as  dipping  at  a  high  angle  to  the 


380  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

northward;  and  in  crossing  the  belt  from  the  south  to  the  north,  after  passing  the 
middle,  one  traverses  a  repetition  of  the  belts  crossed  farther  south,  but  in  an 
inverted  order.  It  would  seem  that  we  have  to  do  here  with  a  case  of  a  synclinal, 
whose  sides  are  folded  close  together  (p.  256). 

The  relations  of  the  Felch  Mountain  range  to  the  rocks  of  the  Menom- 
inee River  and  the  Marquette  district  are  shown  on  a  profile.  The  facts  are 
mentioned  as  having  been  derived  from  an  unpublished  manuscript  report 
(1881  to  1884)  of  Rominger  to  the  Michigan  geological  survey.^ 

1888.' 

Irving,  R.  D.  On  the  classification  of  the  early  Cambrian  and  pre-Cambrian 
formations.  Seventh  Ann.  Rept.  U.  S.  Geol.  Survey,  for  1885-86,  Washington,  1888, 
pp.  365-45-1. 

In  this  article  the  statements  made  above  are  repeated  (p.  435). 

1891. 

Van  Hise,  0.  R.  An  attempt  to  harmonize  some  apparently  conflicting  views 
of  Lake  Superior  stratigraphy.     Am.  Jour.  Sci.,  Vol.  XLI,  1892,  pp.  117-137. 

The  author  of  this  paper,  after  discussing  the  significance  of  the  various 
unconformities  observed  and  after  dividing  the  rocks  of  the  Marquette  dis- 
trict into  a  Fundamental  Complex  and  a  Lower  and  an  Upper  series,  attempts 
to  correlate  the  rocks  of  the  Menominee  region  with  these  divisions. 

Passing  now  to  the  Menominee  and  Felch  Mouutain  districts,  our  information  is 
less  exact.  It  is,  however,  clear  that  in  both  of  these  areas  we  have  the  Fundamental 
Complex — that  is,  the  granites  and  the  gneisses  associated  with  crystalline  schists 
having  the  usual  "eruptive  contacts" — the  equivalence  in  every  respect  of  Lawson's 
combined  Laurentian  and  Coutchichiug  period.  Above  this  complex.  Professor  Pum- 
pelly,  with  whom  this  whole  subject  has  been  discussed,  and  who  has  great  famil- 
iarity with  the  entire  Lake  Superior  region,  suggests  as  exceedingly  probable  that  in 
the  Felch  Mountain  iron-bearing  series  only  the  equivalent  of  the  Lower  Marquette 
occurs,  the  Upper  series,  if  it  once  existed,  having  been  removed  by  erosion  (p.  133). 

1892. 

Van  Hise,  C.  R.  Correlation  papers — Archean  and  Algonkian.  Bull.  U.  S. 
Geol.  Survey,  No.  86,  Washington,  1882,  pp.  549. 

In  a  summary  of  the  literature  on  the  Lake  Superior  region,  the  state- 
ment quoted  above  is  incorporated  without  change  (p.  190). 

I  Published  iu  1895,  Vol.  V. 


PKEVIOUS  WORK  ON  FELGH  MOUNTAIN  KANGE.  381 

18»3. 

Wadsworth,  M.  E.  Report  of  tlie  State  geologist  for  1801-92.  State  Board 
of  C;eol.  Siirv.  for  the  years  1891  and  1892,  Lansing,  1893,  pp.  01-73.  Dated  October 
17,  189L'. 

In  this  brief  report  Dr.  Wadswortli  calls  attention  to  the  granite  which 
is  intrusive  into  the  sedimentary  series  in  the  Felch  Mountain  area. 

In  tbe  Menominee  region,  especially  in  the  Felch  Mountain  district,  these  granite 
dikes  are  well  exposed.  Here  they  are  seen  not  only  to  cut  the  gneiss,  but  to  pene- 
trate the  Republic  or  iron  formation.  Mr.  Wright,  in  1885,  pointed  out  one  of  these 
dikes  cutting  the  iron  series  near  the  Metropolitan  mine,  on  sec.  32,  T.  42,  R.  28  W, 
(p.  101). 

He  includes  the  Felch  Mountain  dolerites  (p.  104)  in  his  Republic  for- 
mation, and  correlates  this  formation  with  Van  Hise's  Lower  Marquette  series. 

1S95. 

RoMiNGER,  C.  Geological  report  on  the  Upper  Peninsula  of  Michigan,  exhibit- 
ing the  progress  of  work  from  1881  to  1884.  Iron  and  copper  regions:  Geol.  Surv.  of 
Michigan,  Vol.  V,  Lansing,  1895,  pp.  1-94. 

Chronologically,  this  is  one  of  the  latest  reports  upon  the  Upper  Penin- 
sula of  Michigan,  yet  in  justice  to  the  author  it  should  be  considered  as 
having  priority  over  any  articles  published  since  Irving's,  m  1887,  to  which 
reference  has  already  been  made,  as  in  that  article  Irving  made  use  of  the 
observations  recorded  in  the  manuscript  of  this  report,  giving  Rominger 
full  credit  for  them.  In  Rominger's  report,  in  the  chapter  on  the  Granitic 
Group,  the  granite  dike  cutting  the  iron-bearing  formation  in  the  Felch 
Mountain  range  is  described  (p.  7).  He  also  describes  a  wedge-like  intru- 
sion into  the  highly  contorted  strata  in  sec.  33,  T.  42  N.,  R.  28  W.,  as  follows: 

We  have  here  evidently  before  us  a  series  of  strata  plicated  into  a  synclinal  and 
another  anticlinal  fold,  the  latter  ruptured  by  an  intruding  granite  mass,  which  rock 
is^there  the  general  surface  rock  and  comes  on  the  south  end  of  the  exposure  in 
contact  with  the  uppermost  ferruginous  strata  of  the  overtilted  anticlinal  fold  (p.  8). 

•  The  portion  of  the  chapter  on  the  iron-ore  group  which  deals  with  the 
Menominee  region  is  devoted  to  the  Felch  Mountain  range  and  to  outlying 
prospects  as  far  north  as  Michigamme  Mountain.  The  description  of  the 
Felch  Mountain  area  is  in  great  part  a  condensation  of  earlier  scattered 
observations.  The  strata  are  given  as  dipping  high  to  the  north  and 
consisting  of  the  following  succession: 

The  underlying  rock  of  the  iron  formation  is  always  formed  of  crystalline  rocks, 
granite  or  diorite.    The  lowest  strata  are  generally  heavy  light-colored  quartzite 


382  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

beds,  with  interlaininated  thinner  ledges  and  schistose  seams,  amounting  to  consider- 
able thickness. 

Above  this  belt  an  equally  large  succession  of  well-laminated,  even-bedded,  often 
fissile,  slate  like  micaceous  quartz-schists  follow,  which  have  a  silvery  luster.  Next 
above  them  comes  a  series  of  micaceoas  argiUites,  amounting  to  a  belt  even  larger 
than  the  former,  which  varies  greatly  in  shades  of  color,  firmness  of  grain,  etc.;  some 
layers  are  whitish,  others  gray  or  bluish  and  greenish,  but  the  greatest  portion  of 
them  is  intensely  red  colored  by  hematitic  pigment.  A  part  is  a  fatty,  impalpably 
fine  mass  of  silky  or  also  pearly  luster,  according  to  the  size  of  the  mica  shales 
incorporated  with  them.  Another  part  is  rough  and  gritty  from  the  prevalence  of 
arenaceous  constituents. 

At  this  horizon  and  rather  in  the  lower  part  of  it  occur  locally  large  bodies  of 
crystalline  limestone  ledges,  some  snowy  white  like  Italian  marble,  but  of  coarser  crys- 
talline grain  and  intermingled  with  radiating  clusters  of  asbestine  fibers  and  larger 
prismatic  crystals  of  colorless  tiemolite,  which  sometimes  forms  larger  concretionary 
seams  in  the  lime  rock,  and  are  then  intimately  associated  with  crystal  masses  of 
sahlite,  one  mineral  penetrating  the  other  in  a  manner  which  suggests  either  a  process 
of  paramorphosis  in  progress,  changing  the  sahlite  into  tremolite,  or  the  original 
conditions,  when  the  calcareous  material  combined  with  the  silica  by  a  slight  modifi- 
cation, induced  simultaneously  the  crystallization  of  the  almost  identical  chemical 
combinations  iu  one  and  the  other  form ;  which  latter  suggestion  is  more  sustained 
by  the  actual  condition  of  the  mingled  minerals  than  the  first,  as  some  of  the  crystals 
of  both  minerals  tightly  grown  together  are  so  perfect  in  form  peculiar  to  each  and  so 
sharply  defined  that  they  must  be  considered  as  crystal  individuals  which  formed  side 
by  side  and  altogether  independent  of  one  another.  In  other  localities  where  such 
crystalline  limestone  belts  occur,  the  tremolite  is  only  sparingly  intermingled,  but  iu 
its  place  colorless  mica  scales  of  nacreous  luster  are  plentifully  disseminated. 

In  the  place  of  marble-like  limestone,  sometimes  also  ordinary  lime  rock  of  dull 
aspect  with  conchoidal  fracture,  and  variously  tinged,  occurs;  it  is  then  usually  full 
of  flinty  siliceous  seams,  resembling  the  limestones  of  the  Quinnesec  range;  the 
quartzose  seams,  locally  even,  prevail  over  the  calcareous.  Incumbent  on  the  above- 
mentioned  micaceous  argellites  succeeds  a  belt,  about  800  feet  in  width,  composed  of 
thinly  laminated,  banded,  ferruginous,  quartzite  ledges  of  dark  purplish  tints  or  hav- 
ing a  metallic  luster  from  intermixture  of  specular  ore  granules.  The  banded  portions 
are  formed  of  an  alternation  of  narrow  seams  of  specular  ore  with  siliceous  seams  not 
so  richly  impregnated  with  the  oxide.  Other  strata  in  the  succession  are  porous 
cherty  rocks  charged  with  ochreous  yellow  or  brown  oxide  of  iron  and  inclosing  i)ockets 
of  the  limonitic  ore.  Also  blood-red  argillitic  seams  occur  in  the  succession,  and  with 
them  sometimes  pockets  of  soft  crumbly  hematite  ore. 

Within  the  first-mentioned  banded  alternation  of  narrow  ore  seams  with  quartz 
seams,  larger  deposits  of  specular  ore  in  slaty  or  in  compact  granular,  or  also  in  the 
soft  friable  condition  of  the  so-called  blue  ore  of  the  Quinnesec  mines  occur,  which 
constitute  the  principal  storage  of  ore  sought  for  by  the  miner,  besides  the  hematitic 
and  limonitic  deposits  mentioned  before.  The  first  impression  of  every  observer  exam- 
iiiing  this  above  described  rock  series  will  induce  him  to  consider  it  as  an  ascending 
succession,  as  the  layers  follow  one  another  in  apparent  conformity;  but  in  some  local- 
ites,  after  having  crossed  this  succession  so  far,  if  we  proceed  farther  in  the  same 


GEOLOGY  OP  FELGH  MOUNTAIN  RANGE.  383 

direction,  we  intersect  the  same  series  again  in  an  inverted  order,  but  retaining  the 
same  di|),  until  we  have  reached  again  a  hirge  beltof  compact  fjuartzite  ledges  in  close 
contiguity  with  granite  or  also  diorite,  as  it  may  happen,  which  latter  rocks  then  form 
the  surface  rock  of  large  areas  on  the  luirth  side  of  the  Felch  Mountain  ore  formation 
(p.  33). 

The  only  satisfactory  explanation  which  I  can  give  of  this  repetition  of  the  rock 
beds  in  an  inverted  order  is  the  suggestion  of  a  folding  of  the  beds  and  the  overturn 
of  tlie  fold  by  a  pressure  acting  principally  from  the  north  side.  If  this  is  the  case, 
we  would  have  to  consider  the  light-colored  quartzite  next  to  the  granite  as  the  most 
recent  deposits  and  the  dark,  ore-bearing,  banded  quartz  beds  as  the  oldest,  which 
would  bring  the  structure  of  the  Felch  Mountain  ore  formation  in  perfect  harmony 
with  that  of  the  Quinnesec  ore  range  (p.  34). 

SECTIOX   II.    GENERAL   SKETCH   OP  THE   OEOL,OGY. 

The  rocks  of  tlie  Felch  Mountain  range  extend  from  the  Archean  to 
the  early  Paleozoic.  The  Paleozoic  is  represented  by  the  Lake  Superior 
sandstone,  of  supposed  Upper  Cambrian  age,  and  the  overlying  Calciferous 
limestone.  These  formations  were  originally  laid  down  over  the  upturned 
edges  of  the  older  rocks  in  flat  sheets  or  with  low  initial  dips,  and  have  not 
since  suffered  relative  displacement  to  any  notable  degree.  As  has  already 
been  stated,  subsequent  erosion  has  to  a  great  extent  removed  this  over- 
lying blanket  and  laid  bare  the  older  rocks,  except  for  the  covering  of 
recent  glacial  deposits.  The  Cambrian  sandstone,  and  to  a  less  extent  the 
Calciferous  limestone,  still,  however,  occupy  considerable  outlying  areas, 
detached  from  one  another  throughout  most  of  the  district,  but  gradually 
coalescing  beyond  the  eastern  end,  where  they  completely  cover  the  older 
rocks  and  limit  all  further  geological  study  of  these  in  that  direction. 

The  Paleozoic  rocks  will  not  be  considered  further  at  present.  On  the 
detailed  Felch  Slountain  map  (PI.  XLIX)  their  known  outcrops  are  repre- 
sented by  appropriate  symbols,  but  except  in  the  larger  areas,  where  they 
so  completely  conceal  the  older  rocks  that  the  distribution  of  these  can  not 
be  determined,  they  are  assunaed  not  to  be  continuous,  and  to  be  non- 
existent, and  in  this  respect  stand  upon  the  same  footing  as  the  Pleistocene 
glacial  covering. 

The  Archean,  which  is  here  made  up  of  granites,  granitic  gneisses,  and 
various  kinds  of  crystalline  schists,  is  the  basement  group  of  the  region. 
The  areas  in  which  these  rocks  are  now  ex])osed  at  the  surface  represent 
the  cores  of  the  larger  arches  which  were  constructed  over  the  whole  region 
by  the  early  manifestations  of  mountain-building  activity,  and  subsequently 


384  THE  CRYSTAL  FALLS  IRON-BEAEING  DISTRICT. 

truncated  by  the  deep  Cambrian  denudation.  Our  studies  have  dealt  with 
the  Archean  only  in  narrow  marginal  zones,  and  have  included  little  more 
than  the  location  of  its  outer  boundaries,  except  when  it  was  necessary  to 
go  deeper  in  order  to  complete  the  work  over  a  full  section.  Consequently 
no  attempt  at  classification  can  be  made  upon  the  map. 

The  rocks,  chiefly  of  sedimentary  origin,  which  are  intermediate  in  age 
between  the  Archean  below  and  the  Paleozoic  above,  and  therefore  fallwithin 
the  system  to  Avhich  the  name  Algonkian  has  been  given  b}'  this  Survey, 
occupy  a  narrow  strip  nowhere  more  than  a  mile  and  a  half  and  usually 
less  than  a  mile  wide,  which  as  a  whole  runs  almost  exactly  east  and  west 
for  a  distance  of  over  13  miles.  This  strip  constitutes  the  Felch  Mountain 
range.  On  the  north  and  south  it  is  bordered  by  the  older  Archean.  The 
lowest  member  of  the  Algonkian  occupies  parallel  zones  next  to  the  Archean 
both  on  the  north  and  south,  and  is  succeeded  toward  the  interior  of  the 
strip  by  the  younger  members.  While  the  general  structure,  therefore,  is 
synclinal,  a  single  fold  of  simple  type  has  nowhere  been  found  to  occupy 
the  whole  cross  section  of  the  Algonkian  formations,  but  usually  two  or 
more  synclines  occur,  separated  by  anticlines,  which  may  have  different 
degrees  and  directions  of  pitch  and  different  strikes,  or  may  be  sunk  to 
diff"erent  depths,  and  complicated  besides  both  by  subordinate  folds  and  by 
faults. 

Among  the  Algonkian  rocks  we  distinguish  two  main  divisions  or  series, 
which  are  probably  separated  from  each  other  by  an  unconformity.  Owing 
mainly  to  the  peculiar  lithological  and  weak  physical  character  of  the 
younger  of  these  two  series,  actual  contacts  between  them  have  not  been 
found,  and  the  evidence  of  unconformability  consequently  consists  not  so 
much  in  observed  discordance  of  structure  as  in  an  inferred  discordance 
based  upon  their  relative  surface  distribution.  This  evidence  will  be  fully 
stated  hereafter. 

In  the  lower  of  these  two  series  are  included  four  formations  which 
clearly  appear  to  be  identical  in  lithological  character  and  order  of  super- 
position  with  the  four  formations  that,  so  far  as  is  known,  make  up  the  lower 
iron-bearing  series  along  the  Menominee  River.  These  are,  reckoning  from 
the  base  upward,  (1)  the  Sturgeon  quartzite,  (2)  the  Randville  dolomite,  (3) 
the  Mansfield  schists,  (4)  the  Groveland  iron  formation. 

Above  this  series  follows  the  younger  series,  which  lithologically  and  in 


AHCHEAN  IN  FELCH  MOUNTAIN  DISTRICT.  385 

its  art'iil  relations  is  very  incompletely  known.  It  inclndes  mica-schists, 
ferruginous  schists,  iuul  thin  interbeddetl  ferruginous  quartzites.  These 
rocks,  which  from  our  imperfect  knowledge  must  for  the  present  be  grouped 
as  a  single  formation,  are  believed  to  have  been  deposited  contempora- 
neously with  the  somewhat  similar  rocks  that  occur  in  the  Menominee 
area,  at  Iron  Mountain,  but  are  most  extensively  exjKised  west  of  the 
Menominee  River,  and  especially  in  the  Commonwealth  and  Florence 
district  in  Wisconsin. 

SECTIOX  III.    THE  ARCHEAX. 

The  Archean  occurs  in  the  Felch  Mountain  district  in  two  belts,  which 
limit  the  Algonkian  rocks  on  the  north  and  on  the  south.  The  north- 
ern belt  for  the  most  part  does  not  fall  within  the  limits  of  the  detail  map 
(PI.  XLIX).  It  occupies  a  triangular  corner  in  sees.  34  and  35,  T.  42  N., 
R.  30  W.,  at  the  extreme  western  end  of  the  area  sm-veyed,  and  even  in 
these  it  has  not  been  directly  observed,  but  its  presence  is  inferred  from 
outcrops  in  the  adjoining  sections  west  and  north  and  from  the  observed 
strikes  in  the  overlying  Algonkian  formations.  For  the  next  11  miles  east 
its  southern  boundary  lies  in  the  tier  of  sections  next  north  of  those  majDped 
in  detail  and  probably  always  less  than  a  mile  away.  This  boiindary  is 
therefore  not  very  accurately  drawn,  as  only  enough  outcrops  were  visited 
to  permit  its  position  to  be  fixed  in  a  general  way.  Our  work  first  touches 
the  southern  area  of  the  Archean,  which  is  much  better  known  on  the  west 
in  sees.  3  and  2,  T.  41  N.,  R.  30  W.,  a  short  distance  south  of  the  township 
line.  Thence  for  3  miles  eastward  the  boundary  follows  the  township  line, 
and  in  sec.  31,  T.  42  N.,  R.  29  W.,  crosses  it  with  a  trend  somewhat  north  of 
east.  From  the  west  line  of  sec.  31  to  the  east  line  of  sec.  36,  T.  42  N., 
R.  29  W.,  the  Archean  occupies  the  southern  third  of  the  south  tier  of  sec- 
tions. Thence  for  a  mile  and  a  half  it  bends  northeast,  and  in  sec.  32, 
T.  42  N.,  R.  28  W.,  reaches  its  farthest  north  in  the  center  of  the  section. 
From  this  point  the  boundary  runs  southeast,  with  a  sinuous  embayment  to 
the  south,  and  passes  outside  the  limits  of  the  map  a  little  north  of  the 
southeast  corner  of  section  33. 

Throughout  the  Felch  Mountain  range  the  southern  Archean  is  much 

better  exposed  than  any  of  the  other  terranes.     In  the  western  portion  of 

the  range,  where  hardly  more  than  the  contact  zone  falls  within  our  limits, 
MON  xxxvi 25 


386  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

outcrops  are  not  especially  numerous;  but  in  the  six  eastern  sections,  which 
include  a  belt  from  a  quarter  to  half  a  mile  wide,  a  very  considerable  por- 
tion of  the  surface  is  bare  rock.  This  exceptional  degree  of  exposui-e  has 
been  brought  about  by  the  forest  fires,  which,  by  loosening  the  thin  soil 
and  destroying  the  protecting  cover  of  vegetation,  have  facilitated  its 
removal  from  the  steep-sided  knobs  that  are  such  characteristic  featm-es  of 
the  Archean  topography. 

TOPOGRAPHY. 

The  Archean  areas,  particularly  the  southern,  are  distinguished  by 
a  characteristic  rough  topography.  The  surface  is  exceedingly  uneven 
on  rather  a  small  scale,  and  has  already  been  described  as  consisting  of 
hunnnocky  elevations  alternating  with  bowl-shaped  depressions.  Both 
hummocks  and  bowls  are  elongated  in  an  east-and-west  direction,  in  accord- 
ance with  the  prevalent  gneissic  foliation. 

While  the  surface  is  thus  so  full  <>f  small  details  that  an  adequate 
delineation  of  it  is  the  despair  of  the  topogra]>her,  the  actual  relief  is 
insiii'nificant.  To  men  bred  on  the  flat  ])lains  of  the  lower  lakes,  as  were 
most  of  the  early  surveyoi's  and  explorei-s,  it  may  naturally  have  appeared 
mountainous,  since  roughness  is  a  quality  particularly  noticeable  in  a  wil- 
derness like  this,  that  can  be  traveled  only  on  foot.  But  from  a  broader 
point  of  view  the  irregularities  are  almost  wholly  lost.  The  higher  summits 
in  the  same  neighljorhood  rise  to  within  a  few  feet  of  each  other.  The 
distant  sky  line  is  even  in  all  directions.  There  is,  however,  a  gentle 
ascent  from  east  to  west,  quite  imperceptible  on  the  ground,  and  made 
evident  only  by  the  general  course  of  the  streams  or  by  leveling. 

Along  the  contacts  between  the  Archean  and  Algonkian  systems  there 
usually  but  not  always  exists  a  topographical  depression,  occupied  by 
swamp  or  streams.  North  of  the  southern  Archean  mass  this  depression 
is  a  well-marked  linear  valley,  extending  with  some  interruption  from 
sec.  33,  T.  42  N.,  R.  28  W.,  on  the  east,  for  6  miles  west  to  sec.  33,  T.  42  N., 
R.  29  W.  For  2  miles  in  tlie  middle  of  this  stretch  the  valley  is  occupied 
by  the  Sturgeon  River;  thence  west  for  2  miles  by  a  small  feeder  of  the 
Sturgeon,  while  the  eastern  third  holds  swamp  with  ill-defined  drainage. 
On  the  south   the  Archean   boundary  of   this  valley   generally  rises  with 


AKCHEAN  IN  FELCH  MOUNTAIN  DISTRICT.  387 

steep  slopes,  which  are  frequently  escarpmeut-like  iu  cliaracter,  and  tor 
short  distauces  present  smooth  faces  to  the  valley.  In  sec.  33,  T.  42  N., 
R.  28  W.,  the  mural  face  which  runs  southeast  across  the  eastern  half 
of  the  section  with  the  regularity  of  a  ruled  line  is  a  true  fault  scarp. 
Toward  the  western  end  of  this  valley,  in  sec.  32,  T.  4-2  N.,  R.  29  W., 
the  Hoor  gradually  rises  ;uid  the  swamp  area  broadens,  penetrating  the 
Archean  in  a  network  of  thicker  and  thicker  mesh  about  the  hiaher  hum- 
mocks,  until  these  are  finally  oveitopped. 

PETROGRAPHICAL  CHARACTERS. 

The  rocks  of  the  Archean  areas  niay  be  di^dded  into  four  quite  distinct 
types,  namely:  (1)  Granites  or  granitic  gneisses,  (2)  gneisses  with  banding 
or  distinct  lamination,  (3)  mica-schists,  and  (4)  hornblende-gneisses  or 
amphibolites.  Between  the  first  two  divisions  there  is  an  extremely  close 
mineralogical  and  chemical  lUieness,  while  in  these  respects  the  fourth 
division  stands  against  all  the  others  in  strong  contrast. 

(1)  The  granites  of  the  first  division  are,  as  seen  in  the  field  or  in  the 
hand  specimen,  holocrystalline  rocks  of  fine  to  medium  grain,  in  which  the 
eye  can  readily  distinguish  the  presence  of  quartz,  pink  feldspar,  muscovite, 
and  biotite.  In  color  they  are  prevailingly  of  pink  or  reddish  tints  of  light 
shades.  Structurally,  they  frequently  appear  in  small  areas  to  be  entirelv 
massive,  but  even  in  the  most  massive  occurrences  the  hammer  can  usuallv 
part  them  along  roughly  parallel  surfaces  which  glisten  with  spangles  of 
mica,  indicating  a  certain  degree  of  alignment  in  these  constituents.  Gen- 
erally, however,  a  rude  foliation  is  more  or  less  distinctly  visible,  and  is 
sometimes  exceedingly  well  developed,  even  to  the  point  of  fissilitv.  It  is 
always  apparently  due  to  the  parallel  arrangement  of  the  micas,  which 
are  more  abundant  as  the  foliation  becomes  more  distinct. 

The  field  relations  show  that  the  massive  and  more  or  less  Ibliated 
varieties  of  this  divi.sion  are  closely  bound  together  by  indistinguishable 
gradations  and,  indeed,  often  constitute  a  visibly  integral  mass.  The  usual 
arrangement  of  the  micas  is  not  parallel  to  a  surface  but  parallel  to  a  line 
which  is  generally  inclined  to  the  horizon  at  angles  varying  between  10° 
and  35°.     A  hand  specimen  when  turned  about  the  direction  of  foliation  as 


388  ■  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

an  axis,  shows  a  parallel  arrangement  of  the  micas  on  all  sides,  and  a  con- 
tinuous glisten  follows  the  revolution;  while  on  a  surface  at  right  angles  to 
this  direction  the  micas  are  not  parallel  and  wind  about  the  other  constitu- 
ents indifferently.  In  the  more  fissile  varieties  the  outcrops  often  have  a 
rough,  channeled  surface,  suggestive  of  the  surfaces  familiar  in  closely 
crenulated  mica-schists,  or  on  the  corrugated  walls  of  a  fault.  Similar  cor- 
rugated surfaces  frequently  j^art  more  massive  from  more  fissile  parts  of  the 
same  outcrop. 

Under  the  microscope  the  essential  constituents  of  the  granites  and 
granitic  gneiss  are  seen  to  be  quartz,  orthoclase,  microcline,  plagioclase, 
biotite,  and  muscovite,  with  the  iron  ores,  titanite,  and  occasionally  apatite 
and  zircon  as  accessories.  In  the  massive  phases  the  general  relations  of 
these  minerals  to  one  another,  and  their  order  of  crystallization,  in  no  respect 
differ  from  those  of  igneous  granite.  The  quartz,  which  is  the  last  mineral 
to  form,  contains  numerous  fluid  and  gas  inclusions,  the  former  often  with 
a  mo^^ng  bubble.  (Jf  the  feldspars  microcline  is  nmeh  the  most  common, 
then  plagioclase,  while  orthoclase  is  generally  comparatively  rare,  although 
sometimes  it  is  more  abundant  than  the  microcline.  The  plagioclase,  from 
its  relief  and  extinction  angles,  is  probably  not  lower  in  the  scale  than  oligo- 
clase.  The  orthocUise  is  usually  clouded  with  alteration  products,  and  some- 
times the  dull  interior  is  surrounded  with  a  narrow  unattacked  rim.  Botli 
micas  are  always  present  as  original  minerals,  and  on  the  whole  biotite  is 
the  more  abundant.  They  occur  in  small  stout  crystals,  often  as  inclusions 
in  the  quartz  and  feldspars.  Magnetite  is  rare,  but  occurs  in  idiomoi-phic 
forms  in  the  later  constituents,  as  do  also  minute  crystals  of  zircon  and 
apatite.  Thin  sections  of  even  the  most  massive-looking  specimens  invari- 
ably show  the  effects  of  pressure  in  the  undulatory  extinction  of  the  quartz 
and  in  the  bending  and  occasional  fracture  of  the  feldspar. 

In  the  foliated  varieties  with  which  these  massive  varieties  are  closely 
associated  the  effects  of  mechanical  stresses  are  the  striking  microscopic 
phenomena.  The  constituent  minerals  are  essentially  the  same  as  in  the 
massive  phases,  but  the  micas  are  relatively  more  abundant.  The  quartz 
and  feldspar  individuals  are  fractured  and  strained,  and  occur  in  irregular 
cores  sepai-ated  by  anastomosing  zones  of  a  fine  quartz-feldspar  mosaic.  In 
these  last,  new  micas,  in  long  curving  individuals  and  clusters,  have  been 
developed  in  great  numbers. 


AROHEAN  IN  FELOH  MOUNTAIN  DISTRICT. 


3H9 


The  following  analyses  give  tlie  clu'Diical  fouijUKsititiu  of  these  granites: 

Analyses  of  granites. 


[liyUr.  H.  N.  Stok 

es,  U.  S.  Geol.  Sun-ey.l 

1.' 

2.» 

3.» 

SiO. 

76.10 

.07 

Noue. 

.02 

12.95 

Noue. 

.65 

.09 

Trace. 

None. 

.12 

.14 

6.50 

2.36 

.17 

.48 

72.17 
.37 

69.69 

.29 

TiO; 

CO. 

P.O. 

AljO, 

CfiOj 

14. 44' 

15.  64' 

Fe  1O3 

1.02 
.99 

.90 
1.62 

FeO 

MnO 

NiO 

CaO 

.69 

.70 
4.84 
3.65 

1.22 

.66 

5.30 

3.34 

MgO 

K2O 

NaO 

H.OatllO'^  

H^O  above  110^ 

Total 

99.65 

I  Ba,  Sr,  Li,  CI,  S,  SO3  were  not  looked  for.         '  Water  not  determined.  '  Includes  PiOr,. 

No.  1.  Specimen  34677,  Lake  Superior  Division,  U.  S.  Geol.  Surv.,  1,935  N.,  1,040  W.,  sec  2,  T.  41  N., 
R.  30  W.,  Upper  Penin.sula  of  Michigan. 

No.  2.  Specimen  34828,  Lake  Superior  Division,  V.  S.  Geol.  Surv..  300  N.,  1,8.50  W.,  sec.  36,  T.  42  N., 
R.  29  W.,  Upper  Peninsula  of  Michigan. 

No.  3.  Specimen  36081,  Lake  Superior  Divi-sion,  U.  S.  Geol.  Surv.,  15  N.,  1,025  W..  sec  31,  T.  42  N., 
R.  28  W.,  Upper  Peninsula  of  Michigan. 

No.  1,  which  is  rather  low  in  alumina,  iron,  and  lime,  is  a  granitic  gneiss 
in  which  the  abundant  secondary  mica,  wdiich  has  grown  in  long  curving 
plates  in  nearly  parallel  zones  of  granulation,  is  wholly  muscovite.  Nos.  2 
and  '6  are  fine-  and  coarse-grained  pink  granites,  which  show  comparatively 
little  crushing  and  development  of  secondary  minerals  in  thin  section. 

The  rocks  of  this  division  therefore  have  the  chemical  composition  as 
well  as  the  physical  and  petrograjjhical  characters  of  igneous  granites. 
The  positive  proof  of  igneous  origin,  hoA^ever — actual  injection  into  older 
rocks — we  have  not  found.  Irruptive  contacts  may  possibly  exist,  and 
may  have  escaped  us,  since  neither  the  Archean  as  a  whole  nor  its  internal 
relations  were  the  objects  of  especially  rigid  scrutiny.  Igneous  granites 
of  Algonkian  or  later  age  ought  to  be  found  within  the  Archean  areas,  for 
several  granite  dikes  are  known  to  penetrate  various  members  of  this  over- 
lying series.  Whether  the  known  granites  within  the  Archean  are  really 
lower-lying  and  larger  masses  witli  which  such  dikes  are  genetically  con- 


390  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

nected  is  not  known,  but  the  possibility  must  be  admitted.  The  banded 
gneisses  are  often  so  faintly  foliated  and  resemble  the  granites  so  closely  in 
color  and  grain  that  the  distinction  can  be  made  with  only  the  microsco])e, 
and  igneous  contacts  between  them  might  easily  be  overlooked.  It  is 
certain,  however,  that  if  the  granites  have  been  injected  into  the  banded 
o-neisses  it  has  not  been  in  the  form  of  narrow  dikes,  and  the  fact  remains 
that  no  case  of  an  igneous  contact  is  recorded  in  our  notes. 

The  gneissic  members  of  this  division  are  merely  crushed  granites,  and 
owe  their  foliation  partly  to  the  crusliing  and  partly  to  the  growth  of  fresh 
mica  in  the  fractured  zones.  The}'  differ  from  the  banded  gneisses  in  fur- 
nishing l»oth  field  and  microscopic  proof  of  the  way  in  which  the  foliation 
was  formed  and  of  the  rocks  from  which  they  were  derived. 

(2 )  The  banded  gneisses  of  the  second  group  have  essentially  the  same 
mineral  composition  as  the  granitic  gneisses  of  the  first.  They  are  distin- 
guished by  the  eye  mainly  by  the  fact  that  the  component  minerals  occur  in 
ra(^re  or  less  distinct  layers,  from  a  fraction  of  an  inch  ujjward  in  thickness. 
The  lamination,  which  only  rarely  is  very  regular,  seems  to  be  caused  in  most 
if  not  in  all  cases  by  the  alternation  of  darker  layers,  which  are  relatively 
rich  in  biotite,  with  lighter  layers,  which  are  comparatively  and  sometimes 
wholly  free  from  it.  The  light  layers  are  almost  always  coarser  in  texture 
than  the  darker,  and  frequently  are  coarsely  pegmatitic.  The  individual  bands 
are  not  indefinitely  persistent,  but  wedge  out  to  knife-edges.  The  banding  is 
sometimes  so  indefinite  as  to  be  lost  in  the  hand  specimen,  the  large  surface  of 
an  outcrop  being  necessary  to  bring  out  the  slight  differences  in  shade.  In 
color  these  rocks  are  light  gray,  dull  ^vhite,  or  pink.  The  banding  shows  great 
variations  in  angle  of  dip,  but  the  strike  is  usually  fairly  constant  within  a  few 
deo-rees  of  east  and  west.  In  a  few  localities  distinct  contortion  was  observed 
in  the  gneis.sic  banding  and  pitching  folds.  The  lamination  of  these  gneisses 
is,  so  far  as  observed,  of  tlie  plane-parallel  type.  The  bands  are  thoroughly 
welded  together,  and  as  a  rule,  the  rock  breaks  indifferently  across  them. 

Under  the  microscope  the  composition  of  these  rocks  does  not  differ 
from  that  of  the  granitic  rocks  of  the  first  division.  The  structural  charac- 
ters, however,  are  in  strong  contrast.  Even  in  those  specimens  which 
possess  the  most  indistinct  foliation  all  the  minerals  are  elongated  in  a 
common  direction.  While  the  individual  grains  in  most  cases  show  more 
or  less  strain  and  are  frequently  fractured,  their  mutual  boundaries  are 
usually  sharp  and  clear,  and  it  is  evident  that  the  forms  are  not  the  direct 


AKGHBAN  IN  FELGU  MOUNTAIN  DISTURIT. 


391 


result  of  the  pressure  tluit  has  ati'eeted  their  optical  properties.  The 
evidence  is  quite  clear  that  tlic  niinerals  now  present  have  crystallized  in 
parallel  elongated  forms,  and  it  is  to  this  they  owe  their  prevalent  lamina- 
tion even  when  the  color  1  landing-  is  indistinct  or  wanting'. 

Subsequent  to  the  time  of  crystallization  they  have  been  exposed  to 
the  action  of  great  stresses,  which  not  onh^  have  left  a  record  in  the  strains 
now  frecpieutly  perceptible  in  the  minerals  of  the  early  crystallization,  but 
also  in  many  cases  have  produced  roughly  parallel  fractures  and  fracture 
zones  sometimes  coinciding  with  and  sometimes  oblique  to  the  early  lami- 
nation. In  these  zones  coarse  micas  have  grown,  reenforcing  the  old 
lamination  when  parallel  to  it,  and  when  oblique  producing  a  less  regular 
secondary  foliation,  which  is  entirely  analogous  and  probably  contempo- 
raneous with  the  foliation  of  the  cnished  granites. 

The  following  analyses  of  these  gneisses  are  interesting  as  showing 
their  striking  chemical  relationship  to  the  granites  (analyses  of  which  are 
given  on  p.  389),  with  which  they  are  intimately  associated : 

Analyses  of  gneiss. 

[By  Dr.  H.  N.  Stokes,  U.  S.  Geol.  Survey.] 


1.1 

2.2 

3.2 

SiO, 

74.37 

.07 

None. 

.01 

13.34 

71.79 
.35 

74.63 
.09 

TiO, 

CO 

P,0,  

AID, 

14.  79' 

13.  951 

CrO,  

Fe,0, 

FeO 

.92 

.21 

Trace. 

1.10 
1.09 

.35 
.32 

MiiO 

NiO 

CaO 

.50 
.27 
6.70 
2.50 
.12 
.44 

1.11 

.71 

3.79 

4.29 

1.08 

.22 

6.73 

2.55 

MgO 

K,0 

Na.O 

H.2O  at  110" 

H.:0  above  llO"^  

Total 

99.45 

'  Ba.  Sr,  Li,  CI,  S,  SO3  were  not  looked  for. 


-  Water  not  determined. 


'  Includes  P»0.r,. 

No.  1.  Specimen  34826,  Lake  Superior  Division,  U.  S.  Geol.  Surv.,  240  N.,  1,250  W.,  sec.  35,  T.  42  N., 
R.  29  W.,  Upper  Peninsula  of  Michigan. 

No.  2.  Specimen  36058,  Lake  Superior  Division,  U.  S.  Geol.  Surv.,  325  N.,  1,225  W.,  sec.  36,  T.  42  N., 
R.  29  W.,  Upper  Peninsula  of  Michigan. 

No.  3.  Specimen  36080,  Lake  Superior  Division.  U.  S.  Geol.  Surv.,  15  N.,  1,025  W.,  sec.  31,  T.  42  N., 
R.  28  W.,  Upper  Peninsula  of  Michigan. 


392  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

(3)  The  mica-scliists  are  not  widel}'  distributed  in  the  portion  of  the 
Arcliean  areas  included  in  the  Felch  Mountain  map.  They  are  well  rep- 
resented in  the  northern  Archean  area  beyond  the  limit  of  the  area  mapjjed, 
but  within  this  limit  they  are  known  only  in  sees.  34  and  35,  T.  42  N.,  R. 
29  W.,  where  an  overthrust  fault  brings  them  into  successive  contact  with 
the  Randville  dolomite  and  Sturgeon  quartzite  for  a  distance  of  three- 
fourths  of  a  mile.  An  excellent  section,  which  includes  the  faulted  con- 
tact with  the  dolomite,  is  exposed  along  the  Sturgeon  River  below  the 
dam  in  the  northern  portion  of  section  35.  Though  so  feebly  represented, 
they  possess  an  unusual  interest  both  in  their  field  relations  and  in  their 
microscopic  characters. 

The  mica-schists  when  fresh  are  dark  gi'ay,  rather  soft  rocks,  of  fine  to 
medium  grain,  with  a  generally  well-developed  schistose  structure.  The 
HKist  noticeable  constituent,  in  spite  of  the  dark  color,  is  muscovite,  which 
occurs  in  pearl}'  flakes  of  large  size  plentifully  sprinkled  along  the  cleav- 
age surfaces,  and  is  especially  characteristic  of  thin  seams,  which  are  much 
more  fissile  than  the  rest  of  the  rock  and  part  it  into  parallel  bands  of  nmeh 
regularity.  Biotite,  however,  is  the  more  abundant  mica,  although  in 
smaller  and  less  conspicuous  plates,  and  to  it  the  dark  color  of  the  rock  is 
due.     Quartz  and  sometimes  feldspar  may  also  be  recognized. 

These  rocks  ofter  little  resistance  to  the  weather.  The  biotite  gives  up  its 
iron  with  great  ease,  staining  the  outcrop  a  dull  red.  The  final  product  is  a 
slightly  coherent  ferruginous  mixture  in  which  the  large  muscovite  plates 
alone  are  recognizable.  At  a  less  advanced  stage  of  weathering  the  alterna- 
tion of  layers  more  rich  in  biotite  produces  color  banding  in  reds  and  grays. 

The  mica-schists  contain  many  intruded  dikes  and  sheets  of  flesh-colored 
pegmatite  and  also  of  amphibolite,  both  of  which  are  generally  parallel  to 
the  foliation.  The  pegmatites  are  typical  "  schrift-granits,"  the  feldspar  being 
microcline.  Both  pegmatites  and  amphibolites  show  ragged  and  intrusive  con- 
tacts with  the  schists  when  these  are  examined  in  detail.    Both  also  are  foliated. 

Under  the  microscope  the  mica-schists  are  thoroughly  crystalline  aggre- 
gates of  quartz,  biotite,  and  muscovite,  always  with  more  or  less  microcline. 
Magnetite  is  always  present  as  a  primary  mineral,  and  hematiteorsomehydrous 
oxide  of  iron  between  hematite  and  limonite  is  very  abundant  in  the  zone  of 
weathering.  Besides  these,  tourmaline  is  an  abundant  accessory  in  some 
slides,  and  apatite,  zircon,  titanite,  pyrite,  and  chlorite  also  commonly  occur. 


AKCDEAN  IN  FELCH  MOUNTAIN  DISTRICT.  393 

Quartz  occurs  in  small  and  often  partly  rounded  areas,  some  of  which 
have  a  very  clastic  appearance.  Except  as  stated  below,  it  is  generally  free 
from  inclusions  of  the  micas,  which  suiTOund  and  terminate  against  it  in 
such  a  way  as  to  indicate  that  it  crystallized  the  earlier.  It  is  often  crowded 
with  fluid  and  gas  inclusions,  and  an  occasional  grain  bristles  Avith  radiating 
clusters  of  rutile  needles.  Minute  crystals  of  magnetite  are  also  frequently 
inclosed.  The  inclusions  of  all  kinds  are  frequently  grouped  in  roughly 
oval  areas  near  the  centers  of  the  grains,  while  between  the  nuclei  and  the 
wandering  perimeters  the  quartz  is  relatively  free  from  inclusions. 

Biotite,  varying  in  color  from  dark  brown  to  light  yellowish  green,  is 
the  predominant  mica.  It  occurs  in  irregular  plates,  generally  much  larger 
than  the  quartz;  the  great  abundance  and  uniform  alignment  of  these  plates 
produce  the  schistose  structure.  As  already  stated,  it  includes  and  is  there- 
fore younger  than  the  quartz  generally,  but  it  is  also  found,  though  rarely 
and  always  in  very  minute  plates,  included  in  the  small  quartz  grains  which 
are  so  abundant  in  the  fresh  microclines.  The  latter  occurrences  belong  to 
an  earlier  generation  than  that  of  the  larger  biotites:  The  chief  interest 
attaching  to  the  biotite  is  in  its  alteration  under  the  attack  of  the  weather. 
The  iron  separates  out  along  the  cleavages  in  little  spheroidal  drops  and 
flattened  plates,  which  are  red  and  translucent,  but  not  quite  of  the  deep 
color  of  hematite.  Doubtless  they  contain  some  water,  and  are  possibly 
close  to  gothite  in  composition.  Between  the  red  globules  the  biotite  sub- 
stance becomes  paler,  its  pleochroism  diminishes,  and  double  refraction 
increases,  and  finally,  in  a  slide  containing  no  basal  sections,  it  can  not  be 
distinguished  from  muscovite.  The  separated  ferric  oxide  remains  in  the 
mica,  and  while  the  rock  remains  firm  does  not  travel  and  stain  the  other 
constituents.  In  these  stages  the  slide  contains  a  very  faintly  colored 
bleached  biotite,  which  is  sprinkled  through  and  through  with  the  little 
dots  of  bright  red  iron  ore. 

Muscovite  is  not  very  abundant.  It  is  sometimes  intergrown  with  the 
large  biotites,  and  occurs  under  similar  conditions,  but  it  chiefly  comes  in 
little  ragged  inclusions  in  the  secondary  microcline.  In  the  form  of  aggre- 
gates of  sericite  it  composes  the  macroscopically  conspicuous  pearly  micas, 
and  also  is  an  abundant  constituent,  and  sometimes  the  only  representative, 
of  the  partly  absorbed  and  older  feldspars  included  in  the  microcline. 

Microcline  is  always  a  secondary  mineral,  and  is  present  in  variable 


394 


THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 


amounts  in  different  sections.  It  incloses  quartz,  the  micas,  niag-netite,  and 
an  older  feldspar.  These  inclosnres  are  usually  small;  they  often  lie  in  par- 
allel alignment  in  the  same  and  adjoining  microclines,  and  the  lines  in  which 
they  are  disposed  sometimes  bend,  apparently  indicating  that  the  original 
i-ock  was  minutely  puckered.  The  inclosed  quartz  sometimes  incloses 
.smaller  flakes  of  biotite  and  muscovite,  as  well  as  magnetite  and  rutile 
needles.  The  inclosnres  in  the  little  grains  of  quartz  are  frequenth^  con- 
centrated in  the  centers,  as  in  the  case  of  some  of  the  quartzes  outside  the 
microclines,  as  described  above.  The  microcline  sometimes  occurs  in  a  few 
scattered  grains;  sometimes  with  its  inclusions  it  makes  up  almost  the  whole 
rock.  In  its  manner  of  occurrence,  its  inclusions,  and  the  way  in  which 
these  are  disposed  within  it,  it  is  strikingly  like  the  secondary  albite  of  the 
Hoosac  schists  of  western  Massachusetts,  described  by  Prof.  J.  E.  Wolff.^ 

The  microclines  are  distinctly  elongated  in  a  direction  parallel  to  the 
foliation,  to  which  they  thus  contribute.  In  a  few  cases  the  elongation  is 
parallel  to  a  line,  and  does  not  appear  in  thin  sections  cut  normal  to  this 
direction.  But  in  most  cases  the  crystals  are  flattened  parallel  to  a  plane. 
These  forms  are  those  of  crystallization ;  except  along  the  secondary  fracture 
planes  the  microline  is  entirely  free  from  breaking  or  granulation. 

The  following  is  a  complete  analysis  of  a  representative  specimen  of 
the  mica-schist : 

Analysis  of  mica-schist, 

[By  ])r.  H.  S.  Stokes,  U.  S.  Geol.  Survey.] 


SiO.        

64.71 

.72 

Noue. 

.02 

16.43 

1.83 

3.84 

Trace. 

TiOj     

CO, 

PC- 

Al,03                       

Fe-Oj 

i     FeO            

MnO 

1 

CaO 

MgO 

K-O 

Na.O 

H.jOat  110°  ... 
H,0  above  110° 

Total 


0.08 
2.97 
.5.63 
.11 
.31 
2.79 


99.44 


No.  1.  Specimen  34822,  Lake  Superior  Division,  U.  S.  Oeol.  Surv.,  1900  N.,  1310  W.,  sec.  35,  T.  42N., 
R.  29  W.,  Upper  Peninsula  of  Michigan. 


'  Mon.  IT.  S.  Geol.  Survey,  Vol.  XXIII,  pp.  59-63. 


ARCHEAN  IN  FELCH  MOUNTAIN  DISTKK  T.  395 

In  its  low  silica  and  lime,  ami  high  iron  and  magnesia,  this  rock  differs 
in  important  partienhirs  from  the  granites,  to  which  in  its  mineral  com- 
position it  is  allied.  In  tliese  respects,  as  well  as  in  the  great  excess  of 
potash  over  soda,  it  closely  approximates  the  composition  of  certain  clay 
slates.' 

The  original  character  of  the  mica-schists  is  indeterminate.  They  may 
be  altered  sediments,  as  the  chemical  analysis  indicates,  but  if  so  they  no 
longer  contain  any  material  which  can  be  proved  to  be  in  its  original  form, 
and  in  view  of  the  complete  recrystallization,  for  which  the  evidence  is  clear 
and  striking,  this  could  not  be  expected.  Their  mineralogical  relationship 
and  close  association  with  the  granites  and  gneisses  is  perhaps  a  reason  for 
regarding  them  as  autoclastic  rocks,  derived  from  originally  massive  granites 
by  dynamic  metamorphism.  If  this  be  true,  then  the  crust  movements 
which  crushed  the  parent  granite  l)elong  to  pre-Algonkiau  time,  for  the  later 
stresses  which  folded  and  l:)rought  the  schists  into  faulted  contact  with  the 
Randville  and  Sturgeon  formations  found  them  with  a  parallel  foliation 
which  it  bent  and  crumpled,  and  no  period  of  great  stress  earlier  than  this 
is  known  in  Algonkian  time.  The  complete  recrystallization  may  be 
referred  with  probability  to  the  period  of  quiescence  following  the  faulting 
and  folding,  during  which  also  occurred  the  recomposition  of  the  older 
Algonkian  formations. 

(4)  The  amphibolites  or  hornblende-gneisses  are  widely  and  abundantly 
represented  in  the  Archean.  Macroscopically  they  are  black  or  dark- 
green  rocks  of  medium  to  fairly  coarse  grain,  the  fresh  fractures  of  which 
glisten  with  the  cleavage  surfaces  of  hornblende,  which  is  much  the  most 
abundant  and  often  the  only  recognizable  constituent.  They  are  universally 
foliated  parallel  to  the  foliation  of  the  associated  gneisses,  and  exhibit, 
but  in  a  more  marked  degree,  the  same  varieties  of  structure.  The  folia- 
tion is  easily  recognized  by  the  eye  as  due  to  the  parallel  arrangement 
of  tlie  hornblende  prisms.  Depending  mainlj^  upon  the  position  of  the 
hornblendes  relative  to  the  other  constituents,  the  structure  is  either " 
of  the  plane-parallel  or  lineai'-parallel  type,  the  latter  often  superbly 
developed. 

The   essential   constituents   of  these  rocks   are   common   ffreen   horn- 
blende,  plagioclase,  biotite,  and  quartz.     The  structure  is  thoroughly  crys- 

'  See  analyses  quoted  by  Kemp,  Handbook  of  Rocks,  p.  107,  noB.  4  and  5. 


396  THE  CRYSTAL  FALLS  IRON-BEAEmG  DISTRICT. 

talliue.  The  lioniblende  occiu-.s  iu  long  prisms  3  to  10  mm.  in  length,  which 
lie  close  together,  and  inclose,  partially  surround,  and  abut  against  smaller 
angular  grains  of  plagioclase.  The  plagioclase  is  quite  unstrained  and  is 
iisually  fresh  and  clear,  and  entirely  without  crystal  boundaries.  Brown 
biotite  is  iiniversalh'  present  in  small  amount,  in  long  plates  parallel  with 
the  foliation.  It  does  not  seem  to  be  an  alteration  product  from  the  horn- 
blende. Quartz  is  the  least  abundant  constituent.  It  is  crowded  with  fluid 
cavities  and  needles  of  rutile,  and  often  incloses  minute  crystals  of  horn- 
blende. The  plagioclase,  from  its  high  extinction  angles  and  alteration 
products,  is  evidently  basic.  A  little  magnetite  is  present,  but  titanite  has 
not  been  observed. 

The  structural  features  are  well  brought  out  in  thin  section  In  the 
linear-parallel  type  the  hornblendes  all  lie  with  their  crystallographic  axes 
parallel  to  a  line.  A  thin  section  parallel  to  the  foliation  cuts  essentially  all 
in  the  zone  of  the  prism  or  near  it;  one  across  the  foliation  gives  only  sec- 
tions across  the  prism.  The  grains  of  plagioclase  are  general!}"  elongated 
without  strain.  Their  outlines  are  most  irregular  and  quite  independent  of 
the  twinning  lamellae.  Their  g-eneral  apjiearance  is  that  which  would  be 
presented  if  numerous  crushed  contiguous  grains  had  united  by  some  proc- 
ess of  annealing  or  absorption  to  form  the  new  individuals.  In  the  plane- 
jjarallel  type  the  only  difference  is  that  the  hornblende  prisms  have  grown 
parallel  to  a  plane,  in  which,  however,  they  may  have  any  orientation  An 
indistinct  banding  is  also  often  observable  in  this  tyjie,  caused  by  a  partial 
grouping  of  the  light  and  dark  constituents  in  parallel  layei's.  The  order 
of  crvstallization  seems  to  have  been  plagioclase  first,  but  nearly  contempo- 
raneous with  the  hornblende  and  biotite,  and  the  quartz  last. 

The  amphibolites  occur  in  comparatively  narrow  bands  of  indefinite 
length  in  the  granites  and  gneisses.  The  width  usually  does  not  exceed  8 
to  10  feet,  and  their  dip  is  always  at  high  angles.  The  boiindaries  are 
invariably  sharp,  and  frequently  cut  the  foliation  of  the  ampliibolite  within 
and  of  the  gneisses  without  somewhat  obliquely.  There  is  a  general  imi- 
formity  of  grain  throughout  the  width;  the  wider  bands  are  not  coarser 
than  the  narrower. 


ARCHEAN  IN  FELGH  MOUNTAIN  DISTRICT. 


397 


The  IbllowiugcoiupU'tc  iuialysi.s  sliows  tliu  cliemical  character  of  a  rep- 
resentative specimen  ofainpliiboHte: 


Analysis  of  amphibolite. 

[Hy  Dr.  H.  N.  Stokes,  U.  S.  Geol.  Survey. 


1.1 


SiOj   50.36 

TiO, 1.77 

CO: None. 

PsOj. .20 

Al.O,  13.26 

Cr,0;, 

Fe  .0:, 6.  30 

FeO 9.34 

MnO Trace. 


1.' 

NiO 1 

CaO 

7.85 
5.  55 
1.14 
2.11 
.16 
1.  55 

MgO 

K.O 

Na.O 

HjO  at  110- 

H-,0  above  1 10^ 

Total 

99.59     i 

'  Ba,  8i-,  Li,  CI,  S,  SO3  were  not  looked  for. 
No.  1.     .Siiecimeii  36407,  Lakr  .Superior  Division,  V.  S.  Cieol.   .Snivey,  1140  N.,  1000  AV.,  sec.  32, 
T.  42  N.,  R.  28  W.,  Upper  Peninsula  of  Michigan. 

From  this  analysis  it  appears  that  the  rock  has  essentially  the  compo- 
sition of  diabase  or  basalt.  The  composition  of  the  amphibolites,  as  shown 
by  the  above  analysis,  and  their  field  relations  leave  little  room  for  doubt 
that  they  are  old  dikes  of  basic  rock. 

Their  present  crystallization  is  of  course  not  that  due  to  original 
cooling,  since  among  other  reasons  it  bears  nO  relation  either  to  their 
thickness  or  t<:)  distance  from  the  walls.  The  evidence  of  complete  recrys- 
tallization  in  place  after  consolidation  which  they  thus  afford,  and  the 
unquestionable  community  of  origin  between  their  foliation  and  that  of  the 
gneisses,  are  significant  facts  in  the  metamorphic  history  of  the  Archean  of 
this  district. 

It  is  for  this  reason  tliat  they  are  described  with  the  Archean  and  not 
with  the  intrusives.  Whether  they  are  really  Archean  intrusions  and  not  of 
Algonkian  age  can  not,  perhaps,  l)e  known  with  certainty.  Basic  rocks 
having  approximately  the  same  composition  are  known  to  have  penetrated 
the  Algonkian,  but  they  have  not  undergone  the  same  recrystallization. 
These  last  besides  have  their  known  analogues,  equally  unmetamorphic  in 
the  Archean  itself.  For  tliese  reasons  it  seems  probable  that  the  amphib- 
olites were  intruded  into  the  Archean  before  the  Algonkian  rocks  of  this 
district  were  deposited. 


398  THE  CRYSTAL  FALLS  IRON-BEAEING  DISTKIOT. 

SECTION   IV.    THE   STURGEOK    QITARTZITE. 

The  lowest  member  of  the  Algoiikiaii  in  the  Felch  Mountain  range 
is  a  formation  consisting  mainly,  but  not  exclusively,  of  coarse  vitreous 
quartzite.  Typical  exposures  of  this  formation,  as  well  as  one  of  the  rare 
contacts  between  it  and  the  underlying  Archean,  occur  along  the  Sturgeon 
River,  and  it  is  therefore  named  the  "Sturgeon  Quartzite." 

DISTRIBUTION,   EXPOSURES,   AND   TOPOGRAPHY. 

The  Sturgeon  formation,  next  to  the  Randville  dolomite,  is  the  most 
widespread  member  of  the  Algonkian  series  in  the  Felch  Mountain  range. 
Its  general  distribution  throughout  the  area  mapped  is  in  two  ])arallel  zones, 
of  varying  width,  immediately  adjoining  the  northern  and  southern  Archean, 
except  wlien  displaced  from  this  position  for  relatively  short  distances  by 
faults.  These  zones  extend  east  and  west  for  the  whole  length  of  the  range. 
Their  surface  width  varies  with  the  complexity  of  the  structure  and  the 
depth  of  erosion.  In  part  of  sec.  35,  T.  42  N.,  R.  29  W".,  the  higher 
formations  have  been  entireh'  removed,  and  the  two  zones  come  together, 
leaving  the  (][uartzite  as  the  onh^  Algonkian  rock  at  the  present  surface. 

On  the  whole  the  Sturgeon  formation  is  fairly  well  but  A^ery  unevenly 
exposed.  Beginning  at  the  west  the  zone  in  contact  with  the  southern 
Archean  furnishes  frequent  outcrops  from  the  south  quarter  post  of  sec.  34, 
T.  42  N.,  R.  30  W.,  to  the  south  quarter  post  of  sec.  36,  T.  42  X.,  R.  30  W., 
a  distance  of  2  miles.  Then  follows  a  gap  of  a  mile  in  which  no  outcrop  . 
have  been  found.  Near  the  north  and  south  quarter  line  of  see.  31,  T.  42  N., 
R.  29  W.,  they  l)egin  once  more,  and  are  supplemented  by  test  pits  as  far 
as  the  West  sixteenth  line  of  section  32,  next  east. 

Then  follows  another  gap  without  exposures,  2^  miles  in  length. 

Near  the  north-and-south  quarter  line  of  sec.  34,  T.  42  N.,  R.  29  W., 
outcrops  begin  again  and  continue  for  a  mile  to  the  east,  with  frequent  inter- 
ruption, as  far  as  the  north-and-south  quarter  line  of  section  35,  where  in 
the  valley  of  the  Sturgeon  the  southern  zone  broadens  and  jt)ins  the  north- 
ern, in  consequence  of  the  general  westward  pitch  which  has  carried  the 
higher  formations  above  the  present  surface  of  denudation.  East  of  this 
point  the  quartzite  is  known  in  only  a  few  scattered  localities.  In  the 
southern  part  of  sec.  36,  T.  42  N.,  R.  29  W.,  it  is  in  contact  with  the 
Archean  on  the  south  l)ank  of  the  Stur"-eon  River.      South  of  Felch  Moun- 


STURGEON  QUAKTZITE  IN  FELCH  MOUNTAIN  RANGE.  39y 

tiiiu,  in  src.  .'52,  T.  42  N.,  Ji.  2S  W.,  it  oiitcntps  iiniiieili;itel\-  soutli  of  the 
abiUidoiied  Northwcstci-ii  mine,  iiud  lifis  also  Ix-eii  t'ouiiil  iu  (lrilliii<>'  (in  the 
west  and  in  test  pits  on  llie  cast  of  the  natnral  exposnres  throngh  a  distance 
of  lialf  a  inih-.  In  sec.  33,  T.  42  W.,  R.  28  W.,  a  small  ledge,  a  few  feet 
square,  occurs  between  the  overlying-  dolomite  and  the  Archean,  200  feet 
east  of  the  road  to  the  Calumet  and  Hecla  (iron)  mines.  East  of  this  the 
contact  between  the  Archean  and  the  Algonkian  is  a  faulted  one,  and  the 
quartzite  is  buried  beneath  the  overlying-  formations. 

The  northern  zone  of  the  Sturgeon  formation  is  not  nearly  so  well 
exposed,  nor  for  the  most  part  does  it  fall  within  the  artificial  line  that 
bounds  our  detailed  work  on  the  north.  Sees.  34  and  35,  T.  42  N.,  R.  30  W., 
on  the  west  contain  a  few  scattered  outcrops,  one  of  which  is  of  exceptional 
})etrog-raphical  interest  and  to  be  noticed  later.  The  next  exposures  are 
o  miles  east,  along-  and  just  north  of  the  north  line  of  sec.  35,  T.  42  N., 
R.  29  W.  The  main  northern  zone  of  the  Sturg-eon  formation  coming  from 
the  west  lies  south  of  these  exposures  and  is  entirely  covered.  Between 
the  two  the  tongue  of  Archean  scliists  already  described  is  faulted  up.  Two 
miles  farther  east  quartzite  again  appears  in  test  })its,  low-lying  outcrops 
and  drill  holes  along  the  northern  border  of  sec.  31,  T.  42  N.,  R.  28  E., 
and  in  section  29,  immediately  north  of  section  32,  is  well  exposed  in  a  broad 
belt  that  reaches  north  almost  to  the  east-and-west  quarter  line. 

The  quartzite  often  forms  distinct  linear  ridges,  which  in  spite  of  the 
chemical  staljility  and  apparent  homogeneity  of  the  rock  seldom  rise  to  the 
mean  altitude  of  the  neighboring  Archean  areas.  An  exception  to  this  rule 
is  the  succession  of  ridges  formed  by  the  southern  zone  in  the  3-mile  stretch 
west  of  sec.  31,  T.  42  N.,  R.  29  W.;  these  frequently  overtop  the  adjacent 
Archean  plateau.  Very  frequently,  also,  the  quartzite  zones  occupy  lower 
ground  not  only  than  the  Archean  but  even  than  the  immediately  overly- 
ing dolomite.  The  southern  zone,  for  some  unknown  reason,  is  a  distinctly 
weak  belt  east  of  sec.  32,  T.  42  N.,  R.  29  W.,  and  for  several  miles  forms 
the  bed  rock  of  the  Sturgeon  and  the  connecting-  valleys. 

FOLDING   AND   THICKNESS. 

It  is  extremel}'  difficult  in  most  cases  to  determine  directly  the  attitude 
of  the  Sturgeon  formation,  owing-  to  its  generally  massive  and  homogeneous 
character.     This  is  due,  as  will  be  shown  hereafter,  to  the  completeness  of 


400  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

the  recrystallization,  in  consequence  of  which  the  ordinary  sedimentary 
features  that  it  originally  possessed  have  been  almost  entirely  obliterated. 
Faint  color  banding,  itself  of  secondary  development,  but  no  doubt  pre- 
serving a  distinction  in  original  composition,  alone  remains,  and  only  here 
and  there,  as  a  guide  to  the  former  stratification.  By  scattered  indications 
of  this  sort,  and  by  the  better  evidence  afforded  by  the  overlying  dolomite, 
often  very  distinctly  banded,  it  is  known  that  the  southern  zone  of 
quartzite  on  the  whole  dips  toward  the  north.  Southward  dips  also  occur  in 
this  belt,  by  which  it  is  known  that  subordinate  folds  occur  within  the  quartz- 
ite itself.  From  the  considerable  variations  in  the  surface  width  of  the  forma- 
tion we  are  led  to  suspect  the  existence  of  more  of  these  little  folds  than 
we  are  able  to  prove.  However,  the  secondary  syncline,  which  extends 
from  the  offset  already  referred  to  in  sec.  35,  T.  42  N.,  R.  30  W.,  for  6  miles 
to  the  east  to  sec.  35,  T.  42  N.,  R.  29  W.,  and  includes  no  formation  higher 
than  the  quartzite,  is  very  definitely  determined. 

In  the  northern  belt  of  the  Sturgeon  formation  the  indications  of  dij^ 
are  generally  northward  at  very  high  angles.  These  indications,  not  in 
themselves  conclusive,  are  reenforced  by  a  corresponding  attitude  in  the 
overlying  dolomite,  and  it  is  therefore  probable  that  there  is  a  general,  or  at 
least  widespread,  overturn  in  the  dip  of  the  northern  belt. 

Since  the  contacts  of  the  Sturgeon  formation  with  the  underlying 
Archean  and  with  the  overlying  dolomite  are  (except  in  one  case)  covered, 
it  is  impossible  to  obtain  the  data  for  very  accurate  determination  of  its 
thickness.  The  uncertainty  in  most  outcrops  as  to  the  dip  of  the  quartzite 
introduces  an  additional  difiiculty.  However,  in  sec.  35,  T.  42  N.,  R.  30  W., 
on  the  west  end  of  the  range,  and  in  sec.  33,  T.  42  N.,  R.  28  W.,  11 
miles  farther  east,  the  covered  intervals  to  the  limiting  formations  are  not 
great,  and  if  the  contacts  are  not  faulted  (which  is  far  from  certain),  the 
minimum  thickness  is  determinable  within  a  reasonable  limit  of  error. 

In  the  western  locality  the  surface  width  of  the  zone  probably  under- 
lain by  quartzite  is  about  500  feet.  The  quartzite  itself  is  structureless,  but 
the  overlying  dolomite  dips  northward  at  an  average  angle  of  about  70°. 
If  the  same  dip  holds  in  the  quartzite,  its  true  thickness  is  about  470  feet. 
In  the  eastern  locality  similar  data  lead  to  a  thickness  of  nearly  430  feet. 
In  these  two  sections  the  quartzite  zone  is  much  narrower  than  it  is  else- 
where, either  because  undetected  faults  have  reduced  it,  or  because  it  is 


STURGEON  QITARTZITE  IN  FELOH  MOUNTAIN  RANGE.         401 

uncomplicated  by  subordinate  folds.  It  is  probably  safe  to  conclude,  in 
view  of  the  uncertainties,  that  the  average  thickness  of  the  formation  is  not 
less  than  450  feet,  and  may  be  considerably  more.  In  a  preliminary  paper 
on  the  district,'  written  before  the  field  notes  were  fully  analyzed,  I  have 
placed  the  thickness  of  the  quartzite  at  about  700  feet;  but  this  figure  is 
probably  too  large. 

PETROGRAPHICAL  CHARACTERS. 

The  Sturgeon  formation  includes  a  few  very  closely  related  rock 
varieties,  of  whicli  quartzite  furnishes  the  great  majority  of  the  exposures. 
The  quartzites  are  usually  light  gray  in  color,  and  break  with  a  coarsely 
granular  or  glassy  fracture.  To  the  eye  quartz  is  often  the  only  recogniza- 
ble constituent  in  the  body  of  the  rock,  although  the  numerous  joint  and 
shearing  planes  shimmer  with  little  silvery  plates  of  muscovite.  Occasion- 
ally a  weathered  surface  is  dotted  with  minute  specks  of  an  opaque  pinkish 
substance,  which  leads  one  to  suspect  the  presence  of  feldspar.  Chlorite 
also  is  now  and  then  visible  in  the  darker  varieties. 

The  quartzites  are  almost  uniformly  massive,  except  for  the  secondary 
fractures  above  mentioned.  At  scattered  localities,  however,  a  faint  color- 
banding,  due  to  the  presence  of  layers  of  a  pinkish  hue,  which  are  inde- 
pendent of  the  secondary  fractures,  seems  to  indicate  the  original  stratifica- 
tion. The  color  bands  are  generally  only  vaguely  defined ;  occasionally, 
however,  they  are  numerous  and  shaq). 

Closely  associated  with  the  massive  quartzites  are  sheared  quartzites,  or 
micaceous  quartz-schists.  These  rocks  are  merely  varieties  of  the  quartzite 
in  which  secondary  shearing  planes,  with  their  attendant  growths  of  new 
muscovite,  are  more  abundant  than  usual.  The  shearing  surfaces  almost 
invariably  intersect,  with  the  result  that  the  new  structure  tends  toward  the 
linear-parallel  type,  and  is  often  as  similar  in  appearance  as  it  is  in  origin 
to  the  structure  already  described  in  connection  with  the  sheared  granites. 

In  a  locality  already  referred  to,  on  the  south  bank  of  the  Sturgeon, 
in  sec.  36,  T.  42  N.,  R.  29  W.,  where  the  Sturgeon  formation  is  in  visible 
contact  with  the  Archean,  the  quartzite  is  underlain  by  a  considerable 
thickness  of  very  fissile  musco^-ite-biotite-gneiss,  which  incloses  rather 
sparingly  obscure  pebbles  of  granite  and  quartz.     This  gneiss,  which  no 

'Relations  of  the  Lower  Menominee  and  Lower  Marquette  series  in  Michigan  (Preliminary)-  Am 
Jour.  Sci.,  Vol.  XLVII,  1894,  p.  217.  .>  ^  • 

MON   XXXVI 26 


402  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT, 

doubt  was  formerly  an  arkose  rich  in  feldspar,  has  recrystallized  and  after- 
wards been  sheared;  the  coarse  micas  to  which  the  fissility  is  due,  together 
with  other  new  minerals,  have  grown  between  the  fractured  surfaces  and 
recemented  the  broken  mass.  It  affords  beautiful  examples  of  foliation 
parallel  to  a  line. 

The  thin  sections  of  the  Sturgeon  quartzite  are  of  exceptional  interest. 
The  principal  constituent  is,  of  course,  always  quartz.  With  the  quartz  are 
associated,  in  nuich  smaller  amounts,  and  not  necessarily  all  in  the  same 
section,  numerous  accessories,  including  muscovite,  biotite,  chlorite,  micro- 
cline,  orthoclase,  plagioclase,  titanite,  rutile,  zircon,  apatite,  and  the  ores. 
The  relations  of  the  quartz  to  the  other  constituents  present  very  unusual 
features,  and  indicate  that  the  metamorphic  changes  by  which  the  present 
completely  crystalline  rock  has  been  made  from  an  original  granitic  sand 
have  proceeded  along  lines  not  hitherto  distinctly  recognized  in  the  forma- 
tion of  rocks  of  this  character. 

Among  the  large  number  of  slides  examined,  a  broad  distinction  can 
at  once  be  made  between  those  which  show  the  effects  of  stress  in  a  pro- 
nounced degree  and  those  in  which  such  effects  are  subordinate  or  hardly 
noticeable.  Connecting  these  two  classes  is  a  pei*fectly  graded  series;  and 
it  is  therefore  certain  that  those  of  the  first  are  merely  the  more  or  less 
modified  varieties  of  an  earlier  stage,  represented  more  nearl)^  by  the  second. 
In  the  slides  in  which  the  effects  of  pressure  are  least  apparent  the  micro- 
scopic characters  are  as  follows:  The  background  is  composed  of  large 
irregular  grains  of  quartz,  the  edges  of  which  interlock  with  the  most  minute 
and  sharp  interpenetrations.  The  longest  dimensions  of  these  grains  range 
from  1.5  to  6  mm.,  avei'aging  perhaps  2.5  or  3.  The"\'  often  have  a  rather 
vague  parallel  elongation,  which  corresponds  to  tlie  alignment  of  the  minerals 
which  diey  inclose.  Scattered  very  abundantly  through  these  large  quartz 
grains  are  the  accessoiy  minerals,  some  predominating  in  one  slide,  others 
in  another,  but  the  micas  and  chlorite  occurring  in  all.  Through  each 
slide  the  accessory  minerals,  with  the  exceptions  noted  below,  lie  with 
their  long  axes  in  a  common  direction,  and  frequently  cross  the  serrated 
boundaries  between  adjacent  quartzes.  The  inclusions  in  many  cases  have 
the  form  and  other  characters  of  clastic  minerals,  and  thus  preserve  the  only 
microscopic  evidence  of  the  original  nature  of  the  rock. 

The  included  micaceous  minerals  are  usually  in  small  plates,  ranging 


STURGEON  QUARTZITE  IN  FELOH  MOUNTAIN  RANGE.         403 

from  0.05  to  0.75  mm.  in  longest  dimensions,  but  few,  however,  exceeding  0.2. 
Many  of  tliese  are  bent  and  split,  the  clejir  mistrained  quartz  of  the  host  jiene- 
trating  from  the  frayed  edges  into  the  interior  between  the  partly  separated 
leaves.  Biotite  and  muscovite,  and  sometimes  chlorite,  occur  in  the  same 
individual,  indicating  alteration  before  inclusion  in  the  quartz  host  took 
place.  Besides  its  conuuon  occurrence  as  an  alteration  j^roduct  of  the 
biotite,  a  few  rounded  areas  of  chlorite,  made  up  of  little  radiating  tufts, 
seem  to  be  pseudomorphs  of  garnet.  Inclusions  of  titanite  and  magnetite^ 
or  a  related  ore,  are  not  uncommon  in  the  larger  micas,  and  the  biotite  and 
chlorite  sometimes  inclose  beautiful  sagenite  webs.  Many  of  the  smaller 
micas,  however,  have  clear  sharp  edges  and  depart  from  the  general  paral- 
lelism of  the  other  inclusions.  These  are  either  contemporaneous  crys- 
tallizations or  else,  perhaps,  were  primary  inclusions  in  former  grains  of 
clastic  quartz  which  has  since  disappeared.  Some  of  the  clastic  jjlates  of 
biotite  are  bleached  and  include  spheroidal  blebs  of  red  iron  ore,  similar  to 
those  described  in  the  case  of  the  Archean  mica-schists. 

The  microcline  inclusions  are  usually  elongated  in  form,  and  frequently, 
jjarticularly  in  the  cases  of  the  larger,  have  well-rounded  clastic  outlines. 
The  long  dimension,  which  usually  coincides  with  one  of  the  cleavages  of 
the  mineral,  rarely  exceeds  0.5  mm.  or  falls  below  0.08  mm.  The  periphery 
is  frequently  partly  sin-rounded  by  a  thin  film  of  biotite.  Within  the  micro- 
clines  are  sometimes  contained  little  blebs  of  quartz,  which  are  not  oriented 
optically  with  the  host,  and  also,  more  rarely,  small  plates  of  biotite.  The 
microcline  individuals  are  sometimes  broken  into  two  or  three  differently 
oriented  parts,  which  may  be  separated  from  each  other,  in  which  cases  the 
quartz  of  the  host  has  completely  filled  the  interspaces.  Fracture  in  the 
feldspar  is  often  unattended  with  the  slightest  appearance  of  strain  in  the 
inclosing  and  cementing  quartz,  which  extinguishes  as  one  individual,  and  is 
therefore  unmistakably  to  be  attriljuted  to  stresses  previous  to  the  crystalli- 
zation of  the  quartz. 

Besides  microcline,  both  orthoclase  and  plagioclase  are  sometimes 
inclosed  in  the  large  quartzes,  but  much  more  sparingly.  They  are  invari- 
ably more  or  less  decomposed,  and  are  sometimes  surrounded  partially  or 
wholly  by  a  film  of  ferruginous  material.  They  show  the  same  phenomena 
of  fracture,  and  occasionally  of  separation  with  penetration  of  the  host,  as 
the  microcline,  and  occur  in  grains  having  a  similar  range  in  size. 


404  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

Titanite  is  of  frequent,  zircon  of  rather  rare,  occuiTence.  The  titanite 
is  found  not  only  inek)sed,  as  ah-eady  stated,  in  biotite  and  chlorite,  but  also 
in  well-rounded  clastic  grains  which  are  often  bordered  with  an  opaque  ore. 
Zircon  occurs  in  broken  grains,  without  doubt  clastic,  and  also  in  small 
crystals  which  show  no  signs  of  wear.  These -last  were  probably  entirely 
embedded  in  original  clastic  grains  of  quartz. 

Besides  the  above  minerals  of  usual  occurrence,  small  quartz  grains  of 
different  orientation  from  the  matrix  are  ^'ery  rarely  found  included  in  the 
large  quartzes  of  the  general  background.  Only  two  or  three  such  cases 
have  been  oljserved,  and  in  these  the  included  grain  is  surrounded  almost 
wholly  with  thin  plates  of  mica.  It  is  believed  that  these  are  original 
clastic  grains  which,  perhaps  because  protected  by  a  film  of  material  now 
represented  by  the  micas,  have  escaped  the  general  fate  of  their  neighbors. 

One  or  two  composite  inclusions,  made  up  of  microcline,  the  micas,  and 
quartz,  have  also  been  noticed.  These  seem  to  represent  original  pebbles 
of  granite  or  a  crystalline  schist. 

The  pressure  effects  begin  with  the  appearance  of  optical  strain  and 
decided  elongation  in  the  large  quartzes  of  the  groundmass.  This  is  fol- 
lowed by  fracture,  either  along  or  quite  independent  of  the  original  sutures, 
the  crack  often  halting  in  the  interior  of  a  grain.  The  fractures  preserve 
very  roughly  the  same  general  direction,  but  frequently  intersect  at  -very 
acute  angles,  or  come  together  in  sweeping  curves.  The  breaking  is  fol- 
lowed by  movement,  and  this  results  in  the  production  of  a  fine-grained 
quartz  mosaic  between  the  parted  surfaces.  In  the  final  stages  shown  in 
the  series  of  slides  in  my  collection,  the  rock  is  made  up  of  long,  narrow 
lenses,  each  of  which  is  an  enormously  strained  qviartz  individual,  separated 
by  narrow  anastomozing  zones  of  very  finely  subdivided  quartz.  After  the 
fracturing  took  place  there  seems  to  have  been  no  further  distortion  of  the 
lenses,  for  the  edges  of  adjacent  individuals  follow  similar  curves,  which  are 
often  reversed,  and  in  many  cases  could  be  brought  together  with  an 
accurate  fit. 

If  the  Sturgeon  quartzite  represents  an  original  sandstone,  it  is  evident 
from  the  facts  stated  alcove  that  the  old  quartz  grains  have  undergone  com- 
plete recrystallization.  The  usual  conception,  since  the  time  of  Sorby,  of 
the  process  by  which  quartzites  are  formed  from  original  deposits  of  sands 
is  that  new  quartz  is  deposited  around  each  original  fragmental  quartz  grain, 


STURGEON  QUARTZITE  IN  FELCH  MOUNTAIN  RANGE.         405 

ill  similiu-  crystallogTaphic  orientation  with  it,  and  that  neighboring  grains 
thus  enlarged  finally  interlock  bv  mutual  limitation  of  one  another's  growth. 
Tliis  explanation  evidently  can  not  account  for  the  background  of  large 
interlocking  qnai-tz  areas  in  these  rocks,  for  if  it  were  true  it  would  be  nec- 
essary to  assume  that  the  quartz  grains  were  less  numerous  in  the  original 
deposit  than  those  of  almost  any  other  mineral,  in  some  slides  even  than  the 
titanite  or  chlorite.  There  seems  to  be  but  one  escape  from  the  conclusion 
that  the  large  quartz  areas  nuist  each  represent  a  nnmber  of  original  frag- 
mental  quartz  grains,  which,  as  deposited,  nuist  have  lain  in  the  rock  with 
their  crystallographic  axes  disposed  entirely  at  haphazard;  and  that  is  the 
hy])othesis  that  this  quartzite  was  not  originall}'  a  sandstone,  but  consisted 
mainly  of  soluble  and  easily  replaceable  material,  such  as  limestone,  with 
the  fragmental  particles  scattered  through  it,  and  that  the  large  quartzes  of 
the  background  have  replaced  this  soluble  substance.  I  have  been  able 
to  tind  no  positive  evidence  to  support  this  hypothesis,  and  I  am  com- 
pelled to  believe  that  the  rock  was  a  sandstone  in  which,  in  some  way 
not  easy  to  understand,  considerable  numbers  of  adjacent  quartz  grains 
have  united  to  form  or  have  been  absorbed  into  a  new  individual,  leaving 
absolutely  no  trace  of  their  former  separate  existence.  The  introduction 
of  new  silica,  or  the  separation  of  silica  from  decomposing  silicates  in  the 
rock  itself,  may  well  have  been  essential  factors  in  the  recrystallization. 
I  shall  make  no  attempt  to  explain  the  process  further  than  to  point 
out  its  probable  analog}'  with  the  ^^rocess  by  which  the  new  microclines 
were  formed  in  the  Archean  mica-schists. 

The  close  alignment  of  the  clastic  minerals  inclosed  in  the  large  quartz 
areas,  their  frequent  fracture,  and  their  occasional  separation,  indicate  that 
the  time  of  crystallization  probably  followed  a  period  of  stress;  while  the 
very  vague  j^arallel  elongation  of  the  individuals  of  the  background  in  the 
unstrained  sections  would  seem  to  show  that  they  crystallized  under  static 
conditions.  Unquestionable  proof  of  a  period  of  stress  later  than  the  crys- 
tallization is  given  by  the  numerous  slides,  in  which  these  grains  are  seen 
to  have  suffered  fracture  and  distortion.  The  microscopical  study  of  the 
quartzites  thus  supplies  important  evidence,  not  afforded  by  the  outcrops, 
as  to  the  erogenic  history  of  the  district. 


406  THE  CRYSTAL  FALLS  lEON-BEARIXG  DISTRICT. 

SECTION  V.    THE  RANDVlLIiE  DOLOMITE. 

The  Sturgeon  quartzite  is  succeeded  by  a  formation  consisting,  so  far 
as  is  known,  almost  wholly  of  crystalline  dolomitic  rocks.  Excellent 
exposures  belonging  to  this  formation  are  situated  within  a  short  distance 
of  Randville  station,  on  the  Milwaukee  and  Northern  Railway,  and  it  may 
therefore  convenienth'  be  named  the  Randville  dolojnite. 

DISTRIBUTION,   EXPOSURES,  AND  TOPOGRAPHY. 

Owing  both  to  its  great  thickness  and  to  its  intermediate  ^^osition  in 
the  series,  the  Randville  dolomite  in  the  Felch  Mountain  range  coveis  a 
larger  share  of  the  surface  than  any  other  member  of  the  Algonkian  suc- 
cession. The  overlying  formations  are  frequently  interrujjted,  because  of 
the  changes  in  direction  of  pitch  of  the  secondary  synclines  in  which  they 
occur.  Ill  these  gaps  the  dolomite  covers  the  whole  interior  of  the  syncli- 
norium.  Wliere  the  higher  formations  are  present,  they  divide  the  dolomite 
into  two  or  more  parallel  east  and  west  belts,  one  of  which  lies  south  of  the 
northern  quartzite  and  the  other  north  of  the  southern.  Only  in  portions 
of  sees.  35  and  36,  T.  42  N.,  R.  29  W.,  wliere  the  rise  in  the  axis  of  the 
main  syncline  has  lifted  it  above  the  present  surface  of  denudation,  is  the 
dolomite  entirely  absent  from  the  main  trough. 

Natural  exposures  of  the  dolomite  are  not  so  numerous  as  of  the 
quartzite,  but  they  are  more  evenly  distributed.  Moreover,  owing  to  its 
proximity  to  the  Groveland  iron  formation,  the  dolomite  has  been  penetrated 
by  many  test  pits  and  diamond-drill  borings  put  down  in  search  of  ore, 
and  these  supply  important  information  in  the  covered  areas.  From  the 
western  end  of  the  map  to  sec.  34,  T.  42  N.,  R.  29  W.,  the  dolomite  is  for 
most  of  the  way  separated  into  two  or  more  parallel  belts.  The  southern 
belt  is  es^jecially  well  exposed  in  sees.  35  and  36,  T.  42  N.,  R.  30  W.,  and 
in  sec.  31,  T.  42  N.,  R.  29  W.,  and  for  2  miles  to  the  northeast,  beyond 
which  it  has  been  found  only  in  test  pits  and  drill  holes.  In  the  middle  of 
sec.  35,  T.  42  N.,  R.  29  W.,  the  base  of  the  formation  is  brought  to  the 
surface  by  the  westerly  pitch  of  the  main  fold  and  is  well  exposed  along 
Sturgeon  River. 

North  of  the  strike  fault,  which,  as  alread's'  described,  has  brought  the 


RANDVILLE  DOLOMITE  IN  FELGH  MOUNTAIN  RAISIGE.         407 

Archean  inica-sclnsts  into  contact  with  tlie  dolomite  and  quartzite  in  the 
northern  part  of  the  same  section,  the  Randville  formation  runs  east  in  a 
single  belt,  which  probably  continuously  widens  as  the  throw  of  the  fault 
diniini-shes.  It  has  been  found 'in  several  places  in  the  north  half  of  sec. 
31,  T.  42  N.,  R.  28  W.,  and  near  the  east  line  of  this  section  the  appearance 
of  the  overlying  mica-schists  again  divides  it  into  two  belts,  which  pass  to 
the  north  and  south  of  the  Felcli  Mountain  syncline.  The  northern  belt 
has  been  proved  by  test  pits  only,  but  the  southern  is  well  exposed  natui'ally 
in  the  neighl)orhood  of  the  Northwestern  mine.  Other  exposures  also 
occur  south  of  the  unconformable  mica-schists  and  quartzite  of  the  upper 
series,  in  the  central  ])ortion  of  sec.  33,  T.  42  N.,  R.  29  W. 

The  dolomite  is  relatively  a  weak  rock,  and  generally  occupies  lower 
ground  than  either  the  quartzite  below  or  the  iron  formation  above  it.  The 
belt  in  contact  with  the  southern  belt  of  quartzite  especially  is  valley 
making  throughout  most  of  its  extent.  The  outcrops  usually  form  low, 
steep-isided  knolls  elongated  with  the  strike  and  of  slight  relief  above  the 
basement;  these  occasionally  unite  into  linear  ridges,  as  in  sec.  35,  T.  42  N., 
R.  30  W.  The  northern  belt  is  one  of  low  general  relief,  from  which,  how- 
ever, similar  isolated  knobs  often  protrude.  The  largest  and  most  prominent 
of  these  is  the  peak  in  the  northeast  quarter  of  NW.  ^  sec.  36,  T.  42  N., 
R.  30  W.,  which  rises  80  feet  above  its  base,  covering  8  or  10  acres. 

No  actual  contacts  between  the  Sturgeon  and  Randville  formations 
have  been  found,  but  from  their  close  association  and  continuity,  as  well  as 
from  the  structural  characters,  Avhen  these  are  determinable,  they  seem 
everywhere  to  be  strictly  conformable.  Near  the  quartzite  the  dolomite 
becomes  distinctly  more  impure  and  contains  a  larger  proportion  of  silicates 
and  quartz.  It  is  altogether  probable  that  between  them  come  transition 
beds,  as  indeed  is  shown  by  some  of  the  drill  records.  In  one  of  these 
"  talckv  mica-schists,  micaceous  limestone,  altered  actinolite-schist,  and 
quartzite"  are  described  as  being  interbedded  near  the  junction. 

The  determination  of  the  thickness  of  the  Randville  formation  is  beset 
with  the  same  difficulties  as  are  encountered  in  the  case  of  the  quartzite, 
namely,  the  uncertainty  as  to  the  exact  position  of  the  contacts  and  the 
possibility  of  faults  and  subordinate  folds  within  the  formation  itself  The 
best  sections  give  a  wide  range  of  values  from  a  mininuim  of  about  500 


408  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

feet  near  Felcli  Mountain  to  a  maximum  of  nearly  1,000  feet  in  the  western 
part  of  the  district.  While  the  discrepancies  may  be  partly  due  to  lack  of 
precision  in  the  data,  it  is  probable  that  the  thickness  of  the  formation  is  not 
uniform,  but  really  increases  from  east  to  west.  On  the  Fence  River,  18 
miles  northwest  of  Randville,  the  thickness  is  probably  about  1,500  feet. 
Accordingly,  accepting-  each  of  these  determinations  as  approximately 
correct,  700  feet  may  be  taken  as  a  fair  estimate  of  the  average  thickness 
of  the  Randville  dolomite  within  the  Felch  Mountain  range. 

PETROGRAPHICAL  CHARACTERS. 

The  outcrops  of  the  Randville  formation  consist  exclusively  of  dolo- 
mite, more  or  less  pure,  and  always  thoroughly  crystalline.  A  few 
comparatively  thin  layers  of  schists,  probably  both  micaceous  and 
amphibolitic,  and  als(i  of  quartzite,  are  mentioned  in  certain  drill  records, 
to  which  I  have  had  access  as  occumng  interbedded  with  the  dolomite; 
and  wliile  the  lithological  determinations  are  perhaps  not  entitled  to  much 
weight,  thev  at  least  prove  the  existence  of  i-ocks  which  are  not  dolomite 
within  the  formation.  In  the  field,  however,  such  interbedded  layers  do 
not  outcro}),  and  they  must  constitute  an  extremely  small  part  of  the  total 
thickness.  From  the  results  of  my  w(irk  the  Randville  formation  appears 
as  a  lithological  unit. 

Macroscopically  the  dolomites  are  rather  coarse-grained  marbles,  of 
various  shades  of  color,  of  Avhicli  jjinkish  or  bluish  white  are  the  most 
common.  They  always  inclose,  more  or  less  abundantly,  large  flakes  and 
aggregates  of  tremolite,  which  are  particularly  noticeable  from  their  projec- 
tion above  the  weathered  surface.  Occasionally  tremolite  and  other  silicates 
are  the  most  abundant,  and  sometimes,  for  small  thicknesses,  are  essentially 
the  only  constituents.  Quartz  and  chlorite  are  also  often  present,  but  in 
much  smaller  amounts.  The  weathered  surface  is  usually  dulled  to  a  light 
brown  or  creamy  yellow  in  a  thin  superficial  skin,  but  is  not  deeply  iron- 
stained,  except  when  the  silicates  containing  ferrous  iron  are  present. 

The  following  partial  analyses  of  three  specimens  from  different  parts 
of  the  range  show  that  the  carbonate  is  normal  dolomite.  The  insoluble 
portion  consists  chiefly  of  tremolite.  These  analyses  were  made  for  me 
by  Mr.  G.  B.  Richardson,  a  graduate  student  in  geology  in  Harvard 
University. 


KANDVILLE  DOLOMITE  IN  PELCII  MOUNTAIN  RANGE. 
Analyses  of  Randville  dolomite. 


409 


I. 

II. 

III. 

Insoluble  in  HCl 

2.0 

1.2 

53.2 

42.3 

9.7 

2.1 

48.9 

38. 0 

29.1 

2.2 

39.3 

27.7 

FOiOi 

CaCO , 

MgCO , 

Total 

98.7 

98.7 

98.3 

The  outcrops,  while  often  entirely  massive,  usually  possess  decided 
structural  features.  These  are  indicated  by  color  banding,  by  differences 
in  texture,  and  by  the  banded  arrangement  of  the  components.  Slight 
variations  in  the  body  color  of  the  rock,  proceeding  from  no  distinguishable 
variation  in  composition,  often  occur  in  alternate  parallel  layers,  which  are 
persistent  within  the  limits  of  observation.  With  the  color  banding  often 
go  variations  in  texture,  which,  however,  are  neither  so  regular  nor  nearly 
so  persistent.  The  characteristic  form  taken  by  these  is  in  thin  layers, 
which  as  they  continue  open  out  into  nodule.s.  Such  layers  consist  of 
closely  packed  crystalline  grains,  very  much  coarser  than  the  body  of  the 
rock,  which  have  grown  normal  to  the  boundaries.  Adjacent  layers  are 
not  strictly  parallel  and  sometimes  cross  each  other.  They  are  believed  to 
represent  ancient  fracture  and  slipping  surfaces,  which  followed  very  closely 
the  original  bedding,  in  which  the  new  carbonate  individuals  have  had 
room  for  larger  growth.  The  arrangement  of  the  accessory  minerals, 
especially  the  tremolite,  also  is  usually  a  banded  one.  Layers  rich  in 
tremolite  alternate  with  layers  poor  in  tremolite,  while  within  the  layers 
the  orientation  of  the  tremolite  individuals  is  usually  at  random.  The 
structure  brought  out  in  these  various  ways  is,  on  the  whole,  a  parallel 
structure.  If  corresponds  with  the  strike  and  dip  in  all  the  localities  where 
these  can  be  independently  confirmed  by  the  attitude  of  the  adjacent  for- 
mations, and  it  also  has  been  tin-own  into  minor  folds.  1  therefore  regard 
the  structm-e  as  having  originated  partly  in  chemical  differences  in  the 
material  originally  deposited  and  partly  in  secondary  growths  in  the  open 
spaces  and  rubbing  zones  determined  by  relative  movements  along  the  sur- 
faces of  easiest  fracture  at  the  time  of  the  earliest  folding,  and  for  both 
reasons  preserving  in  the  subsequent  metamorphism  the  true  stratification 
of  the  formation. 


410  THE  CEYSTAL  FALLS  IRON-BE ARINO  DISTRICT. 

Under  the  microscope  the  dolomites  show  uo  featui-es  of  special  inter- 
est. They  are  thoroughly  crystalline  rocks,  chiefly  composed  of  coarse 
o-rains  of  dolomite  with  which  is  associated  a  considerable  number  of  acces- 
sory  minerals.  Of  these  the  most  important  are  tremolite,  diopside,  chlorite, 
muscovite,  phlogopite,  quartz,  and  rutile,  while  apatite,  tourmaline,  pyrite, 
and  magnetite  are  rare. 

The  dolomite  is  by  tar  the  most  abundant  constituent  in  most  of  the 
slides,  and  furnishes  the  general  background  for  the  accessories.  The  shape 
of  the  grains  in  many  sections  is  decidedly  oval,  and  the  long  axes  lie  in 
the  same  direction,  thus  producing  a  foliation. 

Tremolite  is  abundant  in  some  of  the  sections,  and  is  entirely  absent 
from  none.  It  occurs  in  long-bladed  individuals  and  aggregates,  usually 
bounded  by  the  prism,  but  one  or  both  pinacoids  are  also  sometimes  present. 
It  includes  portions  of  the  carbonate  background.  Diopside  is  rather  rare; 
it  occurs  usually  in  small  single  individuals,  with  sharp  crystal  outlines.  It 
is  sometimes  surrounded  by  tremolite,  from  which  it  is  distinguished  by  its 
high  obliquity  of  extinction  and  its  almost  rectangular  cleavage.  Partings 
parallel  to  both  ])inacoids,  as  well  as  a  transverse  parting  in  prismatic  sec- 
tions, are  also  observable.  Quartz  occurs  in  irregular  grains  completely 
interlocking  with  the  dolomite,  and  in  some  cases  with  tremolite.  In  the 
slides  examined  it  is  in  all  cases  a  secondary  as  well  as  a  rare  constituent. 
In  no  case  is  there  any  indication  that  it  is  clastic.  Chlorite  is  an  abundant 
constituent  of  some  of  the  slides,  while  from  others  it  is  entirely  absent. 
Muscovite  in  little  frayed  plates  is  plentiful  in  some  sections.  Quite  pos- 
sibly some  of  these  may  be  original  clastic  particles.  The  most  interesting 
mica,  however,  is  phlogopite,  which  is  very  abundant  in  one  locality  near 
the  base  of  the  formation.  It  occurs  in  large,  cleanly  bounded  plates, 
each  of  which  is  a  multiple  twin,  and  evidently  a  product  of  secondary 
crystallization.  Some  of  these  plates  have  been  strongly  bent,  thus  showing 
that  the  dolomite,  like  the  quartzite,  has  been  deformed  since  it  crystallized. 

The  thin  sections  therefore  show  that  the  rocks  of  this  formation  have 
experienced  even  more  nearly  complete  reconstruction  than  is  shown  in 
the  case  of  the  quartzites,  for  here  none  of  the  constituents,  except  jjossibly 
some  of  the  smaller  micas,  are  present  in  their  original  form.  Also  the  evi- 
dence for  disturbance  after  crystallization  is  of  similar  character  and  equally 


MANSFIELD  SCHISTS  IN  FELOH  MOUNTAIN  RANGE.  411 

strong.  Accordiiig-ly,  a  close  agreement  in  the  sequence  and  in  the  charac- 
ter of  the  i)rincipal  events  thus  indicated  in  the  history-  of  the  two  rocks 
may  he  recognized.  These  considerations  make  it  quite  certain  that  tlie 
recrystalhzation  of  the  two  formations  was  •  essentially  contemporaneous. 
From  the  character  of  the  accessory  minerals  in  the  dolomite  it  is  probable 
that  the  crystallization  was  not  accompanied  by  the  introduction  of  foreign 
material  from  outside,  in  notable  quantities,  but  consisted  in  a  mineralog- 
ical  rearrangement  of  the  elements  present  in  the  rock  from  the. beginning. 

SECTIOX   VI.     THE   MANSFIELD   SCHISTS. 

Above  the  Randville  dolomite  comes  a  formation  composed  chiefly  of 
fine-  to  medium-gr;iined  mica-schists.  Owing  to  their  exceedingly  soft 
character  and  small  thickness,  these  rocks  are  exposed  naturally  in  only  a 
few  localities  in  the  Felch  Mountain  area.  A  series  of  phyllites  less  meta- 
morphic  but  otherwise  similar,  and  occupying  the  same  stratigraphical 
position,  immediately  above  the  dolomite,  outcrop  characteristically  at  the 
Mansfield  mine,  and  especially  north  of  it,  near  the  Michigamme  River,  in 
T.  43  N.,  R.  31  W.  For  these  reasons  it  is  convenient  to  name  the  forma- 
tion for  the  Mansfield  locality. 

DISTRIBUTION,   EXPOSURES,  AND    TOPOGRAPHY. 

The  existence  of  the  Mansfield  formation  in  the  Felch  Mountain  area 
is  known  mainly  from  test  pits  and  the  records  of  diamond-drill  borings 
and  early  explorations.  Fortunately,  these  are  so  widely  distributed  that 
the  persistence  of  the  formation  is  well  proved.  Many  drill  holes  have 
passed  tlu-ough  it  into  the  dolomite.  Immediately  above  it  comes  the  mag- 
netic Groveland  formation,  which  even  when  covered  betrays  its  presence  to 
the  compass  needle.  With  the  upper  and  lower  limits  thus  determined,  and 
with  the  large  body  of  data  supplied  by  the  test  pits  and  records,  there  is 
no  difficulty  in  indicating  its  approximate  boundaries  for  the  greater  part  of 
the  map. 

On  the  west,  mica-schists  belonging  to  the  Mansfield  formation  have 
been  proved  b}"  diamond  drilling  to  occur  between  the  dolomite  and  Grove- 
land  formations  in  the  south  half  of  sec.  34,  T.  42  N.,  R.  30  W.  Farther 
east  there  is  a  line  of  outcrops  in  the  eastern  portion  of  section  35,  and  the 


412  THE  CRYSTAL  FALLS  IRON  BEARING  DISTRICT. 

schists  have  also  been  found  in  test  pits  on  both  sides  of  the  western  exten- 
sion of  the  Groveland  syncline  in  sec.  36,  T.  42  N.,  R.  30  W.  In  sec.  31, 
T.  42  N.,  R.  29  W.  (the  Groveland  section),  they  have  been  penetrated  in 
10  drill  holes,  besides  numerous  test  pits,  giving  altogether  a  cross  section 
more  than  half  a  mile  in  length  from  north  to  south.  In  the  nortliern  half  of 
sections  32  and  33  numerous  test  pits  have  exposed  the  Mansfield  formation, 
proving  that  it  borders  on  both  sides  the  narrow  syncline,  the  interior  of 
which  for  a  mile  and  a  half  is  occupied  by  the  magnetic  Groveland  jasper. 
Tlu'ough  sees.  34,  35,  and  36,  T.  42  N.,  R.  29  W.,  and  sec.  31,  T.  42  N., 
R.  28  W.,  the  mica-schists  have  not  been  discovered,  probably  both  because 
they  are  but  feebly  represented  and  because  but  few  test  pits  have  been 
sunk  tlu'ough  the  Cambrian  blanket.  In  sees.  32  and  33,  T.  42  N.,  R.  28 
W.,  the  mica-schists  have  been  found  in  scattered  test  pits  and  borings  on 
both  sides  of  the  interior  jasper  of  the  Felch  Mountain  spicline,  and  also 
on  the  south  side  of  section  33. 

The  thickness  of  the  Mansfield  formation  is  so  small — not  more  than 
200  feet — that  it  produces  no  very  noticeable  efi'ects  on  the  general  topog- 
ra]3hy,  in  spite  of  the  ease  with  which  it  weathers.  In  the  western  portion 
of  the  district,  througli  sees.  34  and  35,  T.  42  N.,  R.  30  W.,  with  the  dolo- 
mite it  underlies  a  broad  low-lying  plain,  which  is  bounded  on  the  south 
by  a  ridge  of  the  Sturgeon  quartzite  backed  by  the.  Archean  plateau.  On 
the  north,  a  broad  ridge,  through  which  diagonalljnfass  the  Archean  gran- 
ites and  gneisses,  the  quartzite,  and  the  dolomite,  defines  this  valley  as  far 
east  as  the  middle  of  section  35 ;  in  the  northern  and  central  portions  of  this 
section  it  spreads  out  into  a  swampy  lowland,  diversified  by  glacial  sand 
plains,  expressive  of  the  gradual  widening  of  the  trough  and  of  the  gen- 
erally horizontal  attitude  of  the  soft  rocks  of  the  interior.  The  most  defi- 
nite topographical  feature  directly  due  to  the  Mansfield  schists  is  the  narrow 
steep-sided  valley  which  runs  east  from  this  lowland  for  nearly  2  miles, 
on  the  south  side  of  the  Groveland  syncline.  The  ancient  stream  valley 
filled  with  the  Cambrian  sandstone,  already  mentioned,  follows  along  this 
narrow  belt. 

PETROGRAPHICAL  CHARACTERS. 

The  hand  specimens  from  the  various  test  pits,  the  drill  cores,  and  the 
few  small  outcrops  indicate  that  the  Mansfield  formation  is  quite  uniform 
in  character  throughout  the  Felch  Mountain  area.     The  great  majority  of 


MANSFIELD  SCHISTS  IN  PELOH  MOUNTAIN  KANGE.  413 

the  speciinous  ure  of  fine-grained  mica-schists,  the  color  of  wliicli  varies 
from  light  to  dark,  according  as  muscovite  or  biotite  is  the  })redominant 
mica.  Garnets,  in  some  localities,  are  very  abundant,  especially  near  the 
contacts  with  intrusives.  It  appears  from  the  records  of  explorations  that 
thin  seams  of  jaspery  iron  ore  interlaminated  with  the  schists  have  been 
encountered  in  occasional  drill  holes  and  test  pits,  but  no  specimens  of  such 
occurrences  have  been  obtained.  Their  existence  is  of  interest,  as  showino- 
the  likeness  in  an  imjiortant  character  of  these  more  altered  rocks  with 
the  slates  occupying  the  same  relative  position  in  the  Iron  Mountain  and 
Norway  areas. 

The  outcrops  and  specimens  are  frequently  well  banded  in  lighter  and 
darker  layers,  the  color  banding  in  seme  cases  not  coinciding  with  the 
schistosity.  Just  south  of  the  GroA^eland  mine,  in  a  test  pit  which  was 
sinking  at  the  time  of  my  visit,  the  color  bands  which  mark  the  true  strati- 
fication, as  shown  by  the  contact  with  the  underlying  dolomite,  are  closely 
crumpled  and  cut  by  the  foliation  of  the  rock,  which  is  much  the  more  dis- 
tinct of  the  two  structures. 

Near  the  contact  with  the  overlying  Grroveland  formation  the  mica- 
schists  become  both  more  siliceous  and  more  ferruginous,  and  there  is 
accordingly  a  distinct  passage  between  the  two  formations.  This  does 
not  necessarily  signify  a  transitional  character  in  the  original  sediments, 
but  may  be  altogether  due  to  the  downward  transportation  of  silica  and 
iron  from  the  upper  rock. 

The  mica-schists  are  generally  very  tender  rocks,  and  the  material 
on  the  dumps  of  test  pits  sunk  in  them  is  usually  far  gone  in  decomposi- 
tion after  a  few  years'  exposure  to  the  weather.  From  even  the  freshest 
specimens  the  little  flakes  of  mica  often  rub  off  on  the  fingers.  Where 
penetrated  by  intrusions,  however,  as  in  sec.  36,  T.  42  N.,  R.  30  W.,  and  in 
sec.  31,  T.  42  N.,  R.  28  W.,  they  become  very  much  harder. 

Under  the  microscope  the  rocks  of  this  formation  are  seen  to  be  in  the 
main  thoroughly  crystalline,  though  very  fine-grained,  aggregates  of  biotite, 
muscovite,  chlorite,  quartz,  and  feldspar,  with  the  iron  ores,  rutile,  tourma- 
line, and  apatite  as  the  accessories.  Garnets  are  abundant  in  some  of  the 
sections,  and  with  these  also  occur  actinolite,  epidote,  titanite,  and  an  unde- 
termined colorless  amphibole  in  stout  single  prisms.  In  the  eight  thin  sec- 
tions which  I  have  examined  from  this  formation  I  have  found  no  material 


414  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

which  is  certainly  original  and  fragmental,  although  almost  every  slide 
contains  grains  that  may  possibly  be  such.  On  the  other  hand,  it  is  evident 
that  the  large  majority  of  the  individual  grains  have  formed  in  place. 

The  micas  are  in  most  cases  the  most  abundant  constituent;  sometimes 
muscovite,  though  iisually  biotite,  predominates.  The  two  micas  are  often 
intergrovvn.  The  biotite  is  usually  very  deeply  colored,  both  brown  and 
green,  and,  except  in  the  thinnest  slides,  is  almost  opaque  even  in  cleavage 
sections.  The  larger  mica  flakes  do  not  exceed  0.5  mm.  in  length,  and 
average  not  more  than  0.25  mm. 

Quartz  generally  occurs  in  irregular  grains,  full  of  fluid  inclusions,  and 
inclosing  the  various  accessories.  It  frequently  appears  in  little  triangles 
in  the  interspaces  between  adjacent  flakes  of  mica.  Rarely  part  of  the  peri- 
meter is  rounded  and  embedded  in  a  mica,  thus  suggesting  a  clastic  origin. 

Feldspar  is  very  abundant  in  some  of  the  slides  and  entirely  absent 
from  others.  Both  microcline  and  plagioclase  occur,  and  in  forms  similar 
to  the  quartz.  Biotite  sometimes  penetrates  in  irregular  shredded  edges  and 
filaments  into  the  interior  of  the  feldspars,  and  in  such  cases  may  be  a 
metasomatic  product,  as  described  by  Irving  and  Van  Hise  ^  in  the  mica- 
schists  of  the  Gogebic  district.  But  much  of  the  feldspar,  as  shown  by  its 
form  and  freshness,  has  recrj^stallized.  The  alignment  of  these  minerals  is 
with  the  schistosit)^  of  the  rock,  which  they  thus  determine.  When  the 
schistosity  cuts  the  lines  of  stratification,  as  it  frequently  does,  the  latter  are 
but  faintly  marked  in  the  thin  section  by  very  slight  mineralogical  diff'erences. 
Thus  a  dark  baud,  which  may  be  very  striking  macroscopically,  may  be  due 
merely  to  the  predominance  of  deeply  colored  biotite;  a  light  band,  to  the 
predominance  of  muscovite.  Sometimes,  however,  in  these  bands  a  grain 
of  quartz,  or  a  stout  flake  of  muscovite,  lies  out  of  the  general  orientation 
and  with  the  direction  of  the  band.  Such  grains  are  very  possibly  original. 
The  schistose  structure,  as  has  already  been  stated,  is  determined  by  the 
general  parallelism  of  the  long  axes  of  the  constituent  grains.  Since  the 
greater  part,  if  not  demonstrably  all,  of  these  grains  have  formed  in  this 
position,  and  have  not  been  forced  mechanically  into  it,  the  cases  in  which 
the  schistosity  cuts  the  bedding  support  the  inference  as  to  the  time  of 
the  general  recrystallization  of  the  series  grounded  on  the  facts  observed 

'  The  Penokee-Gogebic  iron-bearing  district  of  Michigan  and  Wisconsin,  by  R.  D.  Irving  and 
C.  R.  Van  Hise :  Mon.  U.  S.  Geol.  Survey,  No.  XIX,  1«92. 


GKOVELAND  FORMATION  IN  FBLGH  MOUNTAIN  RANGE.   415 

ill  the  lower  t'onujition,  luunely,  that  this  time  foHowed  a  period  of  great 
stresses.  Also  a  period  of  still  later  stress  has  affected  the  recrystallized  con- 
stituents of  the  schists,  just  as  it  has  those  of  the  quartzite  and  dolomite.  It 
is  shown  1)>'  lines  of  fracture  crossing  the  slides  along  which  ferric  oxide  has 
infiltrated,  and  by  occasional  straining  and  bending  of  the  quartz  and  mica. 
G-arnetiferous  varieties  of  the  schists  are  found  in  close  proximity  to 
basic  igneous  rocks,  probably  in  every  instance  intrusives,  and  are  evidently 
the  result  of  contact  metamorphism.  With  the  garnets  occur  actinolite  in 
felted  mats  and  clusters,  and  abundant  magnetite  ami  pyrite.  A  colorless 
amphibole  in  large  single  crystals  bounded  by  the  prism  and  clinopinacoid, 
and  giving  low  extinctions,  is  often  associated  with  the  actinolite. 

SECTION   VII.    THE   GROVELAND  FORMATION". 

The  ferruginous  rocks  which  compose  this  formation  are  well  exposed 
in  the  central  portion  of  sec.  31,  T.  42  N.,  R.  29  W.,  in  the  vicinity  of 
the  Groveland  mine,  and  thus  may  properly  be  termed  the  Groveland 
formation. 

DISTRIBUTION,    EXPOSURES,    AND    TOPOGRAPHY. 

The  magnetite,  which  is  always  an  abundant  constituent  of  these 
rocks,  has  made  it  possible  to  trace  them  for  long  distances  throughout  the 
trough,  by  means  of  the  disturbances  effected  in  the  compass  needles.  The 
same  disturbances  had  led  to  the  sinking  of  a  great  number  of  test  pits  on 
the  part  of  former  explorers  for  iron  ore,  and  the  material  thrown  out  of 
these  has  served  to  check  and  substantiate  the  inferences  from  the  magnetic 
attractions.  Finally,  in  several  localities  excellent  natural  exposures  of  the 
iron-bearing  rocks  occur.  So,  altogether,  the  available  data  as  to  the  surface 
distribution  of  the  Groveland  formation  are  fairly  satisfactory. 

On  the  west  the  presence  of  the  Groveland  formation  through  sees.  34, 
35,  and  36,  T.  42  N.,  R.  30  W.,  is  shown  by  one  principal  and  other  minor 
lines  of  attraction,  as  well  as  by  test  pits  and  outcrops.  The  principal  line 
of  attraction  begins  in  section  34,  near  the  southwest  corner,  and  runs  to 
the  northeast,  in  conformity  with  the  strike  of  the  northern  belt  of  dolomite, 
finally  ending  in  the  northeastern  portion  of  section  36.  This  line  of  attrac- 
tion is  very  vigorous  and  strongly  marked.  Two  other  lines,  parallel  with 
the  principal  line,  but  more  feeble  and  much  shorter,  cross  the  boundary 
between  sections  35  and  36,  and  on  the  northern  of  these  ferruginous  rocks 


416  THE  CRYSTAL  FALLS  IRON-BE ARING  DISTRICT. 

outcrop  in  the  western  part  of  section  36.  Near  the  center  of  section  36 
another  hne,  marking  the  western  end  of  the  Groveland  syncUne,  begins 
and  continues  for  a  mile  and  a  half  east  to  the  eastern  portion  of  sec.  31, 
T.  42  N.,  R.  29  W.  Along  the  western  portion  of  this  line  are  many  test 
pits,  and  in  section  31  the  fine  exposures  of  the  Groveland  hill. 

Four  hundred  paces  north  of  the  center  of  sec.  32,  T.  42  N.,  R.  29  W., 
another  line  of  attraction  begins,  and  may  be  followed  toward  the  east 
without  interruption  nearly  to  the  east  line  of  section  33  of  the  same  town- 
ship. Along  this  line,  which  is  comparatively  feeble  and  crosses  wet 
ground,  there  are  but  few  test  pits.  In  the  eastern  part  of  section  33, 
beyond  the  point  at  which  the  attractions  cease,  many  pits  have  been  sunk 
to  and  into  the  Mansfield  formation,  which  is  there  somewhat  ferruginous. 
From  this  point  east  for  4  miles  the  Groveland  formation  has  not  been 
recognized. 

In  the  northern  part  of  sees.  32  and  33,  T.  42  N.,  R.  28  W.,  the  fer- 
ruginous rocks  are  again  well  exposed  on  Felch  Mountain  for  nearly  a  mile 
along  the  strike,  and  may  be  identified  for  half  a  mile  farther  by  the  vigor- 
ous disturbances  produced  in  the  magnetic  needles.  In  the  southeastern 
quarter  of  section  33  the  Groveland  formation  is  again  encountered  in  a 
small  and  much-disturbed  area,  in  faulted  contact  with  the  Archean. 

The  most  conspicuous  hills  within  the  Algonkian  belt  owe  their  relief 
to  the  fact  that  they  are  underlain  by  the  Groveland  formation,  but  else- 
where this  formation  has  left  but  little  impress  on  the  topography,  perhaps 
because  the  local  base-levels  are  cut  nearly  to  the  bottoms  of  the  synclines 
in  which  it  is  preserved.  The  two  lulls  referred  to — Felch  Mountain,  in 
sees.  32  and  33,  T.  42  N.,  R.  28  W.,  and  the  Groveland  hill,  in  sec.  31, 
T.  42  N.,  R.  29  W. — stand  100  feet  or  more  above  the  average  level  of  the 
surrounding  Algonkian  territory,  and  in  both  instances  the  infolded  second- 
ary synclines  are  exceptionally  deep  and  broad.  The  magnetic  lines  which 
indicate  the  other  synclines  pass  through  low  ground,  and  the  Ijelts  of  dis- 
turbance are  much  narrower  than  in  the  cases  of  the  two  principal  hills. 
There  seems  to  be,  so  far  as  the  collected  material  warrants  a  judgment,  no 
litholoarical  difference  between  the  rocks  of  the  naiTOw  and  those  of  the 
broad  and  deep  synclines,  and  accordingly  the  relief  of  the  latter  is  believed 
to  be  caused  by  their  depth  below  the  adjacent  base-levels  and  not  by  their 
more  resistant  character. 


GROVKLAND  FORMATION  IN  FELCH  MOUNTAIN  RANGE.       417 

PETROGRAPHICAL  CHARACTERS. 

The  rocks  of  tlii'  (irntveliind  formation  have  a  general  family  likeness, 
which  makes  it  very  easy  to  distingnish  them  in  tlie  field  from  all  the  other 
members  of  the  Algonkian  series.  Among-  them  two  main  mineralogical 
kinds  may  be  recognized,  the  usnal  one  of  which  consists  of  quartz  and  the 
anhydrous  oxides  of  iron,  while  the  other,  which  is  mucli  rarer,  is  made 
up  essentially  of  an  iron  amphibole,  quite  similar  to  the  griinerite  of  the 
Marquette  range,  with  quartz  and  the  iron  oxides  as  associates. 

As  seen  in  the  field,  the  rocks  of  the  first  kind  are  generally  siliceous, 
heavy,  and  dark  colored,  the  weight  and  color,  which  has  a  tinge  of  blue, 
being  due  to  the  presence  of  abundant  crystalline  ii-on  oxides.  A  large  part 
of  the  silica  is  easily  recognized  as  crystalline  quartz,  in  some  instances, 
indeed,  in  the  form  of  detrital  grains.  The  visible  iron  oxides  occur  both 
as  little  spangles  of  specular  hematite  and  also  in  irregular  dark-blue 
masses  and  single  grains,  the  latter  often  having  the  crystalline  form  of 
magnetite.  Many,  if  not  most,  of  these  last,  however,  seem  to  be  really 
martite,  as  they  give  a  dark  purple  streak,  and  in  fine  powder  are  not 
attracted  by  a  hand  magnet. 

In  the  first  kind  there  is  much  variety  in  external  appearance,  deter- 
mined by  the  varial>le  proportions  in  which  the  chief  constituents  occur 
and  by  the  different  ways  in  which  these  constituents  are  arranged.  Con- 
siderable areas,  for  example,  consist  maiidy  of  granular  quartz  merely 
darkened  by  the  intimately  mixed  iron  oxides,  and  in  these,  so  far  as  the 
eye  can  judge,  the  rock  is  a  ferruginous  quartzite.  Closely  connected  with 
such  occurrences,  or  included  most  iiTegulaidy  in  them,  are  others  in  which 
the  ferruginous  constituents  are  so  abundant  and  the  quartz  so  subordinate 
that  they  would  pass  for  lean  iron  ores.  Between  such  rare  extremes  we 
find  all  intermediate  proportions  of  mixtures  of  the  quartz  and  the  iron 
oxides. 

One  form  of  arrangement  of  the  constituent  minerals  is  in  narrow 
parallel  bands,  in  which  the  quartz  and  the  iron  oxides  alternately  predom- 
inate. Such  alternations  are  sometimes  so  frequent  and  regular  as  per- 
fectly to  reproduce  the  lean  "flag  ores"  of  the  Marquette  range.^  Regular 
banding,  however,  is  not  common.     Usually  the  light  or  dark  bands  are 

'Geol.  Survey  Michigan,  Vol.  I,  Part  I,  by  T.  B.  Brooks,  pp.  93-94. 
MON    XXXVI 137 


418  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

suddenly  cut  oft',  as  if  by  faulting,  or  tajjer  to  thin  edges,  or  occur  in  sepa- 
rated pebble-like  forms.  Neighboring  lenses  and  fragments  of  bands  are 
most  frequently  i-oughly  parallel  with  one  another,  but  often  they  are 
jumbled  together  in  the  greatest  confusion.  They  no  doubt  represent  an 
original  more  continuous  banding,  which  has  suffered  brecciation.  Masses 
thus  shattered  are  also  traversed  and  cemented  by  numerous  small  A-eins 
filled  chiefly  with  quartz,  chalcedony,  and  specular  hematite.  The  posi- 
tions in  which  the  separated  patches  of  the  Groveland  formation  now 
survive,  namely,  in  and  near  the  bottoms  of  synclines,  and  therefore  at  the 
jioints  where  sharp  turning  and  crowding  together  have  taken  place,  suffi- 
ciently explain  the  extensive  brecciation  observed  in  these  brittle  beds. 

Very  prevalent  in  all  the  varieties  of  the  first  kind  of  rock,  in  massive, 
banded,  and  brecciated  alike,  is  the  occurrence  of  some  of  the  constituents 
in  small  roundish  spots,  which  give  to  the  whole  formation  a  very  detrital 
aspect.  In  the  quartzitic  phases,  as  well  as  in  the  most  ferruginous  bands, 
the  eye  recognizes,  besides  the  little  grains  of  clear  quartz,  which  seem  to 
be  unquestionably  detrital,  numerous  small  dots  of  blue  hematite  and  bright 
red  dots  of  jasper.  These  are  more  abundant  in  some  layers  than  in  others, 
but  seem  never  to  be  entirely  absent,  and  are  exceedingly  characteristic  of 
the  formation  wherever  found. 

In  a  few  localities  the  iron  constituent  is  almost  entirely  in  the  form  of 
little  micaceous  scales  of  specular  hematite,  which  have  a  parallel  arrange- 
ment. Hematite-schists,  however,  are  not  very  common.  The  best  exam- 
ples occur  in  the  northern  part  of  sec.  36,  T.  42  N.,  R.  30  W.,  along  the 
northern  syncline. 

The  second  kind  of  rocks  of  the  formation,  the  griinerite-schists,  have 
been  found  in  small  thickness  and  in  one  locality  only,  namely,  in  the 
southern  parts  of  sec.  33,  T.  42  N.,  R.  28  W.,  where  they  underlie,  in  a 
series  of  small  anticlines  and  synclines,  banded  siliceous  beds  composed  of 
quartz  and  magnetite  or  martite. 

Under  the  microscope  the  essential  constituents  of  the  first  or  prevalent 
kind  of  rock  of  the  Groveland  formation  are  quartz,  magnetite,  martite,  and 
hematite.  With  these,  much  smaller  quantities  of  clilorite,  epidote,  and 
apatite  are  generally  associated  as  accessories.  Of  rarer  occurrence  are 
calcite  and  probably  siderite,  sericite,  tremolite,  griinerite,  pyrite,  limonite, 
chalcedony,  rutile,  titanite,  tourmaline,  microcline,  and  plagioclase. 


b 


GKOVELAND  FORMATION  IN  FELCH  MOUNTAIN  EANGE.       419 

Quartz  occurs  in  two  ways — first  as  rouuded  detrital  particles,  and 
secondly  as  grains  which  have  crystallized  in  place.  The  deti'ital  grains, 
which  are  easily  recognized  by  their  form,  size,  and  freedom  from  inclusions 
of  the  ores,  consist  of  single  individuals,  often  surrounded  with  rims  of  later 
growth.  They  are  also  usually  larger  than  the  neighboring  indigenous 
grains.  While  detrital  quartz  is  not  abundant  and,  indeed,  is  often  entirely 
absent  from  the  thin  sections,  its  occurrence  is  of  interest  as  conclusively 
establishing-  the  sedimentary  origin  of  the  iron-bearing  formation! 

The  secondary  quartz  grains  are  the  most  abundant  constituents  of  the 
thin  sections,  and  form  the  genei'al  background  for  the  other  minerals. 
They  always  inclose  separate  crystals  of  the  iron  oxides,  usually  in  gi'eat 
abundance,  and  often  also  chlorite  and  little  prisms  of  apatite.  These 
grains  usuall}'  have  the  shape  of  irregular  polygons  bounded  by  straight 
lines,  frequentl}  with  reentrant  angles,  and  adjacent  grains  completely  inter- 
lock. In  size  the  secondary  quartz  grains  range  from  about  0.03  to  0.4  mm. 
in  diameter.  Grains  of  approximately  the  same  size  occur  together  in  bands 
or  in  the  rounded  areas  to  be  mentioned  later. 

The  iron  ores  include  both  magnetite,  or  martite,  and  crystalline  hem- 
atite, the  former  being  much  the  more  abundant.  The  magnetite  and  martite 
can  not  be  distinguished  in  thin  section,  as  their  color  in  reflected  light  and 
crystalline  form  are  the  same.  They  occur  in  irregvilar  bands  composed  of 
aggregates  of  crystals,  the  edges  of  which  interlock  with  the  adjoining  and 
inclosed  areas  of  quartz,  and  show  the  triangular,  rhombic,  and  square  sec- 
tions of  magiietite  individuals.  Magnetite  also  occurs  in  isolated,  irregular 
aggregates  interlocking  with  the  secondary  quartz  grains,  and  of  similar 
dimensions  to  these,  but  is  especially  abundant  as  single  minute  crystals 
interposed  in  the  grains  of  secondary  quartz,  ranging  in  size  from  such  as 
are  barely  recognizable  under  a  No.  9  objective  to  octahedra  0.03-0.05  mm. 
in  diameter.  A  single  quartz  grain  ^  mm.  in  diameter  may  inclose  a  hun- 
dred or  more  such  minute  individuals.  Hematite  is  much  rarer  than  mag- 
netite, and  seems  to  be  found  only  in  the  secondary  quartz  grains  or  in 
veins.  In  the  former  it  occurs  in  separate  crystalline  plates,  of  deep  red 
color  in  transmitted  light,  under  the  same  conditions  as  to  number  and  size 
as  the  magnetic  crystals.  Throughout  some  sections,  and  in  certain  bands 
and  rounded  areas  in  other  sections,  it  is  more  abundant  as  inclosures  than 
magnetite.     Such  rounded  areas  formed  of  several  quartz  individuals,  each 


420  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

of  which  thus  holds  a  great  number  of  hematite  plates,  appear  macroscopic- 
all)^  as  the  little  jasper  dots  already  described.  Chlorite  and  apatite  are 
also  often  embedded  in  the  secondary  quartz  grains,  the  former  in  thin 
plates  and  the  latter  in  small  hexagonal  prisms.  Epidote  is  quite  common 
in  small  irregular  areas  intercalated  between  the  quartz  gi-ains  or  in  the 
magnetite  bands. 

Many  of  the  slides  contain  a  small  amount  of  rhombohedral  carbonate, 
much  if  not  all  of  which  is  calcite.  It  occurs  chiefly  in  the  quarts  bands, 
in  irreo-ular  grains  which  interlock  with  the  secondary  quartz  grains,  and, 
like  them,  inclose  little  crystals  of  magnetite  and  hematite.  Specimens  the 
slides  from  which  contain  carbonates  effervesce  freely  in  scattered  spots 
with  cold  dilute  acid.  Most  of  the  carbonates  are  clear  white  under  the 
microscope,  and  are  evidently  calcite.  Sometimes,  however,  the  carbonate 
areas  have  a  very  light-brown  tint,  and  are  partially  surrounded  with  a 
limonite  border  and  jjenetrated  by  brownish  filaments  along  the  cleavages. 
In  such  cases  it  is  difficult  to  decide  whether  they  are  calcite  stained  with 
limonite,  or  siderite  partially  oxidized  to  limonite.  However,  if  part  of 
these  areas  are  siderite  it  is  nevertheless  certain  that  the  small  magnetite 
and  hematite  crystals  which  they  inclose  have  not  been  derived  from  them. 
These  little  crystals  are  inclosed  in  the  carbonates  just  as  they  are  in  the 
adjoining  grains  of  secondary  quartz,  while  the  alteration  of  the  siderite,  if 
it  is  siderite,  is  to  limonite.  Carbonates  also  occur  with  tremolite,  quartz, 
chalcedony,  epidote,  and  hematite  in  the  numerous  thread-like  veins  which 
traverse  some  of  the  thin  sections. 

The  feldspars  have  l^een  found  in  onl}^  a  few  thin  sections,  as  well- 
scattered  but  minute  angular  grains  of  microcline  and  plagioclase.  ]\Iany 
slides,  however,  contain  areas  of  matted  sericite  and  quartz  wliicli  probably 
represent  original  grains  of  feldspar. 

Rutile  and  tourmaline  are  also  occasionally  inclosed  with  the  iron  ores 
in  the  grains  of  secondary  quartz.  Small  roundish  areas  of  titanite,  prob- 
ably detrital,  occur  very  sparingl}^  in  a  few  of  tlie  thin  sections. 

The  most  interesting  features  of  the  thin  sections  are  certain  very 
distinct  structural  arrangements  of  the  quartz  and  iron  ores.  In  almost 
every  slide,  in  ordinary  polarized  light  (with  the  analyzer  out),  the  minute 
interpositions  of  the  iron  ores  are  seen  not  to  he  equally  distributed  through- 
out the  background,  Init  to  be  concentrated  in  round  or  oval  areas,  never 


GROVELAND  FORMATION"  IN  FELCII  MOUNTAIN  RANGE.       421 

excuodiug  ;i  inilliiiK'tur  in  (liiunL-ter.  Tliese  oval  forms  are  coiitiiUMl  to  the 
more  siliceous  bands,  and  are  much  more  distinct  in  some  of  the  slides  than 
iu  others.  ( )ften  the  outlines  are  reenforced  b}-  rims  of  closel}'  set  mag- 
netite individuals,  somewhat  coarser  than  the  dust-like  crystals  within.  The 
long'  diameters  of  adjacent  ovals  are  2)arallel  to  one  another  and  to  the  band 
in  which  they  lie,  and  ai'e  often  closely  packed  like  pebbles.  ()cca.sionall3' 
the  little  grains  of  iron  ore  within  the  ovals  have  a  distinctly  concentric 
arrangement. 

Between  crossed  nicols  these  areas  are  seen  to  have  had  in  some 
instances  a  distinct  influence  on  the  crystallization  of  the  secondary  quartz. 
When  they  are  large  and  closely  packed,  each  oval  includes  a  large  number 
of  interlocking  quartz  grains,  and  occasionally  in  such  cases  there  is  some 
difference  in  size  between  the  quartz  grains  inside  and  those  outside  the 
ovals.  In  the  triangular  and  quadrangular  areas  lying  between  the  larger 
ovals,  and  bounded  by  curving  segments  of  their  perimeters,  the  secondary 
quartzes  are  frequently  larger  than  those  within,  and  are  placed  normal  to 
the  boundaries,  precisely  as  if  they  had  grown  outward  from  the  ovals  into 
free  spaces.  Often,  however,  a  single  individual  of  secondary  quartz  lies 
partly  within  and  partly  without  the  oval.  On  the  other  hand,  when  the 
ovals  are  small,  one  or  more  may  be  completely  or  partially  inclosed  within 
a  single  quartz  individual.  The  interlocking  quartz  grains  within  the  large 
ovals  show  no  indications  of  having  formed  in  open  spaces,  even  when  the 
included  iron  ores  have  a  tendency,  as  occasionally  happens,  to  a  concentric 
arrangement.  The  faulting  and  brecciation  so  plainly  seen  in  many  of  the 
thin  sections  have  also  displaced  and  separated  the  oval  areas.  It  seems 
perfectly  clear  to  me  that  these  forms  represent  a  structure  originally 
possessed  by  the  rock  from  which  the  various  phases  of  the  iron  formation 
have  been  derived,  and  which  has  been  preserved  through  the  subsequent 
metamorphism. 

From  the  facts  described  above,  it  is  evident  that  the  Groveland  for- 
mation is  made  up  of  highly  metamorphic  rocks,  which  still,  however,  retain 
some  original  clastic  material  as  well  as  certain  original  structural  characters. 
With  the  exception  of  the  rather  rare  clastic  grains  of  quartz,  titanite, 
feldspar,  etc.,  the  minerals  which  now  chiefly  compose  these  rocks — namely, 
quartz  and  the  crystalline  iron  oxides — are  not  cla.stic,  but  have  crystallized 
in  place.     It  is  a  matter  of  great  interest,  therefore,  to  determine,  if  possible. 


422  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

in  what  form  these  constituents  were  present  in  the  original  deposit.  (.)n  this 
question  the  microscopic  structure  seems  to  me  to  have  a  distinct  bearing. 
Forms  similar  to  the  ovals  in  these  rocks  occur  in  the  iron-bearing 
formations  of  other  districts  in  the  Lake  Superior  region.  In  the  Gogebic 
district  of  Michigan  and  Wisconsin,  R,  D.  Irving  and  C.  R.  Van  Hise^  have 
supposed  that  such  forms  have  resulted  from  pi-ocesses  of  solution  and 
redeposition  after  the  rock  was  formed,  and  are  tlierefore  concretionary. 
They  regard  that  portion  of  the  formation — which  they  have  named  fer- 
ruginous cherts — in  which  such  forms  occur,  as  an  alteration  product  from 
an  original  dejiosit  of  cherty  carbonate  of  iron.  On  tlie  other  hand,  J.  E. 
Spurr'  lias  shown  that  similar  forms  are  exceedingly  abundant  throughout 
the  iron-bearing  formation  of  the  Mesabi  range  of  Minnesota,  and  are  there 
original.  In  the  least-altered  stages  Mr.  Spurr  has  found  that  these  oval 
and  roundish  areas  are  filled  with  a  green  substance,  which  chemically  is 
a  hydrous  silicate  of  iron,  in  composition  A^ery  close  to  glauoonite,  with 
which  it  is  also  optically  identical.  The  oval  and  rounded  forms,  moreover, 
are  those  characteristic  of  glauconite  in  green  sands  of  all  geological  ages. 
Starting  with  this  original  substance,  which  is  very  unstable  when  exposed 
to  oxidizing  and  carbonated  waters,  Mr.  Spurr  has  traced  an  interesting 
series  of  clianges,  the  final  result  of  which  along  one  line  is  the  complete 
oxidation  of  the  iron  to  hematite  or  magnetite  and  the  separation  of  the 
silica  as  chalcedony  and  quartz.  Tlu'oughout  these  changes  the  original 
form  of  the  glauconite  grains  is  jireserved  in  the  new  minerals.  Without 
going  into  the  details  of  these  changes,  and  without  accepting  Mr.  Spurr's 
conclusions  in  their  entirety  as  to  the  steps  involved,  he  has  clearly  shown, 
as  I  have  satisfied  myself  from  the  study  of  the  large  number  of  Mesabi 
slides  in  my  own  collection,  that  the  green  glauconitic  substance  is  the 
source  of  the  iron  and  silica  of  the  ferruginous  cherts  of  the  Mesabi  range, 
and  that  the  peculiar  spotted  structure  of  these  cherts  is  inherited  from  the 
original  forms  of  the  glauconite  grains. 

Between  the  ferruginous  quartzites  of  the  Groveland  formation  and  the 
ferruginous  cherts  of  the  Mesabi  range  there  is  a  very  close  resemblance, 
especially  in  structure.     The  essential  difi"erence  is  that  the  former  contain 

'  Loc.  eit.,  pp.  2.54-257. 

■  The  iiou-ljcaring  rocks  nf  the  Mesabi  rauge  iu  Jliuiiesota,  by  .1.  E.  Spurr:  Bull.  Geol.  aud  Nat. 
Hist.  Surv.  of  Minn.,  No.  X,  1894,  259  pp.,  12  pis. 


UPPER  nURONIAN  OF  FELCri  MOUNTAIK  RANGE.  423 

little  or  IK.  chalcedony,  the  silica  being  crystallized  quartz,  while  the  latter 
have  a  great  deal  of  chalcedonic  silica.  Also  the  former  contain  small 
amounts  of  detrital  material,  which  the  latter  generally  lack,  but  the  essen- 
tial difference  between  them  is  one  of  degree  of  crystallization  only. 

If  the  silica  of  the  Mesabi  cherts  had  originally  crystallized  entirely  as 
quartz,  or  if  after  passing  through  the  stage  of  mixed  chalcedony  and  quartz 
it  had  subsequently  crystallized  as  quartz,  there  would  be  no  essential 
difference  between  the  iron  formations  of  the  two  districts. 

There  are,  then,  at  least  two  possible  forms  in  which  the  iron  and  silica 
of  the  Groveland  formation  may  have  been  deposited  originally,  as  indicated 
by  the  conclusions  of  observers  who  have  studied  the  similar  iron-bearing 
formations  in  other  districts  of  the  Lake  Superior  region  in  which  these 
formations  are  less  altered  than  here.  Either  of  these  forms — namely,  a 
cherty  iron  carbonate,  as  on  the  Gogebic  range,  or  a  glauconitic  greensand,  as 
on  the  Mesabi  range— could  give  i-ise,  under  the  action  of  vigorously  oxid- 
izing waters,  to  rocks  of  the  mineralogical  composition  of  those  in  question, 
and  since  no  trace  of  either  original  form  has  been  found  in  the  Groveland 
formation  the  choice  between  them  may  perhaps  be  regarded  as  still  open. 
My  own  opinion,  based  on  the  microscopic  structure  which,  as  I  interpret  it, 
shows  that  the  Groveland  formation  was  in  the  beginning  largely  made  up 
of  rounded  particles  having  the  same  general  form  as  the  glauconite  grains 
of  the  Mesabi  range,  is  that  the  iron  and  silica  were  originally  present 
largely  in  the  form  of  glauconite. 

SECTION  VIII.    THE   MICA-SCHISTS   AND    QUARTZITES    OF    THE   UPPER 

IIURONIAN   SERIES. 

Through  the  eastern  part  of  sec.  32,  T.  42  N.,  R.  28  W.,  and  entirely 
across  section  33,  runs  a  belt  of  mica-schists  and  thin-bedded  ferruginous 
quartzites  which  seem  to  have  unconformable  relations  with  the  formations 
just  described.  These  rocks  are  seen  on  the  west  in  a  cut  in  the  North- 
western Railway  in  the  SE.  i  of  the  NE.  ^  of  sec.  32.  At  the  western  end 
of  this  cut  the  strike  is  northwest  and  the  dip  northeast  at  an  angle  of  about 
35°.  At  the  eastern  end  there  is  a  decided  bending  in  the  strike  to  a  more 
nearly  east-and-west  direction,  and  the  bedding  surfaces  carry  striations 
which  dip  east  at  an  angle  of  10°,  all  indicating  that  these  outcrops  prob- 
ably lie  on  the  south  limb  of  a  gently  eastward-pitching  synclinal  fold,  and 


424  THE  CEYSTAL  FALLS  IRON-BEAEING  DISTEIOT. 

near  the  axial  plane.  East  from  this  point  similar  schists  and  quartzites 
form  a  ridge,  low  and  flat-topped,  which  extends  immediately  south  of  the 
railway  almost  to  the  east  line  of  section  33,  and  sinks  gradually  beneath  the 
great  swamp  of  the  eastern  portion  of  that  section.  The  formation  notice- 
ably disturbs  the  compass  needles,  and  this  fact,  together  with  the  rusty 
appearance  of  the  outcrops,  has  probably  led  to  the  sinking  of  the  numer- 
ous test  pits  by  which  the  continuity  is  chiefly  established.  But  low-lying 
natural  exposures  are  not  lacking. 

North  of  the  center  of  the  NE.  ^  of  sec.  33  similar  schists  have  been 
found  in  two  test  pits.  Also,  parallel  with  the  outcropping  southern  belt 
and  a  quarter  of  a  mile  or  more  farther  north,  a  faintly  marked  zone  of 
magnetic  disturbances  runs  east  and  west  through  the  swampy  ground 
south  of  Felch  Mountain  and  probaljly  connects  the  last-mentioned  occur- 
rences with  the  exposures  of  the  railway  cut.  It  therefore  seems  likely 
that  the  low  ground  through  the  middle  of  sections  32  and  33  is  wholly 
occupied  by  an  open  syncline  of  these  soft  and  easily  disintegrating 
rocks. 

Between  the  exposed  ^southern  limb  of  this  syncline  and  the  southern 
Archean  the  lower  Algonkian  formations  are  found  in  the  southeastern 
portion  of  section  33.  Actual  contacts  are  not  visible,  but  there  are  note- 
worthy discordances  in  strike  and  dip,  and  especially  clear  proof  of  great 
disturbances  in  the  lower  rocks  in  which  the  upper  have  not  shared.  In 
the  SE.  i  of  the  NE.  4  of  the  SW.  i  of  sec.  33,  about  200  feet  thickness  of 
the  Randville  dolomite,  striking  east  and  west  and  dipping  north  at  about 
70°,  is  exposed  between  the  Sturgeon  quartzite  below  and  the  mica-schists 
to  the  north.  Between  the  dolomite  and  the  schists  is  a  covered  interval 
of  some  40  feet.  The  latter  also  strike  about  east  and  west,  but  dip  north 
at  30°  or  less.  Between  a  quarter  and  three-eighths  of  a  mile  east  of  this 
locality  (the  interval  being  without  outcrops)  the  Mansfield  and  Grroveland 
formations  lie  against  the  Archean  gneisses  with  a  faulted  contact.  They 
have  been  thrown  into  a  series  of  southeastward-pitching  minor  folds,  and 
have  been  intruded  by  a  mass  of  diabase  and  also  by  a  pegmatite  dike. 
The  true  strike  of  these  formations  at  this  locality  is  toward  the  northeast, 
and  the  dip,  as  shown  both  by  the  direction  of  pitch  and  the  order  of  sujjer- 
position,  is  toward  the  southeast.  Five  hundred  feet  north  of  this  disturbed 
area  and  directh'  across  the   strike  of  the  lower  formations  therein,  the 


UPPER  HURONIAX  OF  FP:LCH  MOUNTAIN  RANGE.  425 

upper  schists  and    (niartzites  coiitiiiuc    their  .southeastward   strike  without 
deviation. 

These  general  relations  indicate  tliat  the  ferruginous  mica-schists  and 
quartzites  are  part  of  an  upper  series  which  overlies  unconformably  the 
Groveland  and  all  the  lower  formations.  This  series  lias  not  Ijeen  found 
elsewhere  in  the  Felch  Mountain  area. 

PETROGRAPHICAL  CHARACTERS. 

The  rocks  of  this  formation,  as  seen  in  tlie  outcrops,  are  principally 
soft  and  deeply  iron-stained  mica-schists  in  which  occur  frequent  tliin  beds 
of  ferruginous  and  micaceous  qnartzite. 

Under  the  microscope  tlie  schists  are  composed  mainly  of  biotite, 
quartz,  inuscovite,  and  magnetite.  Chlorite,  as  an  alteration  product  of  the 
biotite,  is  frequently  abundant,  and  garnets  also  occur  in  .some  sections. 
These  schists  are  much  coarser  in  grain  than  those  of  the  Mansfield  forma- 
tion, and  are  wholly  crystalline.  No  clastic  material  has  been  recognized 
in  the  thin  sections. 

The  quartzites  also  are  thoroughly  recomposed  rocks,  without  recog- 
nizable clastic  particles.  Quartz  is  the  most  aljundant  con.stituent,  and  with 
it  muscovite,  biotite,  and  magnetite  are  constantly  associated.  The  micas 
and  the  magnetite  are  frequently  inclosed  in  a  background  of  large  inter- 
locking quartz  grains,  which  is  very  similar  to  the  background  of  the  Stur- 
geon quartzite.  Such  inclosures  lie  in  general  alignment  throughout  the 
thin  sections,  but,  unlike  many  of  the  inclusions  of  the  Sturgeon  quartzite, 
they  seem  not  to  be  clastic  particles  but  to  have  crystallized  in  place.  In 
one  slide  among  the  inclusions  in  the  large  quartzes  of  the  background  is  a 
colorless  isotropic  substance,  of  low  refraction,  occurring  in  large  polygonal 
areas,  but  without  definite  crystal  form.  It  is  usually  stained  with  limonite, 
which  has  penetrated  from  the  margins  along  straight  lines,  as  if  following 
cleavages.  This  interesting  mineral,  which  is  certainly  not  garnet,  and 
probably  not  opal,  deserves  further  investigation 

The  rocks  of  the  upper  series,  like  those  of  the  lower  series,  are  greatly 
altered.  From  their  mineralogical  composition  and  structure  it  is  evident 
that  as  originally  deposited  they  consisted  of  beds  of  mud  separated  by 
thinner  beds  of  sand.  But  as  they  now  stand  they  have  been  as  greatly 
changed  from  their  original  condition  as  tlie  bedded  rocks  below.     Also, 


426  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

since  the  time  of  inetamorphi.sm  they  liave  been  subjected  to  stress,  as  is 
clearly  shown  by  the  optically  strained  condition  of  the  secondary  quartz 
grains  and  the  bending-  and  twisting  of  the  micas. 

From  these  facts  we  may  reasonabl}^  infer  that  the  general  metamor- 
phism  of  both  series  was  accomplished  after  the  deposition  of  the  upper 
series  and  before  the  latter  was  folded.  Reconstruction  so  complete  as 
that  shown  by  the  upper  series  is  not  believed  to  take  place  except  at  con- 
siderable depths  below  the  surface,  and  hence  the  part  of  the  upper  series 
now  visible  must  then  have  been  deeply  covered  by  overlying  rocks,  which 
were  afterwards  entirely  swept  away  before  the  deposition  of  the  Cambrian. 
In  the  earth  movements  which  folded  this  mass  of  material  and  brought  it 
up  within  the  reach  of  denuding  agents,  we  may  recognize  the  causes  which 
have  strained  and  broken  the  secondary  minerals  of  both  series  alike. 

SECTIO]y  IX.    TlIK  IXTRITSIVES. 

The  Alffonkian  formations  of  the  Felch  Mountain  area  have  been  cut 
by  later  intrusives,  among  which  both  acid  and  basic  rocks  are  represented. 
The  latter  have  also  been  recognized  in  the  Archean,  in  which,  indeed,  the 
freshest  and  least-altei-ed  occurrences  have  been  found. 

The  acid  rocks  consist  of  fine-  to  medium-grained  pink  granites, 
occurring  in  narrow  dikes.  A  number  of  these  dikes  have  been  found  in 
the  Stvu-geon  formation,  both  in  the  area  of  tine  exposure  on  the  south  side 
of  sec.  3.5,  T.  42  N.,  R.  30  W.,  and  also  in  sees.  34  and  35,  T.  42  N.,  R.  29  W. 

Two  o-ranite  dikes  are  also  known  in  the  highest  member  of  the  lower 
series,  but  none  have  been  detected  in  the  Randville  or  Mansfield  forma- 
tions. One  of  these  occurs  on  Felch  Moinitain,  the  other,  a  very  coarse 
pegmatite,  is  found  cutting  tlie  Groveland  formation  in  the  southern  part  of 
sec.  33,  T.  42  N.,  R.  28  W. 

Basic  dikes  and  intrusive. sheets  are  found  in  many  localities.  Some 
are  highly  schistose  and  greatly  altered,  others  are  massive  and  but  little 
changed.  They  probably  belong  to  many  eras  of  eruption.  The  least 
altered  are  diabases,  in  one  occurrence  of  which,  from  the  Archean,  the 
aus'Ites  are  almost  intact. 


CHAPTER   IV. 
THE    MICHIGAMME    MOUNTAIN  AND  FENCE   RIVER  AREAS. 

By  reference  to  the  general  map,  PI.  Ill,  it  will  be  seen  that  an  oval- 
shaped  Archean  area,  about  11  miles  long  from  northwest  to  southeast  and 
liaAing  an  extreme  breadtli  of  nearly  4  miles,  runs  through  portions  of 
Ts  44,  45,  and  46  N.,  Rs.  31  and  32  W.  The  country  to  he  described  in 
the  present  chapter  includes  that  portion  of  this  Archean  mass  (together 
with  the  younger  rocks  on  its  eastern  border)  which  lies  east  of  the  line 
between  Ranges  31  and  32  W.,  as  well  as  the  territory  to  the  south  in  the 
prolongation  of  the  axial  line,  as  far  as  the  south  line  of  T.  43  N.,  R.  31  W. 
A  gap  about  6  miles  broad  not  covered  by  oui'  work  intervenes  between 
this  arbitrary  southern  boundary  and  the  western  termination  of  the  Felch 
Mountain  work  at  Randville. 

In  the  northern  portion  of  the  area  now  under  consideration  (which 
lies  along  and  is  twice  crossed  by  the  Fence  River)  the  geological  structure 
is  exceedingly  simple,  while  in  the  southern  portion,  especially  in  the  neigh- 
borhood of  Michigamme  Mountain,  it  is  rather  complex.  The  boundary 
between  these  two  divisions  falls  in  the  neighborhood  of  the  mouth  of  the 
Fence  River  in  sec.  22,  T.  44  N.,  R.  31  W.  It  is  therefore  convenient 
in  what  follows  to  refer  to  the  northern  portion  as  the  Fence  River  area, 
and  to  the  southern  as  the  Micliigamme  Mountain  area. 

By  referring  to  PI.  Ill,  the  broad  geological  structure  of  the  whole 
territory  of  which  the  above-mentioned  Archean  oval  is  the  center  is  evi- 
dent at  a  glance.  It  is  an  anticlinal  dome,  the  core  of  which  is  Archean, 
around  which  the  younger  Algoukian  formations  run  in  a  series  of  concen- 
tric rings,  on  all  sides  dipping  outward  from  the  inner  nucleus.  In  the 
Fence  River  area,  on  the  eastern  long  side  of  the  dome,  the  Algonkian 
formations  have  a  constant  eastward  dip,  and  are  free  from  important 
secondary  folds.  In  the  Michigamme  Mountain  area,  how^ever,  which  lies 
in  the  prolongation  of  the  main  axis  of  the  dome,  these  encircling  forma- 


428  THE  CRYSTAL  FALLS  lEON-BEAEING  DISTRICT. 

tious  fall  away  gently  to  the  south  in  a  series  of  waves,  produced  by 
several  concentric  minor  folds  transverse  to  the  main  axis.  Of  these  minor 
folds  but  one  is  at  all  distinct  to  the  east  of  the  general  anticlinal  axis, 
while  to  the  west  of  this  axis  at  least  three  are  well  made  out  within  the 
Michigarame  Mountain  area.  The  much  greater  bi'eadth  of  the  Algonkian 
formations  on  the  west  side  of  the  dome  than  on  the  east  is  probably  due 
to  the  persistence  of  these  minor  folds  toward  the  northwest. 

The  general  character  and  aspect  of  the  formations  of  the  two  areas 
and  their  succession  is  iii  so  many  respects  identical  with  the  formations  of 
the  Felch  Mountain  range  that  no  doubt  can  be  entertained  that  they  are 
really  the  same  formations.  Nevertheless  certain  differences  mark  these 
rocks  with  a  distinct  individuality.  These  differences  will  be  considered  in 
detail  in  the  descriptions  of  the  several  formations.  In  general  they  may 
be  summarized  as  involving  a  great  reduction  in  thickness  of  the  Sturgeon 
formation,  with  a  corresponding  increase  in  the  Randville  dolomite,  the 
appearance  of  surface  igneous  rocks  at  the  Mansfield  horizon  in  the  Fence 
River  area,  and  a  less  uniform  and  complete  metamorphism  in  the  whole 
Algonkian  series. 

SECTION  I.    THE  ARCHEAN. 

The  rocks  of  the  Archean  core  are  well  exposed  through  the  west- 
central  sections  of  T.  44  N.,  R.  31  W.,  while  farther  north  in  T.  45  N.,  R.  31 
W.,  outcrops  are  few  and  scattered.  Much  less  attention  was  paid  to  this 
area  than  to  the  Felch  Mountain  Archean;  our  work,  as  a  rule,  stopped 
with  the  location  of  the  boundary,  and,  therefore,  the  following  brief  state- 
ments as  to  its  character  embody  observations  along  the  southern  and  east- 
em  margins  only. 

The  prevalent  rock  in  the  Archean  is  granite,  varying  from  medium  to 
coarse  grain,  and  often  carrying  very  large  porphyritic  Carlsbad  twins  of 
flesh-colored  microcline.  Banded  gneisses  and  mica-gneisses  and  mica- 
schists,  such  as  are  so  abundant  in  the  Felch  Mountain  Archean,  are  rare  but 
not  entirely  absent.  While  in  many  localities  the  granites  are  much  crushed 
and  even  sheeted  along  adjacent  parallel  fractures,  their  original!}^  massive 
character  is  sufficiently  evident.  They  have  the  composition  and  structure 
of  typical  igneous  granites.  The  primary  minerals  are  entirely  without 
definite  arrangement. 


AUCIIEAN  OF  MlCeiGAMME  MOUNTAIN  AREA.  429 

111  tlR'  Arcliean  areas  p-raiiites  of  two  af;es  Imv'e  been  found,  the 
younger  in  tlie  form  of  narrow  dikes.  Basic  igneous  rocks,  also  in  dike 
form,  are  rather  abundant.  One  of  these  under  the  microscope  proves  to 
l)e  a  but  little  altered  diabase,  in  which  the  augite  is  almost  intact.  These 
acid  and  basic  intrusions  are  i)rol)ably  connected  with  tlie  surface  flows  of 
like  character  which  are  so  abundant  at  the  ]\Ian.siield  horizon  along  the 
Fence  River. 

Of  mucli  interest  is  the  occurrence  of  a  small  mass  of  quartz-porphyry 
in  contact  with  the  Archean,  and  below  the  lowest  Algonkian  sedimentary 
formation.  The  locality  is  in  sec.  21,  T.  44  N.,  R.  31  W.,  in  the  southeast 
quadrant  of  the  Archean  oval.  The  upper  surface  of  contact  of  this  sheet 
with  the  lowest  sediments  is  covered,  and  hence  it  is  not  entirely  certain 
whether  it  is  intrusive  or  extrusive,  and  therefore  whether  it  belong-s  to 
Archean  or  Algonkian  time.  The  general  relations,  however,  appear  to 
indicate  that  it  is  a  surface  flow  wdiich  suffered  erosion  before  the  deposition 
of  the  ]:)asal  Algonkian  member,  and  is  tlierefore  to  be  classed  with  the 
Archean.  The  exposure  is  250  feet  long  by  100  broad.  The  rock  consists 
of  a  very  finely  granular  matrix  of  a  warm  gray  color,  through  which 
are  sprinkled  quite  uniformly  little  grains  of  blue  quartz,  and  larger  rounded 
grains  of  pink  feldspar.  Flakes  of  biotite  are  scattered  through  the  ground- 
mass  and  coat  the  cleavage  surfaces,  which  are  developed  in  two  distinct 
systems,  intersecting  at  an  angle  of  about  10°.  Immediately  below  the 
porphyry  is  coarse  porphyritic  granite,  sheeted  in  waving  surfaces  parallel 
to  the  contact,  wdiich  dips  eastward  about  40°.  The  lower  portion  of  the 
porphyry  contains  a  number  of  fragments  of  the  underlying  granite,  one  of 
which  is  over  4  feet  in  leuafth. 

Under  the  microscope  the  groundmass  is  a  line-grained  crystalline 
aggregate  of  quartz,  greenish  biotite,  and  a  little  feldspar.  The  quartz 
phenocrysts  are  beautifully  corroded,  and  have  the  characteristic  bipyrami- 
dal  iovm,  while  the  feldspars  are  extensively  altered  to  biotite,  sericite,  and 
quartz. 

Biotite-gneisses  related  to  this  porphyry  in  external  appearance  occur 
among  the  Archean  outcrops  inclosed  in  the  "B"  line  of  magnetic  attraction 
in  sec.  7,  T.  45  N.,  R.  30  W.,  and  may  be  described  here  for  comparison. 
They  are  dark-colored,  fine-grained  rocks,  which  weather  to  light  pink. 
They  are  eminently  schistose,  and  the  cleavage  surfaces  are  coated  with 


430  THE  CRYSTAL  FALLS  IRON-BE ARIXG  DISTRICT. 

biotite  plates  of  medium  size.     Minute  grains  of  blue  quartz  are  occasionally 
distingiiisliable  by  the  eye. 

Under  the  microscope  these  gneisses  have  a  fine  to  medium  grained 
groundmass  composed  of  quartz,  microcline,  orthoclase,  plagioclase,  green 
biotite,  and  nuiscovite,  and  a  little  scattered  epidote.  Within  it  are  large 
roundish  areas  of  quartz  and  feldspar,  sometimes  single  individuals,  but  more 
often  consisting  of  several  fragments.  The  gneissic  foliation  is  pronounced 
and  is  caused  by  a  general  elongation  of  the  constituent  minerals  in  a 
common  direction.  The  onl}"  essential  differences  between  these  gneisses 
and  the  porphyries  described  above  are  this  strong  foliation  and  the  coarser 
groundmass. 

SECTION  II.    THE   STURGEOK  FORMATION. 

The  Sturgeon  formation  as  a  distinct  member  of  the  Algonkiaii  series 
is  hardly  known  in  this  area  apart  from  the  Randville  formation.  Neverthe- 
less, purely  clastic  sediments  unmixed  with  the  carbonates  of  calcium  and 
magnesium  were  deposited  and  are  now  visible  along  one  section  between 
the  Archean  granites  below  and  the  dolomites  above,  and  for  these  it  is 
convenient  to  retain  the  name,  although  their  total  thickness  is  so  small 
and  their  continuity  so  uncertain  that  they  can  not  be  shown  on  the  geo- 
logical map.  The  general  conditions  of  sedimentation  here  were  such, 
perhaps  in  consequence  of  the  low  relief  of  the  neighboring  land,  that  lime- 
stones began  to  form  a  relatively  short  time  after  the  submergence  of  the 
Archean  surface,  so  that  the  two  lower  Algonkian  formations  probably  by  no 
means  represent  equal  periods  of  time  with  the  same  formations  in  the  Felch 
Mountain  range.  The  time  represented  by  both  together  is  perhaps  not 
g-reatly  different  in  the  two  areas,  but  since  in  the  entire  absence  of  fossil 
evidence  it  is  impossible  to  draw  the  line  of  equivalence,  while  at  the  same 
time  the  lithological  break  is  a  shai'p  one,  it  seems  desirable  to  carry  over 
the  Felch  Mountain  names,  extending  the  Randville  dolomite  downward  to 
the  lower  limit  of  limestone  deposition,  and  retaining  the  name  Sturgeon 
formation  for  the  basal  sediments  which  are  free  from  carbonates. 

These  basal  sediments  are  found  only  in  sec.  15,  T.  44  N.,  R.  31  W., 
where  they  are  exposed  in  low-lying  outcrops  in  the  banks  and  bed  of  the 
Fence  River.  Elsewhere  throughout  the  10  or  12  miles  through  which  the 
Archean  extends  in  this  area  no  outcrops  have  been  found  in  the  flat  and 
generally  swampy  belt  which  intervenes  between  it  and  the  dolomite  above. 


KANDVILLE  DOLOMITE  IN  FENCE  RIVER  AREA.  431 

The  expofsuix's  ret'cnxMl  to  consist  of  soft,  light-weathering  slates  and 
gi'aywackes,  with  which  are  interbedded  layers  of  coarser  texture.  They 
are  very  evenly  banded  in  pale  shades  of  yellow,  red,  and  green,  and  the 
structure  thus  brought  out  dips  eastward  at  an  angle  of  52°.  Besides  this 
a  secondary  cleavage  is  quite  prominent,  especially  in  the  finer-grained 
beds,  also  dipping  eastward,  but  at  a  considerably  higher  angle.  At  the 
eastern  side  the  slates  are  overlain  by  the  lowest  marble  beds,  here 
extremely  impure  and  highly  charged  with  chlorite  and  quartz  sand.  The 
thickness  of  slates  exposed  is  about  100  feet,  and  between  the  Archean  and 
the  most  western  outcrops  there  is  room  for  about  as  much  more.  The 
total  thickness,  then,  can  not  exceed  200  feet. 

A  thin  section  of  a  specimen  from  one  of  the  coarser  layers  shows  it 
to  be  a  graywacke,  the  most  prominent  constituent  of  which  is  qi;artz  in 
small  roundish  and  oval  grains.  These  are  embedded  in  a  groundmass 
composed  of  chlorite  in  minute  irregular  plates,  ferric  oxide,  and  kaolin. 
The  quartz  grains  while  having  generally  clastic  shapes  are  bounded  by 
minutely  rough  edges  which  interlock  with  the  fibrous  minerals  of  the 
groundmass.  Evidently  much  new  quartz  has  been  deposited  round  the 
original  grains. 

SECTION  III.     THE  BANDVILLE   DOLOMITE. 

DISTRIBUTION    AND    EXPOSURES. 

In  the  Fence  River  area  the  dolomite,  as  already  stated,  lies  on  the 
east  side  of  the  Archean,  and  occupies  a  belt  over  half  a  mile  in  width, 
which  extends  from  the  mouth  of  the  Fence  River  on  the  south  for  about 
10  miles  to  the  north  and  west,  to  our  western  boundary  near  the  north- 
west corner  of  T.  45  N.,  R.  31  W.  lu  this  distance  it  is  twice  crossed  by 
the  river,  and  on  these  natural  sections  and  in  their  neighborhood  the  only 
known  outcrops  of  the  dolomite  have  been  found.  The  northern  river  sec- 
tion passes  through  sees.  22  and  28,  T.  45  N.,  R.  31  W.,  and  discloses  an 
excellent  series  of  closely  connected  exposures  for  a  distance  of  about 
2,900  feet,  measured  at  right  angles  to  the  strike.  The  southern  section  is 
5  miles  farther  south,  and  is  much  less  continuous,  laying  bare  the  extreme 
upper  and  lower  portions  only  of  the  formation.  Elsewhere  through  the 
dolomite  belt  the  rock  surface  is  concealed  by  swamps  or  glacial  drift^  to 
which  last  it  contributes  but  few  scattered  bowlders  of  noticeable  size. 


432  THE  CEYSTAL  FALLS  IRON-BEARING  DISTRICT. 

South  of  the  Archeaii  dome  in  the  Michigamme  Mountain  area  the 
dolomite  tops  the  hiw  arch  in  a  broad  crumpled  sheet,  in  the  minor  syn- 
clines  of  which  the  higher  formations  are  more  and  more  implicated  as  we 
go  south.  This  broad  sheet,  with  its  included  tongues  of  phyllite,  extends 
to  the  south  line  of  T.  44  N.,  R.  31  W.,  beyond  which  it  disappears  beneath 
the  hig-her  formations,  except  in  a  single  narrow  belt  which  continues  along 
the  main  axis  for  about  a  mile  farther  south.  Exposures  sufficient  in  num- 
ber to  indicate  several  minor  folds  are  found  along  the  Michigamme  River 
and  scattered  through  sees.  28,  32,  and  33,  T.  44  N.,  R.  31  W.,  and  sec.  4, 
T.  43  N.,  R.  31  W. 

FOLDING  AND  THICKNESS. 

In  attitude  the  Randville  formation  in  the  Fence  River  division  of  the 
district  is  an  eastward-dipping  monocline,  the  inclination  of  which  is  gen- 
erally moderate.  The  rocks  are  usually  heavily  bedded  and  nearly  always 
show  distinct  alternations  in  coarseness  and  color,  so  that  structural  obser- 
vations are  made  with  much  more  cei'tainty  than  in  the  Felch  Mountain 
range.  The  more  conspicuous  minerals  secondarily  developed  here — coarse 
carbonates  and  tremolite — have  formed  chiefly  in  the  old  planes  of  bedding. 
Oblique  structures  are  generally  absent  except  in  the  close  vicinity  of  the 
basic  dikes  which  intersect  the  formation  along  the  upper  river  section. 
The  surfaces  of  contact  with  the  dikes  stand  at  high  angles,  and  nearly 
parallel  to  these  the  neighboring  dolomite  has  well-developed  cleavages, 
along  which  new  minerals  have  formed,  intersecting  the  true  bedding.  It 
is  evident  that  the  stronger  igneous  rocks  in  these  cases  have  lurnished 
resistant  surfaces  against  which  the  dolomite  has  been  kneaded  in  the 
general  tilting  of  the  series. 

The  eastward-dipping  monocline  is  a  simple  one,  5^et  the  observed 
angles  of  inclination  are  by  no  means  uniform.  Thus,  along  the  upper  river 
section  the  dip  ranges  from  25°  to  60°,  with  40°  as  the  mean  of  about  a 
dozen  observations.  The  variable  dips  are  so  scattered  through  the  cross 
section  as  to  indicate  no  widespread  roll  in  the  formation  as  a  whole,  but 
rather  a  great  number  of  minor  undulation's  pi'obably  distributed  through- 
out its  thickness.  Such  undulations  are  visible  in  favorable  localities,  as, 
for  example,  on  the  north  bank  of  the  river  in  the  NW.  ^  of  NW.  4»  sec. 
28,  T.  45  N.,  R.  31  W.,  where  fresh  surfaces  have  been  exposed  in  blasting 


FOLDING  AND  THICKNESS  OF  KANDVILLE  DOLOMITE.         433 

for  tlie  dam.  The  light-blue  and  pearly-white  layers  of  tlie  beautiful  mar- 
ble here  seen  are  thrown  into  a  series  of  unsymmetrical  folds.  The  western 
sides  of  the  little  anticlinals  are  short  and  overturned,  while  the  eastern 
sides  are  long  and  gently  inclined.  Evidently,  if  the  same  system  of  sec- 
ondary folding  holds  throughout  the  entire  thickness  of  the  formation,  sur- 
face observations  would  show  everywhere  eastward  dips  at  A'ariable  angles, 
dependent  upon  the  portion  of  the  fold  which  hapi^ened  to  constitute  the 
particular  outcrop,  and  gentle  dips  would  be  more  abundant  than  steep  dips. 
This  would  completely  explain  the  observed  variations. 

Similar  variations  and  lack  of  regular  sequence  in  the  dips  are  found 
in  the  southern  river  section.  Five  good  observations  range  between  20° 
and  58°,  all  eastward,  but  none  of  the  exposures  is  sufficiently  extensive  to 
show  minor  folds.     The  mean  of  these  observations  is  about  40°. 

The  surface  width  of  the  dolomite  zone  on  each  section  is  a  little  less 
than  3,000  feet,  assuming  that  a  fair  proportion  of  the  covered  zones  on 
each  side  is  underlain  by  the  same  formation.  If  the  average  observed  dip 
is  taken  to  represent  the  average  dip  of  the  rock,  the  thickness  in  each 
case  would  be  a  little  over  1,900  feet.  This  is  probably  too  great,  and  is 
certainly  too  great  if  the  same  kind  of  internal  crumpling  visible  in  parts 
of  the  upper  river  section  is  characteristic  of  the  formation  throughout. 
The  average  dip  evidently  would  more  nearly  be  represented  by  the  dips 
of  the  long  eastern  limbs  of  the  little  anticlines.  Assuming  that  these  are 
less  than  the  mean,  we  find  the  average  of  the  dips  below  40°  to  be  30°  for 
each  section.  This  gives  a  thickness  of  about  1,500  feet,  which  still  is  per- 
haps beyond  the  truth,  but  is  probaljly  much  nearer  it  than  the  first  value. 

It  is  interesting  to  compare  this  result  with  the  thickness  of  500-1,000 
feet  obtained  on  the  two  Felch  Mountain  sections.  A  part  of  the  increase 
is  probably  due,  as  already  explained,  to  the  earlier  beginning  of  limestone 
deposition  in  the  Michigamme  area.  But  an  important  part  of  it  is  pi'ob- 
ably  not  depositional  at  all,  but  is  the  result  of  plications.  The  whole 
series  here  is  but  gently  tilted  as  compared  with  the  walls  of  the  Felch 
Mountain  trough,  and  hence  the  strong  horizontal  pressures  have  acted 
in  a  direction  but  slightly  inclined  to  the  bedding.  The  result  has  been 
the  secondary  crumpling  within  the  formation  which  must  contribute  in  an 
important  degree  to  its  present  apparent  thickness. 

In  the .  scattered    outcrops   of  the    Michigamme    Mountain    area   the 

MON  XXXVI 28 


434  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

dolomite  strikes  and  dips  toward  all  points  of  the  compass.  This  irregularity 
is  cansed  by  the  gentle  arching  over  the  general  northwest-southeast  axis, 
combined  with  much  sharper  local  folding  about  a  series  of  axes  which  run 
more  nearly  east  and  west.  The  l)est-defined  east-aud-west  folds  occur 
west  of  the  main  axis  in  sec.  32,  T.  44  N.,  R.  31  W.,  in  which  three  syn- 
clines  and  three  anticlines  are  found  along  a  north-and-south  section  4,000 
feet  long.  Tlie  two  southern  syuelines  are  sufficiently  deep  to  include  the 
overlying  Mansfield  phyllites.  The  secondary  folds  die  out  toward  the 
main  nortli-and-south  axis  and  bi'oadeu  toward  the  west.  East  of  the 
main  axis  Ijut  one  secondary*  fold  has  been  recognized,  namely,  the  syncline 
which  forms  Michigamme  Mountain.  This  is  tlie  deepest  of  the  secondary 
folds,  and  the  only  one  containing  the  Groveland  formation. 

PETROGRAPHICAL  CHARACTERS. 

The  Randville  formation  in  this  area  is  richer  in  litliological  varieties 
than  in  the  Felch  JMountain  range.  As  originally  deposited,  a  much  larger 
proportion  of  sand  and  mud  was  mingled  with  the  carlionates,  and  tlie  prog- 
ress of  subsequent  metamorphism  also  has  been  less  uniform.  Depending 
upon  the  interaction  of  these  two  factors,  we  find,  as  the  extremes  of  variation, 
on  the  one  liand  coarse  saccharoidal  marbles,  sometimes  very  pure,  but 
most  often  filled  with  secondary- silicates,  and  on  the  other  hand  fine-grained 
little-altered  limestones,  which  occasionally  are  ho  impure  as  to  be  rather 
calcareous  or  dolomitic  sandstones  and  shales.  The  more  impure  varieties 
occur,  as  might  be  expected,  near  the  contacts  with  the  adjacent  formations. 

On  the  Fence  River,  in  sec.  16,  T.  44  N.,  R.  31  W.,  the  base  of  the 
dolomite  rests  on  the  Sturgeon  formation.  The  rock  is  filled  with  grains  of 
quartz  and  feldspar  and  scales  of  chlorite,  and  is  so  soft  that  it  ma}^  be 
crushed  between  the  fingers.  In  sec.  32,  T.  44  N.,  R.  31  W.,  the  toj)  of  the 
formation  is  in  contact  with  the  Mansfield  slates,  and  between  them  is  a  com- 
plete sei'ies  of  transition  beds.  Near  the  junction  the  limestone  becomes  dark 
colored  and  contains  thin  bands  in  which  the  clayej'  material  greatly  exceeds 
the  carbonates.  These  are  succeeded  by  alternating  beds  of  slate  and  impure 
limestone  in  nearly  equal  volume,  and  it  is  only  high  up  in  the  slate  member 
that  the  calcareous  bands  completely  disappear.  Apart  from  these  belts  of 
extreme  impurity  at  the  base  and  top  of  the  formation,  the  presence  of  scat- 
tered fragmeutal  grains  of  quartz  and  feldspar  is  j-ather  general  tln-oughout. 

The  prevalent  colors  are  white,  various  shades  of  pink,  both  light  and 


PBTKOGKAPUIOAL  CUAUACTKKS  OF  KANDVILLE  D(JLOMITE.      435 

(U't-p  l)hu',  iuifl  pale  green.  Where  weathered,  the  u.sual  colons  are  light 
brown  or  buff.  The  lighter-colored  rocks  in  general  are  chai-acteristic  of 
the  Fence  River  area  where  metaniorphisni  is  more  uniform  and  more 
intense,  and  the  (hirker  colors  of  tlie  Michigamme  Mountain  area  to  which 
the  less  crystalline  forms  are  wholly  contined.  Bands  differently  colored 
are  nearly  always  present  in  the  same  outcrop. 

In  the  Michigamme  jMountain  area  the  torsional  strains  attendant  upon 
the  formation  of  folds  in  two  directions  have  developed  two  systems  of  frac- 
ture in  the  dolomite.  In  these  secondary  quartz  has  formed,  occasionally 
in  large  amount.  Of  nuich  interest  is  the  occurrence  in  close  connection 
witli  such  vein  quartz  of  occasional  thin  bauds  of  pegmatite,  doubtless  aris- 
ing from  the  action  of  deeply  derived '  waters.  In  similar  spaces  coarse 
secondary  carbonates,  tremolite,  and  oxides  of  iron  also  have  commonly 
formed.  Over  the  small  anticlinal  axes  and  domes  of  this  area  the  orieinal 
bands  of  the  rock  have  often  been  shattered,  and  are  now  recognizable  only  in 
displaced  fragments  cemented  together  b}'  the  new  minerals.  In  the  Fence 
Ri^'er  area  the  general  secondary  folding  has  been  attended  Avith  differential 
movements  along  the  bedding,  which  left  narrow  open  spaces  where  the 
adjacent  surfaces  failed  to  fit  in  their  final  position  of  rest.  These  sjiaces  are 
now  indicated  by  coarsely  crystalline  carbonates  and  silicates  arranged  nor- 
mal to  the  original  walls.  Where  the  space  was  a  wide  one  the  outer  walls 
are  usually  lined  with  coarse  calcite,  while  the  interior  is  filled  with  quartz. 

In  the  Michigamme  Mountain  area  certain  pink  bands  of  the  dolomite 
have  a  beautiful  oolitic  texture,  which  is  most  clearly  brougjit  out  in  Aveath- 
ering  by  the  geometrical  regularity  of  distribution  of  the  harder  shells  or 
cores  of  the  little  rounded  grains.  The  forms  are  not  different  from  and  are 
quite  as  distinct  as  those  in  the  oolitic  limestones  of  recent  deposition. 

The  chemical  composition  of  the  dolomites  is  illu.strated  by  the  follow- 
ing partial  analyses  by  Mr.  R.  J.  Forsythe,  of  Harvard  University: 

'  Analyses  of  dolomites  frovi  Michigamme  Mountain  area. 


I. 

II. 

III. 

14.25 
11.15 

47.18 
18.  48 

9.34 

.  12.  57 

45.98 

•19.22 

Al2(Fe..)03 

5.38 
36.60 

16.38 

CaCOj 

MgCOs 

436  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

The  ratio  of  CaCOg:  MgCOg  is  too  great  for  normal  dolomite,  but 
approximates  that  for  2CaC03+MgC03. 

Under  the  microscope  the  chief  differences  in  the  various  thin  sections 
are  in  the  degree  of  metamorphism  and  in  the  quantity  and  character  of  the 
foreign  fragments.  The  least  altered  varieties  are  those  highest  in  the  series 
from  the  Michigamme  Mountain  area.  These  consist  of  a  background  of 
extremely  fine-grained  calcite,  with  a  few  rounded  fragmental  quartz  grains, 
and  scattered  particles  of  chalcedony.  Mixtures  of  small  quartz  particles, 
chalcedony,  and  calcite  slightly  coarser  than  the  background  occur  in  short 
vein-like  gashes.  The  prevalent  deep  color  of  these  rocks  is  due  to  the  even 
sprinkling  through  the  background  of  a  black  opaque  pigment,  which  may 
be  carbonaceous.  Altogether  the  microscopic  characters  are  those  of  a 
little-altered,  slightly  cherty  limestone. 

The  more  crystalline  varieties  of  the  dolomite  contain  several  secondary 
minerals,  namely,  tremolite,  diopside,  chlorite,  muscovite,  phlogopite,  pyrite, 
and  the  oxides  of  iron.  Of  these,  tremolite  is  very  common  and  abundant, 
especially  in  the  Fence  River  area,  where  the  rarer  pyroxene,  diopside,  also 
is  found.  Phlogopite  comes  in  but  two  of  the  thin  sections,  while  muscovite 
occurs  in  nearly  all.  The  general  habit  of  these  siHcates  is  precisely  the 
same  as  in  the  dolomite  of  the  Felcli  ]\Iountain  range.  They  are  developed 
pari  passii  with  the  passage  of  the  unaltered  dolomite  into  marble. 

The  fragmental  inclusions  within  the  dolomite  are  of  interest.  These 
are  little  pebbles  of  quartz,  feldspar,  mica,  titanite,  magnetite,  and  augite; 
and  are  evidently  derived  mainly  from  preexisting  granites  or  gneisses. 
Titanite  and  augite  are  very  i-are;  the  others  are  represented  in  almost 
every  slide.  The  quartz  grains  are  seldom  more  than  a  millimeter  in  diam- 
eter and  commonly  are  much  smaller.  While  the  general  shape  is  oval  or 
rounded  in  most  cases,  the  perimeters  are  usually  extremely  irregular  and 
interlock  with  the  carbonate  grains  of  the  Ijackground,  which  indicates  that 
they  have  been  enlarged  since  deposition  by  the  formation  of  new  silica. 
This  is  very  evident  in  the  few  instances  in  which  the  original  smooth  out- 
line, or  part  of  it,  is  preserved  by  a  film  of  difi"erent  material  inside  the 
present  perimeter.  The  feldspar  pebbles  include  orthoclase,  microcline,  and 
plagioclase,  nncrocline  being  the  common  species.  They  are  usually  much 
decomposed  and  iron  stained.  The  feldspars  are  especially  abundant  in  the 
slides  from  the  Fence  River  area. 


PETIUKJRAI'HICAL  GHAKAGTKRS  OF  UANDVILLE  DOLOMITE.      437 

Tho  clastic  |)cl)l)les  give  us  striking'  proof  of  the  general  and  severe 
internal  strains  suffered  by  the  dolomite,  the  etfects  of  which  have  healed 
over  without  a  scar  in  the  carbonate  matrix.  The  pebbles  are  always 
Optically  strained.  Very  often  they  are  fractured  and  the  parts  separated, 
and  sometimes  they  have  been  reduced  to  small  fragments.  In  tliese  cases 
the  breaks  have  been  completely  healed  by  the  flow  or  redeposition  of  the 
Si'roundmass  in  the  interstices.  These  effects  are  foitnd  in  g-reater  or  less 
degree  in  every  thin  section. 

The  oolitic  varieties  are  very  interesting  under  the  microscope.  They 
consist  of  little  oval  or  round  areas,  averaging  2  mm.  in  diameter,  packed 
together  as  closely  as  possjble.  Each  oval  consists  of  a  single  or  compound 
nucleus,  surrounded  by  several  thin  and  very  even  concentric  layers.  The 
nucleus  in  a  few  cases  is  a  single  roundish  quartz  individual,  evidently  a 
clastic  grain.  In  most  cases,  however,  it  is  composed  of  a  great  number  of 
minute  quartz  grains,  or  of  several  coarse  calcite  grains,  with  films  of  iron 
oxide  between.  The  arrangement  of  these  separate  quartz  and  calcite  indi- 
viduals is  such  as  to  indicate  that  they  have  filled  interior  cavities.  The 
surrounding  thin  layers  are  calcite  in  all  cases.  Sometimes  two  adjoining 
nuclei,  each  within  its  own  rim  of  several  layers,  are  together  included 
within  a  common  series  of  shells.  In  one  such  case  the  outside  rim  trav- 
ersed the  edges  of  the  rings  surrounding  one  of  the  nuclei  with  a  decided 
unconformity,  as  if  the  latter  had  been  eroded  before  the  deposition  of  the 
former.  The  oolitic  structure,  I  believe,  has  not  hitherto  been  noted  in 
limestones  of  undoubted  pre-Cambrian  age. 

SECTION  IV.     THE  MAIVSFIELD   FORMATION. 

The  typical  locality  of  the  Man.sfield  formation  is  the  Michigamme 
River  valley  in  the  vicinity  of  the  Mansfield  mine,  which  lies  a  mile  west 
of  the  border  of  my  field  of  work,  and  is  described  by  Mr.  Clements.  The 
same  formation,  however,  is  present  in  the  Michigamme  Mountain  area, 
where  its  relations  to  the  adjacent  formations  are  clearly  defined.  In  the 
Fence  River  area  rocks  of  very  different  character  and  derivation  occur  at 
the  Mansfield  horizon.  These  occur  in  typical  development  to  the  west,  on 
the  Hemlock  River,  and  are  hence  called  the  Hemlock  formation. 


438  THE  CRYSTAL  FALLS  IKON-BEARING  DISTRICT. 

DISTRIBUTION,   EXPOSURES,  AND  TOPOGRAPHY. 

The  Mansfield  rocks  of  the  Michigamme  Mountain  area  consist  of 
phylHtes  or  mica-slates  of  various  colors.  They  are  found  in  the  series  of 
east-west  synclines,  which  have  already  been  described  in  connection  with 
the  Randville  formation.  The  best  exposures  occur  in  sec.  32,  T.  44  N., 
R.  31  W.,  between  the  center  and  the  west  quarter  post,  and  still  farther 
north  along-  the  south  bank  of  the  ^lichigamme  in  the  northwest  quarter  of 
the  section.  The}"  are  also  found  round  the  western  edge  of  the  Michi- 
gamme Mountain  syncline  in  sec.  33,  T.  44  N.,  R.  31  W.,  and  in  sec.  5, 
T.  43  N.,  R.  31  W.,  but  here  the  exposvu-es  are  mainly  in  test  pits.  Test 
pits  have  likewise  penetrated  them  in  sec.  10,  T.  43  N.,  R.  31,  where  they 
succeed  the  dolomites  as  the  surface  rock  over  the  general  arch.  Their 
extent  iu  the  covered  portions  of  this  area  is  probably  considerable,  but 
the  structure  is  so  complex  and  the  outcrops  so  few  as  to  forbid  any  but  the 
most  approximate  outlining  of  their  general  boundaries. 

The  geological  position  of  the  Mansfield  rocks  is  free  from  doubt.  In 
the  princii)al  syncline  of  section  32  they  are  seen  to  overlie  the  dolomites 
and  to  pass  downward  into  them  by  a  relatively  slow  gradation,  while  on 
the  Ijorders  of  the  Michigamme  ^Mountain  syncline  they  are  jJi'oved  to 
underlie  the  Groveland  formation.  The  passage  to  the  higher  formation 
likewise  is  graded,  though  more  rapidly,  and  is  marked  in  certain  bands  by 
an  increase  in  clastic  quartz  grains  and  by  changes  in  the  character  of  the 
matrix  in  which  these  are  set. 

The  portions  of  the  surface  underlain  by  the  Mansfield  formation  are 
without  special  features,  and  are  indistinguishable  topographically  in  the 
gently  rolling  plain,  the  greater  portion  of  which  is  formed  in  the  dolomites. 
In  section  32  the  outcrops  are  miniature  ridges  elongated  with  the  strike, 
the  height  of  which,  however,  is  less  than  the  contour  interval  of  the  map. 

FOLDING    AND    THICKNESS. 

The  folding  of  the  Mansfield  rocks,  so  far  as  it  can  be  determined 
in  this  area,  has  already  been  described  in  the  account  of  the  preceding 
formation,  which  they  overlie.  The  rocks  are  known  only  in  the  sec- 
ondary synclines  winch  lie  transverse  to  the  general  direction  of  the 
main  axis  south  of  the  Michigamme  River.  In  the  southern  of  these  syn- 
clines, in  sec.  32,  T.  44  N.,  11.  31  W.,  between  the  limestone  rims  on  the 


FOLDING  AND  T[IICKNESS  ()1<    MANSFIELD  FORMATION.        439 

north  ;in<l  soutli,  a  .superfiriiil  widtli  of  iiltout  1,S()()  feet  of  pliyllitcs  is 
exjjoseil.  The  most  southern  exposures  (lip  northward  at  a  low  angle.  On 
the  northern  rim  the  true  beddinj;-  is  nearly  \-ertical.  P]lsewhere  the  ver- 
tical cleavage  structure  alone  is  distinguishable.  The  ixpper  limit  of  tlie 
formation  is  not  found  in  this  syncline.  Making  the  iii()>t  liberal  estimate 
for  ])ossible  minor  crumples,  it  is  improbable  that  a  less  thickness  than  300 
to  400  feet  occurs  here.  On  the  eastern  side  of  the  main  axis  the  phyllites 
below  the  Groveland  formation  are  very  mvicli  thinner  than  this,  the  thick- 
ness at  the  Interrange  exploration,  for  example,  being  only  about  100  feet; 
but  there,  as  well  as  along  the  whole  western  edge  of  the  Michigamme 
Mouulaiu  syncline,  the  lower  contact  with  the  dolomite  is  ])robably  faulted. 
It  seems  entirely  safe,  therefore,  to  place  the  average  thickness  of  the 
Mansfield  formation  in  the  Michigamme  Mountain  area  ;it  not  less  than 
400  feet. 

PETROGRAPHICAL  CHARACTERS. 

The  ]\Iansfiield  formation  consists  almost  entirely  of  ^'ery  tine  grained 
mica-slates  or  phyllites.  The  prevailing  colors  are  dark  green,  black,  and 
light  olive-green.  These  are  often  mottled  irregularly  with  red,  due  to  the 
infiltration  of  iron  oxides  along  the  secondary  cleavages.  The  cleavage 
surfaces  have  a  dull  luster,  caused  by  the  ])arallelism  of  the  micaceous 
minerals,  which  are  too  minute,  however,  to  be  distinguished  by  the  eye  or 
lens. 

The  phyllites  are  often  finely  banded  in  different  colors  and  shades. 
Near  the  base  of  the  formation  bands  of  limestone  and  near  the  toj)  thin 
bands  of  graywacke  are  interbedded,  as  has  already  been  stated.  Quartz 
and  calcite  lenses  are  not  unusual  in  the  minutely  puckered  portions  of  the 
formation. 

The  secondary  cleavage  is  the  jjrominent  structure,  and,  indeed,  the 
only  structure  of  the  outcrops  where  the  color  and  texture  bandings  do  not 
appear.  Its  general  direction  is  transverse  to  the  main  arch,  or  nearly  east 
and  west,  and  its  dip  is  almost  vertical.  The  north-south  compression  thus 
appears  to  have  been  the  stronger,  or  to  have  been  active  somewhat  later 
in  point  of  time  than  the  east-west  compression. 

Under  the  microscope  the  phyllites  are  seen  to  be  composed  principally 
of  fine  leaves  of  muscovite  and  chlorite,  often  also  with  a  little  biotite,  and 
with  a  variable  and  usually  small  amount  of  quartz,  feldspar,  and  sometimes 


440  THE  CRYSTAL  FALLS  lEON-BEAEING  DISTRICT. 

calcite.  Magnetite,  ilmeuite,  and  limonite  are  usually  rather  abundant. 
Pyrite  also  occurs  in  a  few  grains  in  nearly  every  slide.  The  differences  in 
color  depend  mainly  upon  the  relative  proportions  of  the  chlorite  and  nuis- 
covite,  the  former  being  characteristic  of  the  dai'k-colored,  the  latter  of  the 
light-colored,  rocks.  The  very  dark-green  or  black  varieties  contain  also 
an  opaque  and  jn'obabl}'  organic  pigment  in  very  minute  particles.  The 
quartz  and  feldspar  grains  are  usually  verj-  small  and  irregularly  shaped. 
The  larger,  however,  of  which  a  few  occur  in  the  slides  froin  the  less  com- 
pressed rocks,  have  well-rounded  contours.  In  other  cases  extremely 
flattened  and  strung-out  lenses  composed  of  many  small  particles  represent 
what  were  doubtless  <:)riginally  single  clastic  grains. 

Two  varieties  of  cleavage  are  well  illustrated  in  the  thin  sections, 
namelv,  that  caused  by  the  parallelism  of  the  component  minerals,  and 
"  ausweichungs-clivage."  The  former  is  characteristic  of  the  coarser- 
grained  varieties,  and  the  latter  of  the  finer  grained,  where  the  direction  of 
pressure  has  made  a  large  angle  with  the  bedding.  In  some  cases  the  little 
leaves  of  muscovite  outline  parallel  and  equal  folds,  less  than  0.2  mm.  from 
crest  to  crest,  each  of  which  is  ruptured,  sometimes  with  slight  displace- 
ments, sometimes  with  none,  entirely  across  the  slide.  The  structure  is  most 
distinct  in  the  red  phyllites,  in  which  the  fractures  and  the  arrangement  of 
the  muscovite  plates  afe  clearly  outlined  by  the  ferruginous  stain.  Each 
kind  of  cleavage  in  a  different  way  tells  the  storj-  of  extreme  pressure. 

SECTION  V.    THE  HEMLOCK  FOR3IATION. 

The  Mansfield  formation  of  the  Michigamme  Mountain  area  changes 
along  the  strike  into  rocks  of  an  entirely  different  character,  which,  as 
already  said,  have  been  named  the  Hemlock  formation. 

DISTRIBUTION,   EXPOSURES,  AND  TOPOGRAPHY. 

* 

The  Hemlock  formation  in  the  Fence  River  area  consists  of  several 
varieties  of  schists  which  occupy  a  belt  between  2,000  and  3,000  feet  in 
width  between  the  dolomite  on  the  west  and  the  Groveland  formation  on 
the  east.  The  best  exposures  occur  on  the  two  river  sections  already 
referred  to  (p.  431),  but  outcrops  are  by  no  means  lacking  elsewhere.'  At 
the  northwest  corner  of  the  area,  in  sec'  6,  T.  45  N.,  R.  31  W.,  the  schists 
are  found  striking  N.  60°-70°  W.,  and  dipping  northeast  about  40°.     East 


THE  UEMLOOK  FORMATION.  441 

of  the  center  of  sec.  Hi,  '1\  46  N.,  li.  31,  a  few  exposures  occur,  tlie  sti'uc- 
ture  of  which  strikes  a  few  degrees  west  of  north  and  dips  eastward  at 
angles  of  45°  to  50°.  In  sections  21  and  28,  a  mih-  and  a  lialf  south,  numer- 
ous outcrops  in  siniihn-  attitudes  are  found  ah)ng  the  northern  river  .section. 
In  sees.  3  and  4,  T.  44  N.,  H.  31  W.,  a  few  scattered  outcrops  only  have 
been  found,  but  throufiliout  section  10  they  are  very  abundant.  For  the 
next  2  miles  south  tln-ough  sees.  15  and  22,  T.  44  N.,  K.  31  W.,  onlv  a 
few  small  exposures  have  been  discovered  which  have  the  same  northerlj' 
strike  and  eastward  dip.  Thus  for  a  distance  of  1 1  miles  along  the  strike 
exposures  occur  at  comparatively  short  intervals,  the  longest  gap  being  3 
miles. 

In  general  this  belt  is  one  of  slight  elevation  above  both  the  dolomite 
country  on  the  west  and  the  iron  formation  country  on  the  east.  The  areas 
of  best  exposure  are  characterized  by  very  rough  topographical  details, 
which  are  entirely  lost  in  the  generalized  curves  of  the  map.  Abrupt  strike 
ridges,  separated  by  narrow  ravines,  succeed  one  another  at  short  intervals. 
In  the  covered  areas  the  surface,  while  retaining  its  general  elevation,  has 
been  leveled  off  by  the  deposition  of  till  in  the  hollows,  and  has  the  smoothly 
undulating  contours  characteristic  of  till-covered  areas. 

FOLDING    AND     THICKNESS. 

No  secondary  folds  have  been  detected  within  the  Fence  River  area  of 
the  Hemlock  formation,  and  on  account  of  the  metamorphism  and  cleavage 
structural  observations  are  not  possible  from  wdiich  they  might  be  inferred. 
The  only  clear  evidence  as  to  the  attitude  of  the  rocks  was  afforded  by  the 
contact  at  one  locality  between  beds  of  amygdaloid  and  agglomerate. 
There  the  dip  is  eastward  at  an  angle  of  50°.  The  surface  width  of  the 
formation  varies  between  2,000  and  3,000  feet.  If  50°  is  taken  as  the  dip, 
the  thickness  would  be  from  1,500  to  2,300  feet.  If  the  average  dip  is 
assumed  to  be  40°,  or  the  average  of  the  observed  dips  of  the  underl^-ing 
dolomite,  the  thickness  would  be  from  1,300  to  1,900  feet.  Or  if  30°  be 
taken,  the  average  of  the  lower  dips  of  the  dolomite,  the  thickness  would 
be  1,000  to  1,500  feet.  We  may  say,  therefore,  that  the  thickness  north 
of  the  southern  river  section  is  probably  not  less  than  1,000  nor  jn-obably 
more  than  2,300  feet.  South  of  the  southern  river  section  the  thickness 
diminishes  rapidly. 


442  THE  CRYSTAL  FALLS  IRON-BEARII^G  DISTRICT. 

PETROGRAPHICAL  CHARACTERS. 

The  exposures  tlu-ougli  section  10  and  the  northern  ])art  of  sec.  15, 
T.  44  N.,  R.  31  W. — the  southern  river  section — give  us  a  nearly  complete 
seqv;ence  across  the  Hemlock  formation,  the  principal  gaps  being  on  the 
extreme  east  and  west,  thus  leaving  the  details  of  the  relations  with  the 
dolomites  below  and  the  iron  formation  above  undisclosed.  In  this  section 
of  3,000  feet  in  length,  the  rocks  are  chiefly  chloritic  and  epidotic  schists, 
with  which  are  associated  schists  bearing  biotite,  ilmenite,  ottrelite,  and 
araphibole,  greenstone  conglomerates  or  agglomerates,  and  amygdaloids. 
These  rocks  ai-e  characterized  by  a  generally  fine  and  even  grain,  by  a  lack 
of  sedimentary  characters,  and  by  a  double  structure.  In  most  of  the 
varieties  minerals,  which  have  formed  quite  independently  of  and  later  than 
these  structures,  are  macroscopically  conspicuous.  The  prevailing  color  is 
green,  passing  to  dark  purple  and  black  in  the  varieties  in  which  biotite, 
hornblende,  and  magnetite  abound. 

The  distinction  made  in  the  field  between  the  several  varieties  of  the 
schists  is  a  rough  one,  indicating  the  predominating  minerals  rather  than 
iniplving  the  absence  of  the  others.  In  fact  all  the  varieties  are  intimaxely 
related.  The  chloi-ite-schists  are  very  fine-grained  green  rocks,  usually 
from  their  color  evidently  very  epidotic;  they  weather  to  greenish  or  pink- 
ish white.  The  cleavage  surfaces  are  often  plentifully  sprinkled  with  little 
flakes  of  Ijiotite.  Frequently  also  black  needles  of  ihnenite,  brilliant  plates 
of  ottrelite,  and  large  clusters  of  actinolite  run  irregularly  through  them, 
quite  independent  of  the  cleavages.  The  biotite-schists  are  nuicli  darker, 
and  lack  the  green  coloring  Through  them  also  the  same  metamorphic 
minerals  are  frequentl}'  interlaced.  By  an  increase  in  these  minerals  the 
passage  to  the  other  varieties  in  limited  exposures  is  a  very  eas)^  one. 

Greenstone-conglomerates  and  amygdaloidal  rocks  occur  in  a  few 
exposures.  In  the  former,  light  green  or  gray  aphanitic  inclusions,  of 
angular  shapes,  ranging  from  an  inch  to  2  or  3  feet  in  long  diameter,  are 
inclosed  in  a  matrix  of  chlorite-schist  or  biotite-schist.  The  chlorite-schists 
often  hold  round  or  lens-shaped  eyes  of  epidote,  and  epidote  and  quartz. 
That  these  are  filled  cavities  can  in  most  cases  be  shown  only  by  the  micro- 
scope, vet  some  of  the  larger  amygdules  have  a  banded  structure  evident 
to  the  naked  eye.     These  rocks  are  of  structural  interest  since  they  are  the 


PETROGRAPHICAL  CHARACTERS  OF  OEMLOC^K  FORMATION.      443 

only  Hiciiihers  of  rlic  urea  wliicli  ])ossess  undoubted  bedding-.  The  i)lane  of 
contact,  between  an  aniyg-daloid  and  a  layer  of  greenstone-conglomerate  in 
SE.  \  sec,  10,  T.  44  N.,  R.  31  W.,  dips  eastwanl  at  an  angle  of  SO^. 

Two  well-marked  systems  of  cleavage  traverse  all  the  rocks  of  the 
southern  river  se(;tion.  The  angle  betv/een  their  strikes  is  always  acute 
toward  the  north,  varying  from  F)°  to  as  high  as  34°  in  different  exposures, 
while  the  direction  of  the  bisectrix  is  almost  constant  at  N.  8°-10'^  W. 
The  dip  of  both  systems  is  toward  the  east  at  about  the  same  angle, 
namely  50°  to  60°.  The  two  systems  are  usually  both  well  developed,  so 
that  the  outcrop  edges  break  down  by  weathering  along  zigzag  lines. 
The  character  of  the  cleavages  varies  from  fine  partings  which  divide  the 
surface  into  rhombs,  sometimes  extremeh'  i-egular  in  the  more  aphanitic 
rocks  to  a  single  perfect  scliistosity  capable  of  minute  subdivision,  along 
which  the  com])f)nent  minerals  are  visibly  aligned,  in  the  more  crystalline. 
Along  the  cleavages  seams  of  quartz  and  calcite  have  frequently  formed. 

Along  the  upper  river  section  the  rocks  of  the  area  are  distinctly  more 
crystalline,  and  are  chiefly  biotite-schists  and  biotite-hornblende-schists,  the 
latter  often  veiy  coarse.  They  are  sometimes  banded,  but  very  irregularly, 
the  lenticular  character  of  tlie  banding-  sug-o-estinp-  the  rhondjic  cleavag-es 
of  the  southern  section.  Some  of  the  finer-grained  biotite-schists  contain 
round  or  elongated  areas  of  quartz  and  epidote,  which  resemble  amygdules. 
With  these  are  associated  considerable  thicknesses  of  sericite-schists,  full  of 
little  eyes  of  blue  quartz;  these  are  evidently  metamorphic  acid  eruptives. 
The  width  of  the  northern  section  is  about  2,000  feet. 

Under  the  microscope  the  Hemlock  schists  of  the  Fence  River  area 
have  a  general  porphyritic  habit.  Two  main  divisions  only  are  clearly 
distinguished.  One  of  these  is  the  fine-grained  mica  (sericite)  schists, 
which  are  characterized  Ijy  the  presence  of  muscovite  as  well  as  biotite  in 
the  microcrystalline  groundmass,  and  true  phenocrysts  of  feldspar  and 
bipyramidal  quartz,  while  the  other  embraces  all  the  other  varieties,  which, 
diverse  as  they  undoubtedly  are,  have  yet  certain  important  characters  in 
common  and  are  connectf^d  by  gradations.  The  sericite-schists  are  obviously 
metamorphosed  acid  lavas,  and  need  not  be  described  in  detail  here. 

The  origin  of  the  second  division,  however,  is  far  more  obscure.  The 
least  altered  of  these  rocks  possess  an  exceedingly  fine  grained  microcrys- 
talline groundmass,  made  up  of  very  pale  chlorite  and  a  colorless  aggregate 


444  THE  CRYSTAL  FALLS  IRON-BEARIXG  DISTRICT. 

with  feeble  double  refraction,  which  seems  to  be  quartz.  Between  crossed 
nicols  the  groundniass  is  almost  isotropic,  and  it  is  by  no  means  improbable 
that  certain  reddish  patches  here  and  there  may  really  be  glass.  Little  crys- 
tals of  magnetite  are  abundantly  scattered  through  the  groundmass,  and  are 
often  arranged  in  parallel  curving  lines,  very  suggestive  of  the  flowage 
lines  brought  out  on  the  surface  of  weathered  rhyolites  by  the  ferruginous 
stains.  In  man}-  sections  the  groundmass  includes  minute  lath-shaped 
plagioclase  feldspars,  much  altered  and  with  indistinct  boundaries,  which 
are  often  arranged  in  pa,i-allel  lines.  The  groundmass  also  is  generally 
sprinkled  with  little  irregular  grains  of  epidote  and  calcite. 

In  this  groundmass  are  included  in  variable  combinations  and  propor- 
tions much  larger  crystals  and  grains  of  common  hornblende,  actinolite, 
biotite,  ottrelite,  calcite,  ilmenite,  epidote,  and  zoisite.  Of  these  biotite, 
calcite,  ilmenite,  epidote,  and  zoisite  are  the  most  constant  and  abundant. 

Biotite  is  present  in  all  or  nearly  all  of  the  thin  sections.  It  is  always 
brown,  and  is  characteristically  developed  in  stubby  individuals,  very  thick 
for  their  basal  dimensions.  These  individuals  are  large  and  lie  scattered 
through  the  slides.  They  frequently  inclose  portions  of  the  groundmass. 
The  mica  cleavage  most  frequently  stands  across  the  cleavage  of  the  rock. 
In  many  of  the  darker-colored  schists,  however,  biotite  plates  intermediate 
in  size  between  the  large  porphyritic  individuals  and  the  small  chlorite 
plates  of  groundmass  are  present  in  large  numbers,  constituting  a  sort  of 
secondary  groundmass.  These  are  generally  aligned  with  the  cleavage 
of  the  rock  and  are  sometimes  gathered  in  bands,  but  in  color  and  stubby 
habit  are  similar  to  the  phenocrysts. 

Ilmenite  in  brownish-black  prismatic  sections  is  a  common  constituent. 
It  usually  lies  at  random  through  the  slide.  It  incloses  the  quartz  and 
epidote  grains  of  the  groundmass.  Epidote  and  zoisite  are  exceedingly 
abundant,  often  in  well-formed  crystals.  Manj^  of  the  epidote  and  zoisite 
individuals  contain  darker  colored  inner  nuclei,  the  nature  of  which  is 
uncertain.  In  some  cases  the  nuclei  are  irregular  in  shape  and  have 
the  characteristic  pleochroism  of  epidote,  but  are  more  strongly  colored 
than  the  surrounding  zones.  In  other  cases  they  have  sharp  crystal 
boundaries,  isomorphous  with  e])idote,  are  brown  in  color,  and  inclose 
grains  of  magnetite;  these  may  be  allanite.  The  nuclei  are  too  small, 
however,  for  determination.     Generally  tliey  do  not  extinguish  exactly  witli 


PETKOGRAPIIICAL  CHAKACTERS  OF  HEMLOCK  FORMATION.      445 

the  .suiToiindinii-  zoiu's.  It  is  prohahlo  that  luauy  of  these  nuclei  represent 
an  early  g'eneration  of  e[)idote,  like  the  small  irregular  grains  of  the  ground- 
mass,  which  were  subsequently  enlarged  to  porphyritic  size.  Inclosui'es  of 
zoisite  are  not  uncoiumon  in  tlie  large  epidote  individuals.  Large  lenticular 
aggregates  of  epidote  with  calcite,  chlorite,  and  biotite  are  found  partially 
replacing  feldspar  individuals,  which  were  no  doubt  original  phenocrysts. 
Similar  aggregates  unmixed  with  the  remains  of  feldspar  are  not  infrequent, 
and  may  reasonably  be  attributed  to  the  same  source.  Epidote  with  quartz 
is  also  the  common  tilling  of  the  amygdaloidal  cavities. 

Common  hornblende,  actinolite,  and  ottrelite  are  very  common  as 
porphyritic  constituents  of  the  schists.  Hornblende  occurs  in  very  large 
well-formed  single  crystals  and  clusters  placed  at  random  through  the 
gToundmass.  It  is  characteristically  associated  with  ottrelite  and  biotite, 
and  often  has  formed  somewhat  later  than  the  latter.  It  is  always  crowded 
with  inclusions,  which  in  the  laminated  varieties  carry  the  structure  through 
without  reference  to  the  position  of  the  host.  Ottrelite  is  abundant  in  some 
of  the  sections,  and  is  distinguished  by  its  characteristic  pleochroism.  It 
occurs  in  large  individuals  and  multiple  twins,  and  like  the  large  horn- 
blendes and  biotites  is  full  of  inclusions. 

The  general  characteristics  of  these  schists  then  are,  lir.st,  a  groundmass 
composed  of  chlorite,  quartz,  magnetite,  epidote,  and  in  some  cases  contain- 
ing plagioclase  microlites,  and  secondly  the  presence  in  this  groundmass  of 
much  larger  porphyritic  individuals  of  several  secondary  minerals.  The 
varieties  are  determined  by  the  varying  ratio  of  the  porphyritic  constituents 
to  the  groundmass,  by  the  nature  of  the  predominant  secondary  minerals, 
and  also  by  the  diiferences  in  grain  of  the  groundmass.  This,  while  gen- 
erally extremely  fine  grained  is  much  coarser,  Ijut  without  minei'alogical 
change,  on  the  northern  river  section  where  the  schists  are  more  distinctly 
crystalline.  The  cleavage  of  the  schists  is  determined  by  the  aiTangement 
of  the  minute  particles  of  the  groundmass,  and  not  by  the  parallelism  of 
the  large  secondary  minerals.  These  last,  further,  are  never  faulted  or 
broken,  and  in  general  are  unstrained  optically.  They  must  have  formed 
then  after  the  compression  and  tilting  of  the  series. 

The  origin  of  these  schists,  I  think,  is  not  doubtful.  As  important 
points  of  evidence  we  have,  first,  the  absence  of  rocks  possessing  any  sedi- 
mentary characters    throughout    the  whole    section.     Next   we    have    the 


446  THE  CRYSTAL  FALLS  IRON-BEAKmU  DISTRIC3T, 

undoubted  presence  of  lavas  in  the  series,  shown  ))y  the  sericite-schists, 
amyg'daloids,  and  greenstone  conglomerates  or  agglomerates.  Furthermore, 
the  minerals  which  compose  the  schists  are  those  which  would  result  from 
the  alteration,  in  connection  with  dynamic  metamorphism,  of  igneous  rocks 
of  basic  or  intermediate  chemical  composition.  Finally,  the  grain  and 
character  of  the  groundmass,  and  in  some  slides  the  presence  therein  of 
plagioclase  microlites  disposed  in  flow  lines,  point  directly  to  an  igneous 
orio-in  and  to  consolidation  at  the  surface. 

Mr.  Clements  has  reached  similar  conclusions  for  the  formation  above 
the  Randville  dolomite'  on  the  western  side  of  tlie  Archean  dome.  There 
metamorphism  seems  to  have  progressed  less  far  than  in  the  Fence  River 
area,  and  among  the  more  basic  rocks  he  has  recognized  andesites  and 
basalts. 

I  conceive,  then,  that  the  Hemlock  rocks  of  the  Fence  River  area  are  a 
series  of  old  lava  flows,  varying  in  composition  from  acid  to  basic,  which 
first  underwent  dynaiBic  disturbance,  which  developed  the  secondary  cleav- 
ages, and  afterwards,  in  a  state  of  rest,  the  porphyritic  minerals  were 
formed.  It  is  an  interesting  fact,  for  which  I  can  suggest  no  explanation, 
that  metamorphism  is  further  advanced  in  tlie  northern  part  of  the  area 
than  in  the  southern,  and  the  schists  more  distinctly  crystalline.  This  is 
also  true  of  the  underlying  dolomite. 

SECTION  VI.    THE  GROVELAND  FORMATIOIf.^ 

DISTRIBUTION,  EXPOSURES,  AND  TOPOGRAPHY. 

The  Groveland  formation  in  this  area,  as  in  the  Felch  Mountain  range, 
consists  mainly  of  siliceous  iron-bearing  rocks,  which  hold  much  fragmental 
material,  together  with  certain  subordinate  schists.  While  it  is  of  wide 
extent  throughout  the  area,  its  known  outcrops  are  limited  to  three  local- 
ities, namely:  The  vicinity  of  Michigamme  Mountain,  in  sees.  33,  T.  44  N., 

'  Volcanics  of  the  Michigamme  district  of  Michigan,  by  J.  Morgan  Clements:  .lour.  Geol.,  Vol. 
Ill,  1895,  No.  7,  p.  801. 

-This  formation  was  originally  named  by  me  the  Michigamme  Jasper  (Am.  Jour.  Sci.,  March, 
1894).  The  name  Michigamme  was  subsequently  used  for  one  of  the  Upper  Marquette  formations,  in 
the  Preliminary  Report  on  the  Marquette  District,  l.'ith  Ann.  Rept.  U.  S.  Geol.  Survey.  I  now  aban- 
don the  old  name,  although  it  is  entitled  to  stand  by  the  rules  of  priority,  in  order  to  avoid  the  con- 
fusion which  would  necessarily  arise  from  its  retention. 


THE  GitOVELAND  EOKMATION,  447 

Iv.  31  W.,  and  3,  '1\  43  X.,  \\.  31  W.;  the  exposures  and  test  pits  at  the 
Shohleis  exploration  in  sec.  21,  T.  45  N.,  R  31  W.,  and  the  test  ])its  at  tlie 
Doane  exploration  in  sec.  K;,  T.  45  N.,  K.  31  W.  The  last  two  localities 
are  1  mile  apart,  and  tlie  inoiv  soutliern  is  8  miles  north  of  Michiffamme 
Mountain. 

In  spite  of  the  poverty  of  the  formation  in, outcrops,  its  distribution 
throug-hout  the  area  has  been  well  determined  throug-h  its  magnetic  jjroper- 
ties  (following  the  methods  described  in  Chapter  II).  Adjacent  to  the 
Fence  River  area  of  the  Hemlock  formation  it  gives  rise  to  a  strong  mag- 
netic line  which  passes  through  the  outcrops  and  test  pits  of  the  Sholdeis 
and  Doane  explorations.  To  the  north  this  line  was  followed  to  the  south- 
ern side  of  sec.  32,  T.  46  N.,  R.  31  W.,  where  it  is  said  to  connect  with  a 
magnetic  line  followed  by  Mr.  Clements  from  the  western  to  the  northern 
side  of  the  Archean  dome.  To  the  south  it  continues  into  the  Micliii>-amme 
Mountain  area  to  within  a  mile  of  the  outcrops  of  Michigamme  Moun- 
tain. There  the  magnetic  line  gives  way  to  a  broad  zone  of  disturbances, 
feeble  and  difficult  to  interpret,  but  consequent  I  believe  mainly  upon  the 
flattening  of  the  foi'mation  as  it  begins  to  pass  over  the  general  northwest- 
southeast  anticlinal  axis.  This  zone  connects  directly  with  the  exposures 
of  Michigamme  Mountain  which  produce  similar  irregular  distui-bances  of 
the  needles  and  which  visibly  constitute  a  thin  crumpled  sheet,  on  the 
whole  but  gently  inclined. 

For  the  stretch  of  13  miles  just  described  the  Groveland  formation 
occupies  a  continuous  belt  on  the  east  side  of  the  main  anticlinal  axis.  In 
the  Fence  River  area  it  lies  east  of  and  upon  the  Hemlock  formation,  while 
in  the  Michigannne  Mountain  area  it  holds  the  same  relations  to  tlie  Mans- 
field formation. 

The  eastern  belt  was  not  traced  farther  than  a  mile  soixtheast  of  Michi- 
gamme Mountain.  .  In  the  central  and  southeastern  portions  of  T.  43  N., 
R.  31  W.,  however',  in  the  direct  prolongation  of  the  anticlinal  axis,  we 
found  a  broad  belt  of  slight  magnetic  disturbance,  along  the  western  mar- 
gin of  which  lie  volcanic  rocks,  dipping  west.  In  sec.  26,  T.  43  N.,  R.  31  W., 
this  magnetic  belt  splits  into  two  branches,  one  of  which  runs  directly  east 
for  a  mile  and  then  southeast  indefinitely,  while  the  other  maintains  a  general 
southerly  course  to  the  south  line  of  the  township.     In  section  26  large 


448  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

angular  bowlders  evideiith'  derived  from  tlie  Grovelaiid  formation  are  found 
in  the  zone  of  magnetic  disturl^ance,  but  no  outcrops  have  been  discovered. 
There  can  be  little  doubt  that  these  disturbances  roughl}'  outline  the  position 
of  the  Groveland  formation  in  the  axial  region. 

Except  in  ^[ichigamme  Mountain,  the  most  elevated  point  of  the  dis- 
trict, the  Groveland  formation  is  not  topographicall}'  ^^I'ominent.  In  the 
Fence  River  area  it  produces  a  more  subdued  and  somewhat  lower-lying 
surface  than  the  underlying  formation,  but  the  difference  is  slight  and  is  of 
little  moment  in  comparison  with  the  confusing  effects  of  glaciation. 

FOLDING    AND    THICKNESS. 

In  the  Fence  River  area  there  is  no  reason  to  suppose  that  the  Grove- 
land formation  contains  within  itself  minor  folds  of  any  importance.  Our 
knowledge  of  its  attitude  is  supplied  almost  wholly  by  the  magnetic  obser- 
vations, and  these  indicate  that  it  has  a  general  eastward  dip  like  the  under- 
lying members  of  the  succession.  Here  and  there  it  may  be  divided  into 
two  or  more  parts  by  sheets  of  intrusive  material,  and  also  may  be  slightly 
criimpled,  but  on  the  whole  it  must  be  regarded  as  a  single  persistent  sheet, 
having  a  general  eastward  dip. 

At  Michigamme  Mountain  the  Groveland  formation  caps  tlie  hill  in  a 
well-marked  syncline,  the  axis  of  which  runs  northwest  and  southeast. 
The  structure  is  distinctly  shown  by  the  attitude  both  of  the  ferruginous 
rocks  and  of  the  underlying  ])hyllites.  At  the  Interrange  exploration  half 
a  mile  south,  a  secondary  embayment  of  the  same  syncline,  but  more  open, 
is  found.  Tliese  are  the  only  folds  of  the  Michigamme  Mountain  area 
sufficiently  deep  to  include  the  iron-bearing  rocks.  The  thickness  of  the 
formation  can  onlv  be  guessed  at,  as  no  complete  section  is  exposed,  and 
the  data  for  determining  its  upper  limit  are  decidedly  shadowy.  The  mag- 
netic observations  indicate  a  breadth  of  from  400  to  600  feet,  and  as  in  the 
Fence  River  area  it  is  certainly  nuich  thinner  than  the  two  lower  forma- 
tions, its  thickness  may  be  approximately  500  feet. 

PETROGRAPHICAL  CHARACTERS. 

In  general  aspect  the  iron-bearing  formation  in  this  area  is  strikingly 
like  that  of  the  Felch  Mountain  range,  and  all  the  varieties  there  found  are 
represented  here  also.  It  is  therefore  unnecessary  to  repeat  the  detailed 
descriptions  already  given.     By  way  of  broad  comparison,  however,  it  may 


PETKOGRAPHIOAL  CHAHACTEKS  OF  GROVEL  AND  FORMATION.      449 

he  .said  that  tliu  toriuatiou  coutaius  less  iron  than  in  tlie  Felch  .Mountain 
range,  and  consequent!}-  the  lighter-colored  varieties  are  more  abundant, 
that  it  contains  niort'  detrital  material,  and  that  in  tlie  ]\Iichigamme  Moun- 
tain area  the  texture  is  generally  closer  and  less  granular.  Moreover,  in 
passing  north  from  the  Michigamme  Mountain  area  to  the  Fence  River  area 
we  find  at  the  Sholdeis  and  Doane  explorations  that  the  lower  portion  of 
the  formation  is  com])osed  of  ferruginous  quartzite,  which  is  succeeded 
higher  up  by  actinolite-schists  and  grihierite-schists  similar  in  all  respects 
to  the  characteristic  rocks  of  the  Negaunee  iron  formation  in  the  western 
Marquette  district.  In  this  change  in  character  as  the  Marquette  district  is 
approached  is  found  the  lithological  support  for  the  view,  first  suggested  by 
the  distribution  of  the  lines  of  magnetic  attraction,  that  the  Groveland 
formation  is  the  southern  equivalent  of  both  the  Ajibik  quartzite  and  the 
Negaunee  formation  of  the  Marquette  district.  The  passage  to  a  more 
crystalline  condition  in  going  from  south  to  north  is  also  in  accord  with  the 
like  changes  already  noted  in  the  lower  formations. 

Under  the  microscope  the  close  texture  of  the  Groveland  rocks  of 
Michigamme  Mountain  is  seen  to  be  due  to  the  minuteness  of  the  quartz 
grains  of  the  groundmass,  and  to  the  abundance  therein  of  chalcedony. 
The  coarse  quartz  grains  are  all  detrital  and  are  often  beautifully  enlarged. 
In  many  slides  feldspar  pebbles  occur,  and  in  many  also  sericite  and  chlorite 
are  prominent  in  the  groundmass.  The  iron  oxides,  including  both  mag- 
netite and  hematite,  in  single  crystals  and  also  in  agg-regates,  are  well  dis- 
tributed, as  in  the  Felch  Mountain  sections.  A  similar  grouping  of  these 
in  round  j^ebble-like  areas  as  in  the  Felch  Mountain  range  is  also  beauti- 
fully shown.  In  one  slide  tlie  matrix  is  a  rhomljohedral  carbonate,  prob- 
ably calcite,  in  which  are  embedded  quartz  grains  and  the  iron  ores  in 
single  crystals  and  irregular  aggregates. 

The  most  interesting  features  of  the  thin  sections  from  Michigamme 
Mountain  are  the  pressure  eff'ects.  In  many  slides  the  detrital  quartz 
grains  are  strained  to  an  extraordinary  degree.  In  one  case  the  stage  was 
rotated  45°  before  the  black  wave  of  extinction  completely  traversed  a 
little  pebble  0.3  mm.  in  diameter.  Almost  every  section  is  crossed  in  sev- 
eral directions  by  fractures  healed  by  the  deposition  of  coarse  quartz 
and  the  iron  oxides. 

MON  XXXVI 29 


450  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

In  the  Fence  River  area  the  lower  portion  of  the  formation  consists  of 
quartzite,  more  or  less  ferruginous  and  micaceous.  It  contains  beautifully 
rounded  and  enlarged  grains  of  quartz,  and  also  less  abundantly  rolled 
grains  of  microcline.  Muscovite,  biotite,  and  epidote  occur,  with  the  gen- 
eral background  of  interlocking  later  quartz.  The  more  ferruginous  laj^ers 
have  a  groundmass  almost  exclusively  of  hematite,  in  which  the  clastic 
particles  are  set.  The  hematite  is  in  parallel  micaceous  scales,  which  com- 
pletely cover  the  cleavage  surfaces.  Above  these  layers  come  crystalline 
actinolite-schists  and  griinerite-schists,  the  former  with  garnets  and  Ijoth 
carrying  particles  of  the  iron  oxides.  These  rocks  are  not  distinguishaljle 
in  the  field  or  in  thin  section  from  certain  varieties  of  schists  of  common 
occurrence  in  the  Negaunee  formation  of  the  Marqiiette  district. 


50 


n 


I 


m 


!l 


CHAPTER    V. 

THE  NORTHEASTERN  AREA  AND  THE  RELATIONS  BETWEEN 
THE  LOWER  MARQUETTE  AND  LOWER  MENOMINEE 
SERIES. 

From  the  northerninost  outcrops  of  the  Fence  River  area  to  the  north- 
ern end  of  the  Repubhc  trough  the  air-lhie  distance  is  about  1 1  miles.  This 
intervening  territory,  on  one  side  of  which  we  find  the  typical  formations 
of  the  Menominee  district  and  on  the  other  the  typical  formations  of  the 
Marquette  district,  remains  to  be  described  in  this  chapter.  It  may  con- 
veniently be  referred  to  as  the  Northeastern  area. 

As  was  shown  in  the  report  on  the  Marquette  district,  in  the  pro- 
ductive portion  of  the  Marquette  range  between  Negaunee  on  the  east  and 
Repulilic  on  the  west,  the  lower  Marquette  series  consists  of  two  or  three 
clearly  marked  formations,  which,  perhaps,  may  further  be  subdivided 
according  to  individual  taste. ^  The  lowest  of  these,  the  Ajibik-  quartzite, 
which  rests  on  the  Archean  complex,  is  fragmental  in  origin,  and  is  prevail- 
ingly a  white  vitreous  quartzite,  which  in  one  or  two  localities  is  conglom- 
eratic near  the  base.  Often  it  is  represented  by  a  muscovite-schist  as  the 
result  of  the  dynamic  metamorphism  of  the  original  arkose.  In  the  eastern 
part  of  the  productive  area  of  the  Marquette  district  and  along  the  northern 
side  of  the  main  fold,  in  the  western  part  of  the  district,  this  formation  is 
overlain  by  the  Siamo  slates.^  Elsewhere  the  slates  are  not  present,  or  are 
not  known. 

The  next  formation  is  the  Negaunee  iron  formation,*  which  has  already 
been  referred  to  in  Chapter  II.  This  rock,  which  has  many  phases,  as  there 
noted,  is  clearly  marked  oif  from  the  lower  quartzite  by  its  great  richness 
in  iron  and  by  the  fact  that  over  the  whole  Mai-quette  district  it  nowhere 
appears  to  contain  fragmental  material,  except  in  the  transitional  zone 
between  it  and  the  lower  formations. 

'The  Marquette  iron-bearing  series  of  Michigan,  by  C.  E.  Van  Hise  and  W.  S.  Bayley,  with  a 
chapter  on  the  Republic  trough,  by  H.  L.  Smyth:  Mou.  U.  S.  Geol.  Survey,  Vol.  XXVIII,  1897,  p.  221. 
2  Op.  eit.,  pp.  528-529.  '  Op.  cit.,  pp.  313-315.  '  Op.  cit.,  pp.  328-407. 

451 


452  THE  CRYSTAL  PALLS  IRON-BEAEING  DISTRICT. 

Above  these  conformable  formations  comes  the  unconformably  placed 
Upper  Marquette  series,  tlie  base  of  which  rests  now  on  one  member,  now 
on  the  other,  or  on  the  Archean. 

East  and  south  of  Negaunee,  and  extending  thence  to  the  shore  of 
Lake  Superior  at  Marquette,  is  a  series  of  rocks  which  resemble  lithologic- 
ally  neither  the  Upper  nor  the  Lower  Marquette  series  in  the  productive 
area.  It  consists,  in  ascending  order,  of  quartzite  with  basal  conglomer- 
ates, dolomite,  and  slates,  and  thus  bears  a  close  resemblance  lithologically 
and  stratigraphically  to  the  three  lower  members  of  the  Menominee  series. 
This  series,  named  by  Wadsworth  the  Mesnard  series,  has  been  regarded 
b»y  him  as  belonging  with  the  Upper  Marquette  series,  or  at  least  as  over- 
lying the  Lower  Marquette  formations  just  described.  Maj.  T.  B.  Brooks 
had  earlier  correlated  the  dolomite  with  the  Lower  Marquette  quartzite, 
and  had  supposed  that  there  was  a  gradual  passage  from  one  into  the  other 
along  the  strike.  Mr.  C.  R.  Van  Hise  has  recently  stated  that  its  position  is 
below  the  Ajibik  quartzite. 

This  series  is  found  only  in  the  eastern  part ,  of  the  Marquette  area, 
between  Goose  Lake  and  Lake  Superior,  a  distance  of  about  6  miles. 
Elsewhere,  over  by  far  the  greater  part  of  the  Marquette  district,  no  member 
of  it  has  been  recognized. 

The  geological  structure  of  the  Marquette  railge  presents  the  general 
features  of  an  east-west  striking  complex  syncline  or  synclinorium.  The 
pre-Cambrian  sedimentary  rocks,  with  their  associated  intrusive  and  extru- 
sive igneous  rocks,  occupy  the  trough,  in  which  there  is  much  local  com- 
plexity of  structure.  The  trough  is  flanked  on  the  north  and  south  by  the 
•older  Archean  crystallines. 

At  the  western  end  of  the  district  the  peculiar  Republic^  trough 
branches  from  the  main  synclinorium,  and  runs  southeast  into  the  Archean 
rocks  for  6  or  7  miles,  having  a  nearly  constant  width  of  about  one-half  to 
three-quarters  of  a  mile.  In  this  trough  the  Algonkian  rocks  have  been  so 
closely  compressed  that  they  stand  essentially  on  edge.  The  interior  is 
occupied  by  the  younger  Upper  Marquette  quartzites  dud  schists,  betAveen 
which  and  the  underlying  Archean  walls  the  older  Lower  Marquette  iron 
formation  and  quartzite  here  and  there  occur. 

The  northwestern  end  of  the  Republic  trough  is  about  the  western 

'  Op.  cit.,  p.  525. 


THE  NOETHEASTEEN  AEEA.  453 

rmiit  of  iniuing'  develop lueiit,  tliough  not  of  exploration,  on  the  wouth  side 
of  the  Marquette  synclinorium.  Up  to  this  point  outcrops,  producing 
mines,  and  old  explorations  are  sufficiently  abundant  to  permit  the  separate 
formations  to  be  traced  and  mapj^ed  with  comparative  ease,  and  to  indicate 
at  least  the  larger  structural  features. 

At  this  northwestern  end  of  the  Republic  trough  the  Lower  Marquette 
sei'ies  makes  an  abrupt  turn  to  the  south,  and  may  be  followed  for  a  mile 
or  more  by  occasional  outcrops  and  test  pits.  The  Negaunee  iron  formation 
is  persistently  present  beneath  the  U]3per  Marquette  .quartzite,  and  gives 
rise  to  a  very  strong  and  persistent  line  of  magnetic  atl  action,  which  was 
followed  in  our  work  for  about  12  miles  to  the  south  and  southeast  into  the 
Northeastern  area.  For  about  4  miles  from  the  sharp  turn  at  the  mouth  of 
the  Republic  trough  it  runs  nearly  due  south;  afterwards  it  turns  somewhat 
to  the  east  of  south,  and  follows  that  course  for  about  6  miles,  after  which 
it  turns  more  and  more  toward  the  east,  and  finally,  where  we  left  it,  its 
course  w^as  only  slightly  south  of  east.  That  this  magnetic  line  is  caused 
by  and  marks  the  position  of  the  Negaunee  iron  formation  there  can  not  be 
the  slightest  doiibt,  for  that  rock  outcrops  in  a  few  scattered  localities,  occurs 
abundantly  in  the  i:lrift,  aiul  has  been  found  in  occasional  test  pits  and  drill 
holes  throughout  this  distance.  The  underlying  quartzite  outcrops  beneath 
the  iron-bearing  formation  near  the  northern  end  of  the  line,  but  farther 
south  it  is  entirely  covered  by  the  drift,  so  far  as  the  territory  has  been 
examined.  The  overlying  Upper  Marquette  rocks  are  also  known  to 
be  present  just  west  of  the  Negaunee  formation  as  far  south  as  sec.  19, 
T.  46  N.,  R.  30  W. 

The  magnetic  line  which  accompanies  the  Negaunee  formation  may  be 
called  the  A  line.  Taking  into  account  the  connected  Republic  trough  and 
its  exposures  of  the  Lower  Marquette  rocks,  it  is  seen  that  the  A  line  par- 
tially surrounds  a  dome  of  the  Archean  crystallines,  ai^J  that  in  going 
from  the  interior  of  this  dome  outw^ard  across  the  A  line  we  pass  from  older 
to  younger  I'ocks.  The  dip  along  the  A  line  is,  therefore,  on  the  whole, 
toward  the  west,  although  the  observed  dips  at  the  few  localities  where 
determinations  have  been  made  are  either  vertical  or  slightly  inclined  from 
the  vertical  toward  the  east.  The  southern  part  of  the  A  line,  as  far  as  it 
has  been  traced,  passes  through  sees.  5,  8,  9,  15,  and  16  of  T.  45  N.,  R.  30 
W.     In  section  5  it  is  just  5  miles  east  of  the  Grroveland  formation,  which. 


454  THE  CRYSTAL  FALLS  lEON-BEAElNG  DISTRICT. 

as  was  shown  in  earlier  chapters,  is  a  magnetic  rock  occupying  a  definite 
place  in  the  Menominee  succession,  and  is  underlain  by  other  typical 
Menominee  formations,  and  finally  by  the  Archean. 

Between  the  A  line  and  the  magnetic  line  caused  by  the  Groveland 
formation,  which  may  be  called  the  C  line,  is  a  third  magnetic  line,  which 
may  be  called  the  B  line.  This  was  traced  parallel  to  the  A  line  and  less 
than  half  a  mile  away,  from  near  the  south  end  of  the  latter  to  the  north 
end,  and  finally  entirely  round  an  elliptical  area,  closing  again  upon  itself 
at  the  starting  point,  the  perimeter  of  the  ellipse  being  25  miles  in  length. 
Throughout  this  entire  distance  not  a  single  outcrop  could  be  discovered 
along  the  B  line.  Within  the  inclosed  area,  however,  in  sees.  6  and  7, 
T.  45  N.,  R.  30  W.,  and  in  sec.  19,  T.  46  N.,  R.  30  W.,  several  exposures 
of  granites  and  crystalline  schists  were  found,  which  left  no  doubt  that 
the  greater  part  of  the  area  inclosed  by  the  B  line  is  occupied  by  Archean 
rocks  of  the  same  general  character  as  those  partially  inclosed  by  the  A 
line  on  the  east  and  entirely  by  the  C  line  on  the  west.  The  area  between 
the  A  and  B  lines  as  far  south  as  sec.  19,  T.  46  N,  R.  30  W.,  has  been 
proved  to  contain  the  basal  member  of  the  Upper  Marquette  series.  The 
southwestern  quadrant  of  the  B-line  ellipse  is  nearly  parallel  to  the  C  line 
and  only  IJ  miles  away. 

The  known  facts  with  reference  to  the  B  line,  then,  are  these:  (1)  It 
represents  a  magnetic  rock;  (2)  this  magnetic  rock  completely  encircles  an 
Archean  core.  It  may  further  be  inferred  with  practical  certainty  that  this 
formation,  which  carries  such  constant  magnetic  properties  for  25  miles, 
must  be  sedimentary.  With  regard  to' its  structure,  the  foregoing  con- 
siderations would  necessarily  involve  the  conclusion  that  it  dips  away  from 
the  Archean  core  on  all  sides,  and  this  conclusion  is  fortified  by  the 
unsymmetrical  separation  of  the  horizontal  maxima  on  the  magnetic  cross 
sections.  It  follows,  therefore,  that  on  the  eastern  side  of  the  oval,  where 
the  formation  is  parallel  to  the  A  line,  it  dips  toward  the  east,  and  on  the 
western  side,  where  it  is  parallel  to  the  C  line,  it  dips  toward  the  west. 
This  conclusion  is  further  supported  by  the  dips  within  the  ellipse  in  the 
outcropping  Archean  rocks  that  show  structure.  These  all  happen  to  lie 
east  of  the  major  axis,  and  all  dip  toward  the  east. 

East  of  the  B  line,  and  between  it  and  the  A  line,  is  found  the  basal 
member  of  the  Upper  Marquette  series.     The  rock  which  is  manifest  in  the 


THE  NORTHEASTERN  AREA.  455 

B  line  must,  therefore,  be  older  than  anj-  member  of  the  Upper  Marquette 
series.  The  Negaunee  iron  formation,  represented  in  the  A  line,  dips  west, 
while  the  rock  of  the  B  line  dips  east.  They  are  both  older  than  the  basal 
member  of  the  Upper  Marquette  series,  and  are  both  j^'ounger  than  the 
Archean.  Thej'  are  both  strongly  and  persistently  magnetic.  For  8  or  1 0 
miles  they  run  parallel  to  each  other  less  than  half  a  mile  apart.  Their 
broad  structural  relations  to  the  Archean  basement  of  the  region  are  pre- 
cisely similar.  Therefore,  although  the  rock  that  gives  rise  to  the  B  line 
has  never  j-et  been  seen,  it  may  be  concluded  with  the  utmost  confidence 
that  it  is  the  Negaunee  iron  formation,  and  that  the  A  and  B  lines  represent 
this  rock  brought  up  in  the  two  limbs  of  a  narrow  and  probably  deep 
synclinal  fold. 

This  conclusion  carries  the  Negaunee  iron  formation  3  J  miles  farther 
to  the  west,  and  in  the  northeast  part  of  T.  45  N.,  R.  31  W.,  leaves  a  gap  of 
but  1^  miles  between  the  Lower  Marquette  and  the  Menominee  sei'ies. 

Here,  between  the  B  and  C  lines,  is  precisely  the  same  situation  as 
between  the  A  and  B.  One  magnetic  rock,  represented  by  the  B  line,  dips 
west;  the  other,  the  Grroveland  formation,  represented  by  the  C  line,  dips 
east.  Between  them  no  magnetic  disturbances  can  be  found.  The  area 
between  them  must  have  a  synclinal  structure,  and  if  they  are  not  one  and 
the  same  formation  each  must  undergo  an  extremely  rapid  and  precisely 
similar  change  in  lithological  character  (namely,  the  loss  of  magnetite)  in 
a  very  short  distance  and  be  represented  on  the  opposite  side  of  the  syn- 
clinal fold  by  a  nonmagnetic  formation.  Each  of  these  rocks  is  ^Dersistently 
magnetic  in  the  direction  of  the  strike  for  great  distances.  That  each  should 
independently  lose  its  magnetite  in  the  direction  of  the  dip  in  this  particular 
locality  is  very  improbable.  And,  therefore,  the  grounds  for  the  conclusion 
that  the  B  and  C  lines  represent  one  and  the  same  formation  are  quite  as 
firm  as  those  upon  which  rests  the  conclusion  that  the  A  and  B  lines  repre- 
sent the  same  formation. 

The  greater  portion  of  the  Northeastern  area  is  without  outcrops,  yet 
through  the  structural  and  lithological  residts  of  the  magnetic  work  we  are 
able  to  bridge  over  the  gap  and  to  show  with  a  high  degree  of  j^robability 
that  the  Negaunee  iron  formation  of  the  Marquette  range  is  identical  with 
the  Groveland  iron  formation  of  the  Felch  Mountain  range.  Further, 
when    we   recall   the   differentiation   of    the   Groveland    formation   in    the 


456  THE  CRYSTAL  FALLS  lEON-BEARING  DISTRICT. 

northern  part  of  the  Fence  River  area  into  ferruginous  quartzite  at  the 
base  and  griinerite-schist  in  the  upper  portion,  it  would  seem  probable  that 
the  Groveland  formation  represents  the  underlying  Ajibik  quartzite  as  well 
as  the  Negaunee  formation  of  the  western  part  of  the  Marquette  range. 

This  conclusion  has  an  important  bearing  on  the  interpretation  of  the 
early  geological  history  of  what  is  now  the  Uj)per  Peninsula  of  Michigan. 
If  the  formations  which  constitute  the  whole  of  the  Lower  Marquette  series 
over  the  25  miles  or  more  of  the  productive  and  best-known  portion  of  the 
range  are  represented  in  the  Menominee  district  and  the  intervening  area 
by  a  single  formation,  and  that  the  highest  in  the  Felcji  Mountain  succes- 
sion— namely,  the  Groveland  formation — the  formations  below  the  Grove- 
land  formation  are  all  older  than  the  Marquette  rocks  and  do  not  occur  at 
all  within  the  productive  portion  of  the  Marquette  range.  Why  are  these 
lower  formations  absent? 

To  this  question  there  seem  to  be  two  answers  which  are  a  priori 
possible.  It  is  conceivable  that  the  quartzite,  dolomite,  and  slates  of  the 
south,  or  some  of  them,  may  have  been  deposited  in  a  succession  of 
unbroken  sheets  over  the  whole  Marquette  area,  in  continuity  with  the  simi- 
lar Mesnard  formations  on  the  east,  and  that  afterwards  the  main  Marqtiette 
area  was  elevated  above  the  sea  and  entirely  stripped  of  these  formations 
by  long-continued  denudation.  Finally,  when  the  time  of  deposition  of 
the  Groveland  formation  came  round,  this  elevated  area  had  again  been 
reduced  to  sea  level,  and  subsided  below  it,  so  that  the  Ajibik  quartzite  and 
the  Negaunee  iron  formation,  and  their  southern  equivalent,  the  Groveland 
formation,  were  deposited  in  an  unbroken  sheet  over  the  whole.  If  this 
hypothesis  is  correct,  two  consequences  should  follow  from  it:  First,  we 
ought  to  find  some  discordance  between  the  Groveland  formation  or  the 
Lower  Marquette  quartzite  and  the  lower  formations  in  the  marginal  areas 
between  the  Menominee  and  Marquette,  and  the  Mesnard  and  Marquette 
areas,  respectively,  or  at  least  a  gradual  cutting  out  of  these  lower  forma- 
tions by  the  iron-bearing  members  and  the  lower  qviartzite,  and,  secondly, 
we  ought  to  find,  in  the  lack  of  discordance,  rocks  present  in  the  areas  of 
continuous  deposition  which  represent  the  time  of  denudation. 

With  regard  to  the  first  of  these  consequences  no  verification  is  possible, 
at  least  in  the  territory  between  the  Marquette  and  Fence  River  districts, 
from  lack  of  outcrops.     Throughout  the  Northeastern  area,  from  the  north- 


THE  NORTHEASTERN  AREA.  457 

western  end  of  the  Republic  trough  in  T.  47  N.,  R.  30  W.,  to  the  C  hue  in 
T.  45  N.,  R.  31  W.,  there  are  no  exposures  whatever  of  the  Algonkian  rocks 
which  underhe  the  Groveland  formation.  Somewhere  in  this  distance  of 
about  11  miles  the  lower  formations  disappear,  but  whether  by  unconformity 
or  overlap  is  an  unanswerable  question;  nor  (for  the  same  reason)  can  it 
be  definitely  settled  whether  elsewhere  farther  to  the  south  there  is  any 
discordance.  That  there  is  general  parallelism  between  the  Groveland 
formation  and  the  lower  rocks,  and  strict  conformity  in  some  places,  is  true. 
But  this  is  not  at  all  inconsistent  with  a  period  of  erosion  between  them,  if 
that  erosion  antedated  the  later  and  more  severe  orogenic  disturbance. 

In  the  Mesnard  area  the  observed  relations  have  been  interpreted  by 
Van  Hise  to  mean  that  the  lower  formations  disappear  by  overlap.  The 
facts  at  present  known  on  the  Felch  Mountain  side  are  capable  of  the  same 
interpretation,  but  they  are  not  sufficiently  definite  to  exclude  the  possibility 
of  a  period  of  erosion  below  the  iron-bearing  formation. 

With  regard  to  the  second  consequence — the  deposition  in  the  sub- 
merged areas  of  formations  which  would  represent  the  erosion  period  in  the 
elevated  area — the  evidence  at  hand  is  decidedly  against  the  existence  of 
such  formations. 

The  alternative  hypothesis  is  that  the  lower  quartzite,  dolomite,  and 
slate  formations  of  the  Menominee  area  were  not  deposited  over  the 
western  Marquette  area  at  all,  but  disappear  toward  the  north  and  east  by 
overlap,  and  this  hypothesis  is  much  more  likely  to  be  the  true  one.  We 
can  suppose,  as  I  have  already  pointed  out,^  that  this  part  of  the  Upper 
Peninsula  was  a  slowly  subsiding  area,  the  central  portion  of  which,  now 
occupied  by  the  Marquette  rocks,  stood  initially  at  a  greater  elevation 
above  the  encroaching  sea  than  the  rest.  While  the  quartzite-dolomite-slate 
triad  was  going  down  in  the  Mesnard  area  on  the  east  and  the  Menominee 
area  on  the  south  and  west,  the  central  Marquette  area  still  remained 
above  the  sea.  At  last,  when  the  Groveland  foi'mation  began  to  be  deposited, 
the  Marquette  high  land  was  finally  submerged  and  covered,  as  the  sea 
marched  over  it,  first,  with  a  sheet  of  arkose  made  up  of  its  own  disinte- 
grated ddbris,  and,  finally,  with  the  same  nonclastic  sediments  as  chiefly 
compose  the  Groveland  and  Negaunee  formations. 

'  Relations  of  the  Lower  Menominee  and  Lower  Marquette  series  in  Michigan,  by  H.  L.  Smyth : 
Am.  Jour.  Soi.,  Vol.  XLVII,  1894,  p.  222. 


CHAPTER    VI. 
THE  STURGEON  RIVER  TONGUE. 


By  William  Shirley  Bayley. 


DESCRIPTION    AND    BOUNDARY    OF    AREA. 

The  Sturgeon  River  area  of  Algonkian  sediments,  like  the  Felch 
Mountain  area,  is  an  east-west  tongue  of  conglomerates,  slates,  and  dolo- 
mites, very  narrow  at  its  eastern  extremity  and  widening  out  toward  the 
west  until  it  finally  plunges  under  drift  deposits  that  separate  it  from  the 
large  Huronian  area  of  the  Crystal  Falls  district.  The  tongue  occupies 
the  western  portion  of  T.  42  N.,  R.  27  W.,  the  central  and  northern  portions 
of  T.  42  N.,  R.  28  W.,  T.  42  N.,  R.  29  W.,  and  T.  42  N.,  R.  30  W.,  and 
the  southern  parts  of  T.  43  N.,  Rs.  28  W.,  29  W.,  and  30  W.  The  best 
exposures  of  the  rocks  constituting  the  tongue  are  found  in  sees.  7,  8,  17, 
and  18,  T.  42  N.,  R.  28  W.,  and  in  sees.  1  and  3,  T.  42  N.,  R.  29  W.,  on  or 
near  the  northwest  branch  of  the  east  branch  of  the  Sturgeon  River;  hence 
the  name  Sturgeon  River  tongue  (PI.  LI). 

On  the  south  the  sedimentary  rocks  are  bounded  by  an  area  of  granites, 
gneisses,  hornblende-schists,  and  mica-schists,  that  are  cut  by  granite  and 
quartz  veins,  by  dikes  of  diabases,  and  by  other  greenstones.  This  area 
separates  the  Sturgeon  River  tongue  of  sediments  fi-om  the  Felch  Mountain 
tongue  lying  from  2  to  3  miles  farther  south.  The  exact  line  of  demarca- 
tion between  the  granite-schist  complex  and  the  sedimentary  rocks  is  difficult 
to  draw,  because  for  the  eastern  7  miles  the  latter  are  bordered  by  green- 
stones whose  position  in  the  granite-schist  complex  or  in  the  sedimentary 
series  can  not  be  determined  at  present.  The  line  as  drawn  on  the  map 
places  the  greenstones  in  the  Archean.  It  begins  near  the  south  side  of  sec. 
7,  T.  42  N.,  R.  27  W.,  and  runs  a  little  south  of  west  to  the  quarter  post 
between  sections  17  and  18,  in  the  next  town  west,  then  northwest  to  near 

458 


51 


U.S  GEOLOGICAL  SURVEY 


MONOGRAPH    XXXVI  PL  LI, 


GEOLOOK^AL    MAP  OF    STIHGEOX  HIVJ-JU  TONOUE 

SCALE  i  INCH  -  i  MILE     COT^TOUR    INTKRVAL  2()  FEET 

'^  Outci'ops  williout  observed  strike  or  dip  =r   Outoroijs  vvilli  deternuned  strike  and  dip 

TOutcrope  with  slaltnesH  or  3(.'hiBtosity.  ^  ■      Teel  pilH  bottomed  in  rock, 

\^nTI('AL    SCAIiE    or  SE(-/riON  1  INCH -i:i2<J  FEET, 

ARCHKAN ,AI^G0^11IAN__ PLEISTOCENE 

Granite  Stut;^eonquarLzite         Raixdville  dolamilft  


A-Arhos* 

SI'SIate 
^-(iuart7ite 


THE  STUKGEON  RIVER  TONGUE.  459 

the  N.  quarter  post  of  sec.  13,  T.  42  N.,  R.  29  W.,  and  westward  to  near  the 
southwest  corner  of  sec.  12,  T.  42  N.,  R.  30  W.  From  this  pomt  the  Hne 
leaves  the  Sturgeon  River  tongue,  curves  southward,  and  returns  east  on 
the  north  side  of  the  Felch  Mountain  tongue.  The  eastern  boundary  of  tlie 
Sturgeon  River  Algonkian  area  is  even  less  definitely  determinable  than  its 
southern  boundary,  because  of  the  thick  drift  covering  the  rocks.  This 
boundary  is  placed  at  about  the  east  lines  of  sees.  6  and  7,  T.  42  N.,  R.  27  W. 
because  just  east  of  this  line,  in  the  NW.  ^  sec.  5,  ledges  of' Paleozoic 
limestone  occur.  The  northern  boundary  is  the  most  indefinite  of  all. 
The  southern  portions  of  T.  43  N.,  Rs.  28,  29,  and  30  W.,  are  so  deeply 
drift  covered  that  but  few  ledges  can  be  found  in  them,  and  these  are  widely 
separated.  In  sec.  6,  T.  42  N.,  R.  27  W.,  and  in  sees.  13  and  24,  T.  43  N., 
R.  29  W.,  are  exposures  of  granite.  These,  so  far  as  is  known,  mark  the 
southern  limit  of  an  Archean  area  which  stretches  some  miles  northward 
and  separates  the  Sturgeon  River  fragmentals  from  those  of  the  Marquette 
district.  The  line  marking  the  northern  boundary  of  the  Sturgeon  River 
tongue  begins  at  the  southeast  corner  of  sec.  6,  T.  42  N.,  R.  27  W.,  and  is 
assumed  to  run  a  few  degrees  north  of  west  from  this  point  until  it  reaches 
the  west  line  of  R.  29  W.,  where  it  turns  north. 

Between  the  northern  and  the  southern  boundaries  of  the  sedimentary 
area  as  defined,  and  in  the  midst  of  the  sediments,  are  two  areas  of  granite, 
the  rock  of  one  of  which  is  unquestionably,  and  that  of  the  other  presum- 
ably, older  than  the  conglomerates  within  the  tongue.  The  best  defined  of 
these  two  areas  lies  in  the  northern  portions  of  sees.  7  and  8,  T.  42  N., 
R.  28  W.,  and  sec.  12,  T.  42  N.,  R.  29  W.  It  measures  about  2J  miles  in 
length  and  less  than  one-half  mile  in  width.  The  extent  of  the  second 
area  can  not  be  so  accurately  outlined.  It  occupies  about  three-fourths  of 
a  square  mile  and  is  entirely  within  sec.  3,  T.  42  N.,  R.  29  W. 

lilTBKATURE. 

But  few  references  to  the  existence  of  fragmental  rocks  in  this  portion 
of  the  Upper  Peninsula  of  Michigan  can  be  found  in  the  literature  of  the 
region. 

The  early  United  States  surveyors^  reported  tlie  occurrence  of  talcose 

T.  ■R^  *^^°'"''^^  observations  upon  the  geology  aad  topography  of  the  district  south  of  Lake  Superior, 
by  Bela  Hubbard:  Thirty-first  Congress,  first  session,  Executive  Documents,  1849-50,  Vol.  Ill  No  1 
pp.  846,  847,  848,  855.  '       ■    > 


460  THE  CEYSTAL  FALLS  IRON-BEARING  DISTRICT. 

and  argillaceous  slates  in  Ts.  42  and  43,  Rs.  29,  30,  etc.,  of  mica-slates  in 
Ts.  41  and  42,  Rs.  29,  30,  etc.,  and  of  "calciferous  sandrock"  near  the  south 
boundary  of  T.  42  N.,  Rs.  27  and  28  W. 

In  a  list  of  specimens  gathered  from  these  townships  Burt^  mentions 
sienitic  greenstone,  trap,  granite,  granulite,  and  talco-micaceous  slate.  On 
the  land  plats  made  by  these  surveyors  conglomerate  is  noted  on  the  west 
line  of  sec.  8,  T.  42  N.,  R.  28  W.,  and  marble  at  the  south  corner  between 
sections  3  and  4  in  the  same  township. 

In  1851  Messrs.  Foster  and  Whitney  reported^  the  existence  of  an  arm 
of  Azoic  rocks  about  18  miles  in  length  and  10  in  breadth,  extending  east- 
erly into  Ts.  42  and  43  N.,  R.  28  W.,  and  located  its  position  on  their  map 
of  the  Upper  Peninsula. 

Brooks,^  in  his  description  of  the  northern  iron  belt  of  the  Menominee 
district,  refers  to  the  existence  of  outcrops  of  hornblendic  rocks,  mica- 
schists,  and  gneisses,  cut  by  trap  dikes,  which  he  regarded  as  equivalents 
of  the  various  greenstone-schists  exposed  along  the  Menominee  River. 
"Near  the  center  of  this  hornblendic  belt,  in  the  north  part  of  sees.  22,  23, 
and  24,  T.  42  N.,  R.  29  W.,  a  line  of  weak  magnetic  attraction  was  observed. 
This  is  regarded  as  an  indication  here  of  the  existence  of  an  iron-ore  belt." 

The  gneiss,  granite,  etc.,  north  of  the  north  quarter  post  of  sec.  31, 
T.  42  N.,  R.  29  W.,  he  declares  to  have  the  appearance  of  typical  Laurentian 
rocks.  "If  future  investigations  prove  them  to  be  Laurentian,  a  very 
troublesome  structural  problem  would  be  presented  here,  as  we  would  have 
Laurentian  rocks  conformably  overlying  beds  unmistakably  Huronian."* 

The  only  distinct  reference  made  by  Brooks  to  the  sedimentary  beds 
of  the  district  is  in  the  following  paragraph:^ 

A  range  of  marble  associated  with  quartzite,  chloritic  and  talcose  rock,  and 
overlaid  by  a  chloritic  gneiss,  with  beds  of  chloritic  schist  and  gneissoid  conglom- 
erate, the  whole  dipping  at  a  high  angle  to  the  south,  passes  about  5  miles  north  of 

'  Geological  report  of  the  survey  of  a  district  of  township  lines  in  the  State  of  Michigan,  in  the 
year  1846,  by  Wm.  A.  Burt:  Thirty-first  Congress,  first  session,  1849-50,  Senate  Documents,  Vol.  Ill, 
No.  1,  p.  84. 

^  Report  on  the  geology  and  topography  of  the  Lake  Superior  land  district,  hy  J.  W.  Foster 
and  J.  D.  Whitney,  Part  II,  The  Iron  Region:  Thirty-second  Congress,  special  session,  1851,  Senate 
Documents  Vol.  Ill,  No.  4,  p.  14. 

3  Iron-Bearing  Rocks  (economic),  by  T.  B.  Brooks:  Geol.  Survey  of  Michigan,  Vol.1,  1869-1873, 
N.Y.,  1873,  p.  161. 

*  Op.  cit.,  p.  175.  '  Op.  cit.,  p.  176. 


LITBRATLTRB  ON  STUEGEON  RIVER  TONGUE.  461 

the  North  belt  (i.  e.,  the  Pelcb  Mountain  tongue).  These  may  represent  the  north 
side  of  the  trough  or  basin,  of  which  this  iron  belt  is  the  south  outcrop.  No  iron 
has,  however,  been  found,  so  far  as  I  know,  on  this  range. 

In  Romiuger's  first  I'eport  on  the  Menominee  district  only  a  single 
reference  is  made  to  this  area.  He  declares  that  a  series  of  test  pits  put 
down  in  the  W.  ^  sec.  26,  T.  42  N.,  R.  29  W.,  and  in  the  SW.  i  sec.  14, 
T.  42  N.,  R.  30  W.,  are  in  decomposed  granite.^ 

A  specimen  of  the  conglomerate  referred  to  by  Brooks  as  overlying 
the  marble  in  the  belt  5  miles  north  of  Felch  Mountain  is  described  and 
pictured  by  Van  Hise^  in  his  paper  on  the  Principles  of  North  American 
pre-Cambrian  Geology  (see  also  PI.  LIII).  It  is  stated  to  be  from  the 
Felch  Mountain  district.  The  more  exact  location  of  the  ledge  from  which 
it  was  obtained  is  near  the  northwest  corner  of  sec.  17,  T.  42  N.,  R.  28  W., 
in  the  Sturgeon  River  tongue. 

Thus  the  only  distinct  reference  to  a  tongue  of  sediments  north  of  the 
Felch  Mountain  range  is  that  of  Brooks,  although  the  existence  of  sedi- 
mentary rocks  in  this  portion  of  the  Menominee  district  was  reported  by 
Hubbard  and  Burt.  Brooks  believed  that  the  Sturgeon  River  rocks  repre- 
sented the  northern  rim  of  a  syncline  whose  southern  i-im  constitutes  the 
I'dch  Mountain  range,  although  both  he  and  Rominger  discovered  a 
granite-schist  complex  underlying  the  country  between  the  two  areas  of 
fragmental  rocks. 

RELATIONS  BETWEEN"  THE  SEDIMENTARY  ROCKS  AND  THE  GRANITE- 
SCHIST  COMPLEX. 

As  has  already  been  stated,  the  country  between  the  Sturgeon  River 
tongue -of  sediments  and  the  Felch  Mountain  tongue  is  underlain  by  a  com- 
plex of  granites  and  various  schists,  traversed  by  fresh  and  altered  diabases 
and  by  granite  and  quartz  veins.  Brooks  recognized  these  rocks  as  pre- 
senting a  Laurentian  aspect,  although  he  felt  constrained  to  call  them 
Huronian,  because  of  the  supposed  structural  difficulties  involved  in  any 
other  view  of  their  age. 

No  contacts  of  this  granite-schist  complex  with  the  bedded  rocks  of 
the  Sturgeon  River  tongue  have  been  discovered.  Nevertheless,  there  can 
be  little  question  as  to  the  relative  ages  of  the  two  series.     As  has  been 

1  Geol.  Survey  of  Michigan,  by  C.  Rominger,  Vol.  IV,  1881,  pp.  198-199. 

2  Sixteenth  Ann.  Kept.  U.  S.  Geol.  Survey,  1896,  p.  801  and  PI.  CXV. 


462  THE  CRYSTAL  FALLS  IROISr-BEAEING  DISTEIOT. 

stated,  the  granites  and  schists  extend  southward  to  the  Felch  Mountain 
fragmentals,  and  here  they  are  unconformably  beneath  the  latter.  Moreover, 
since  the  Sturgeon  River  rocks  and  the  lower  members  of  the  Felch  Moun- 
tain series  are  identical  in  character,  it  is  probable  that  they  are  of  the 
same  age,  in  which  case  the  granites  and  schists  that  are  older  than  the 
Felch  Mountain  rocks  are  older  also  than  those  of  the  Sturgeon  River 
tongue. 

The  relations  of  the  sedimentary  series  to  the  granites  on  the  north 
have  not  been  determined,  because  no  contacts  are  exposed.  The  granites, 
however,  can  be  traced  northward  until  they  are  found  unconformably 
beneath  the  rocks  of  the  Lower  Marquette  series  at  Republic,  and  these,  so 
far  as  is  known,  are  the  oldest  sediments  in  Upper  Michigan.  There  can 
be  little  doubt,  therefore,  that  the  relations  of  the  sediments  to  the  northern 
granites  are  the  same  as  those  with  the  southern  schist  complex. 

The  granites  of  the  two  areas  surrounded  by  the  sediments  are  prob- 
ably of  the  same  age  as  the  northeni  and  southern  granites.  The  rocks  of 
the  area  in  sees.  7  and  8,  T.  42  N.,  R.  28  W.,  and  sec.  12,  T.  42  N.,  R.  29  W., 
are  demonstrably  beneath  the  conglomerates,  though  their  relations  with  the 
dolomites  have  not  been  determined.  A  well-marked  contact  between  the 
granites  and  the  conglomerates  is  exposed  at  the  south  base  of  a  small  hill 
of  granite  in  the  NE.  |  sec.  7,  T.  42  N.,  R.  28  W}  The  conglomerate  here 
is  well  bedded.  Its  strike  is  N.  60°  W.,  and  its  dip  almost  vertical.  It 
consists  largely  of  pebbles  and  bowlders  of  granite  identical  with  the  granite 
composing  the  hill,  and  a  matrix  constituted  entirely  of  granitic  debris. 
The  contact,  though  exposed  for  only  a  short  distance,  seems  to  be  an 
erosive  one.     It  is  certainly  not  an  igneous  one. 

From  a  consideration  of  the  facts  as  given  above,  there  can  be  little 
doubt  that  the  rocks  of  the  granitic  areas  within  the  Sturgeon  River  tongue 
and  of  those  bounding  it  on  the  northern  and  the  southern  sides  are  older 
than  the  sediments  within  the  tongue,  though  this  has  not  been  proved  for 
the  granites  with  respect  to  the  limestones. 

From  the  lithological  similarity  of  the  Sturgeon  River  fragmentals 
with  those  of  the  Felch  Mountain  district,  and  from  the  structural  relations 
exisiting  between  the  rocks  of  the  two  districts,  it  is  practically  certain  that 
the  Sturgeon  River  sediments  are  of  the  same  age  as  the  Felch  Mountain 

1  The  exact  location  of  the  coutaot  is  400  paces  N.,  280  W.,  of  the  southeast  corner  of  section  7. 


BASEMENT  COMPLEX  OP  STURGEON  RIVER  TONGUE.    463 

ones — i.  e.,  Meuomiuee  (Huroiiiau) — while  the  granites  and  schists  belong 
to  the  Basement  Complex  on  which  the  Lower  Algonkian  beds  throughout 
Michigan  have  been  laid  down. 


THE   BASEMENT   COMPLEX. 


•  The  Basement  Complex  rocks  in  the  area  studied  comprise  gneissoid 
granites,  biotite-schists,  and  hornblende-schists,  cut  by  dikes  of  greenstone 
and  by  veins  of  quartz  and  granite.  The  granites  are  best  exposed  in  the 
NE.  4  sec.  7  and  the  NW.  ^  sec.  8  and  the  NE.  i  sec.  7,  T.  42  N.,  R.  28  W., 
where  they  occur  as  bare  knolls  of  a  fairly  coarse  pink  rock,  separated 
from  one  another  by  stretches  of  sand.  The  best  exhibition  of  rocks  with 
the  typical  aspect  of  the  Basement  Complex  is  along  the  west  half  of  the 
east-west  quarter  line  of  sec.  19,  T.  42  N.,  R.  28  W.,  and  south  of  the  center 
of  this  section.  Here  we  find  hornblende-schists  and  hornblende-gneisses 
cut  by  veins  and  dikes  of  red  granite  and  by  greenstones  that  are  usually 
schistose.  Near  the  west  quarter  post  of  the  section  is  a  high  hill  bare  of 
vegetation.  On  this  hill  the  rocks  are  especially  well  exposed.  In  addi- 
tion to  the  types  already  mentioned,  there  is  present  here  a  coarse  white 
pegmatitic-looking  granite  that  apparently  cuts  the  hornblende-gneiss. 

All  the  members  of  the  Basement  Complex  in  this  area  are  so  similar 
to  the  corresponding  members  of  this  complex  elsewhere  in  the  Lake  Supe- 
rior region  that  they  demand  but  little  description.  They  are  described 
here  only  in  sufficient  detail  to  establish  their  character. 


THE  GNEISSOID  GRANITES. 


The  gneissoid  granites  north  of  the  fragmental  tongue,  and  those  of 
the  two  areas  surrounded  by  the  sedimentary  rocks,  are  mediumly  coarse 
aggregates  of  a  dark-red  feldspar,  white  quartz,  and  a  dirty  green  chloritic 
substance.  The  red  feldspar  is  in  excess,  sometimes  to  the  exclusion  of  the 
other  components,  when  the  hand  specimen  resembles  a  dense  red  felsite. 
Almost  all  specimens  are  gneissoid.  The  constituents  are  usually  lenticu- 
lar, but  in  a  few  specimens,  particularly  those  taken  from  near  the  contacts 
with  the  sedimentary  rocks,  they  are  drawn  out  into  long  slender  string-like 
masses,  giving  the  specimens  a  streaked  appearance. 

The  microscopical  features  of  all  the  granites  are  those  common  to  these 
rocks  elsewhere  in  the  Basement  Complex.     They  consist  of  clouded  ortho- 


464  THE  CRYSTAL  FALLS  lEON-BEAEING  DISTEICT. 

clase,  some  plagioclase  and  a  little  microcline,  quartz  in  varying  quantity, 
and  more  or  less  green  chlorite  that  seems  to  have  been  derived  from 
biotite.  All  the  constituents  present  abundant  evidence  of  the  efFects  of 
pressure.  In  the  least-crushed  rocks  the  quartz  shows  undulatory  extinc- 
tions, and  the  feldspar  grains  granulation  around  their  edges.  As  the 
criTshing  action  increased,  the  granulation  increased,  so  that  the  most 
crushed  granites  now  consist  of  large  grains  of  feldspar  and  of  quartz  in  an 
aggregate  of  broken  fragments  of  orthoclase,  quartz,  plagioclase  and  micro- 
cline, and  a  few  wisps  of  green  chlorite.  Movement  in  the  crushed  rock 
mass  has  drawn  out  the  granulated  aggregate  between  the  large  grains  of 
feldspar  into  bands  and  lines,  thus  producing  the  schistose  structure  noted 
in  the  hand  specimens  and  in  the  ledges.  In  the  more  highly  schistose 
granites  a  considerable  quantity  of  new  microcline  and  a  small  quantity  of 
new  plagioclase  have  developed  within  the  granulated  aggregate,  and  in  a 
few  instances  muscovite  has  been  found  in  fairly  large  plates  of  pale-yellow 
color.  This  muscovite  occurs  on  the  contact  between  the  larger  quartz  and 
orthoclase  grains,  but  more  particularly  in  the  granulated  matrix. 

The  granites  in  the  area  between  the  Sturgeon  River  and  the  Felch 
Mountain  tongues  are  not  so  abundant  as  those  in  the  northern  area  of  Base- 
ment Complex  rocks,  or  in  the  areas  surrounded  by  the  sediments,  but  in 
their  essential  features  they  are  identical  with  these.  Occasionally  the  sur- 
face of  a  fresh  fracture  through  these  southern  granites  shows  the  outlines 
of  porphyritic  orthoclase  crystals,  but  these  crystals  are  riot  sufficiently 
numerous  to  impart  a  porphyritic  aspect  to  the  rock. 

Some  of  the  granite  specimens  examined  from  this  district  are  so  com- 
pletely granulated  that  they  can  with  difficulty  be  distinguished  in  the  hand 
specimens  from  the  schistose  arkoses  near  the  base  of  the  fragmental  series. 
In  thin  section  they  differ  from  the  latter  in  containing  no  rounded  quartz 
grains  and  in  the  possession  of  very  little  niica.  The  feldspathic  constitu- 
ents are  nearly  all  decomposed,  and  very  much  of  the  quartz  present  in  the 
granites  is  of  secondary  origin. 

Thus  in  all  essential  respects  the  gneissoid  granites  of  this  district  are 
like  those  in  the  Marquette  district  elsewhere  described.^ 


'The  Marquette  iron-bearing  district  of  Michigan,  hy  C.  E.  Van  Hise  and  W.  S.  Bayley,  with  a 
■chapter  on  the  Republic  trough,  by  H.  L.  Smyth :  Mon.  U.  S.  Geol.  Survey,  Vol.  XXVIII,  1897,  pp. 
171-176. 


BASEMENT  COMPLEX  OP  STURGEON  EIVEE  TONGUE.  465 

THE  AMPHIBOLE-SCHISTS. 

In  additiou  to  gneissoid  granites,  the  southern  area  of  the  Basement 
Complex  contams  a  number  of  ledges  of  dai-k-colored  schistose  rocks. 
These,  in  some  instances,  are  cut  by  dikes  of  granite  similar  to  the  granite 
already  described. 

These  dark  schists  may  be  classed  as  greenstone-schists  and  as  horn- 
blende-schists. The  former  are  heavy  rocks,  with  dull  greenish-gray  luster 
and  distinct  schistose  structure.  They  resemble  closely  in  their  micro- 
scopic as  well  as  in  their  macroscopic  features  the  schistose  dike  greenstones 
to  be  referred  to  later.  They  are  doubtless  altered  or  squeezed  diabases  or 
gabbros. 

The  hornblende-schists  are  usually  line-grained,  bluish-black  rocks, 
with  a  very  even  schistosity,  closely  resembling  slaty  cleavage.  On  the 
surfaces  of  cross  fractures  may  be  seen  long  slender  prisms  of  glistening 
black  hornblende  aiTanged  in  as  distinct  lines  as  the  lines  of  particles  in  an 
evenly  bedded  sedimentary  rock.  Often  the  cleavage  surfaces  are  coated 
with  thin  layers  of  golden-yellow  mica  scales.  In  most  specimens  there 
may  also  be  noticed  a  fine  banding  parallel  to  the  foliation. 

In  thin  section  these  hornblende-scliists  differ  from  the  schistose  dike 
diabases  and  from  the  greenstone-schists,  referred  to  above,  in  the  presence 
of  large  quantities  of  quartz,  and  of  some  biotite,  and  to  some  extent  in 
structure.  The  greenstones  owe  their  schistosity  to  the  flattening  of  their 
components,  while  in  the  hornblende-schists  this  structure  appears  to  be  due 
largely  to  the  crystallization  of  the  hornblende  in  elongated  prisms  with 
their  major  axes  parallel.  The  parallel  arrangement  of  the  amphibole  in 
the  latter  rocks  is  thus  much  more  pronounced  than  in  the  schistose 
greenstones. 

The  hornblende-schists  are  comjDosed  of  hornblende,  quartz,  biotite, 
plagioclase,  magnetite,  and  sphene. 

The  hornblende  is  in  long  prisms  of  the  usual  yellowish  green  color. 
The  mineral  is  compact,  but  it  is  full  of  inclusions  of  quartz  grains  similar  to 
those  constituting  a  large  part  of  the  matrix  lying  between  the  amphiboles. 
It  was  evidently  formed  in  situ  as  an  original  crystallization,  and  not,  like 
much  of  the  hornblende  of  the  schistose  greenstones,  by  the  alteration  of 
augite  or  of  some  other  component  of  a  basic  crystalline  rock.     The  biotite 

MON  XXXVI 30 


466  THE  CRYSTAL  FALLS  IRON  BEARING  DISTRICT. 

is  in  small  dark  greenisli-brown  flakes  interspersed  between  quartz  and 
feldspar  grains,  wliich  together  constitute  a  matrix  surrounding  the  other 
components.  The  quartz  is  unusually  free  from  inclusions.  It  contains  a 
few  liquid  inclosures  and  occasionally  a  few  flakes  of  biotite  and  needles  of 
hornblende.  The  plagioclase,  where  present,  is  in  irregular  grains  with 
ragged  outlines,  as  though  a  newly  formed  mineral.  It  apjjears  to  act  the 
part  of  a  cement  surrounding  the  other  minerals  with  which  it  is  in  contact. 
Small  round  grains  of  sphene  and  magnetite  occur  very  abundantly  scat- 
tered through  the  matrix.  Often  the  magnetites  are  surrounded  by  borders 
of  sphene ;  hence  it  is  probable  that  this  mineral  is  a  titaniferous  variety  and 
that  the  round  grains  of  sphene  are  pseudomorphs  after  mag'netite  grains 
that  have  been  completely  altered. 

In  a  few  specimens  large  colorless  areas  with  the  outlines  of  porphy- 
ritic  crystals  are  observed  in  the  midst  of  the  finer-grained  groundmass  of 
schist.  Between  crossed  nicols  these  break  up  into  a  coarse-grained  aggre- 
gate composed  of  the  same  minerals  that  constitute  the  rest  of  the  rock, 
except  that  in  it  altered  plagioclase  is  common  and  amphibole  is  rare. 
These  probably  represent  phenocrysts  of  plagioclase  which  have  suffered 
alteration  into  quartz  and  new  plagioclase  that  may  differ  somewhat  from 
the  feldspar  of  the  original  crystal. 

The  banding  of  some  of  the  hornblende-schists  has  already  been 
referred  to.  Under  the  microscope  the  only  differences  noted  in  the  bands 
are  the  quantity  of  hornblende  present  in  them  and  a  variation  in  the 
coarseness  of  grain.  The  coarsest  of  the  bands  have  the  composition  and 
structure  of  the  schistose  greenstones.  They  contain  large  quantities  of 
plagioclase,  both  fresh  and  altered,  and  large  grains  of  hornblende  that  are 
not  in  the  definite  prismatic  form  characteristic  of  this  mineral  in  the  main 
mass  of  the  rocks. 

OEIGIN   OF   THE   AMPHIBOLE-SCHISTS. 

From  the  gradations  often  observed  between  the  hornblende-schists 
and  the  greenstone-schists,  it  is  plain  that  the  two  rocks  are  genetically 
related.  The  latter,  from  their  similarity  to  schistose  dike  greenstones  in 
composition  and  structure,  are  believed  to  have  been  derived  from  massive 
diabases  or  gabbros.  The  hornblende-schists  are  in  all  probability  derived 
.from  similar  basic  rocks,  though  the  presence  in  them  of  what  appears  to 


BASEMENT  COMPLEX  OF  STURGEON  EIVEK  TONGUE.  467 

have  once  been  plagioclase  pheiiociysts  may  indicate  that  the  original  rocks 
were  in  the  form  of  hxvas. 

The  principal  difference  between  the  hornblende-schists  and  tlie 
greenstone-schists  seems  to  be  in  the  nature  of  the  amphibole  in  the  two 
rocks  and  in  the  presence  of  quartz  and  newly  formed  plagioclase  in  the 
first  named.  The  materials  of  the  greenstone-schist  were  derived  from  the 
alteration  of  those  of  the  original  rock,  as  were  also  those  of  the  hornblende- 
schist,  but  the  former  now  consist  mainly  of  the  direct  jjroducts  of  this 
alteration,  whereas  in  the  latter  the  substances  now  existing  liaA^e  been 
worked  over  and  entirely  recrystallized. 

THE  BIOTITE-SCHISTS. 

Mica-schists  are  not  common  in  the  Sturgeon  River  tongue.  They 
constitute  by  no  means  so  large  a  part  of  the  Basement  Comj)lex  in  this  dis- 
trict as  they  do  in  the  other  portions  of  the  Lake  Superior  region  that  have 
been  studied.  Indeed,  only  a  few  ledges  of  this  rock  have  been  observed 
in  the  country  between  the  Sturgeon  River  and  the  Felch  Mountain  sedi- 
mentary tongues,  and  most  of  these  are  along  the  southern  edge  of  a  g'reen- 
stone  knob  300  to  400  paces  north  of  the  southeast  corner  of  sec.  17, 
T.  42  N.,  R.  28  W. 

The  mass  of  this  knob  is  a  dark  hornblende-schist.  On  the  south  side 
of  the  top  of  the  knob  this  rock  is  in  contact  with  a  A^ery  evenly  banded  or 
streaked  rock  of  a  general  dark-gray  color.  In  the  hand  specimen  it  resem- 
bles very  closely  a  fine-grained  banded  aug-en-gneiss.  Near  its  contact 
with  the  hornblende-schist  the  rock  is  apparently  porphyritic,  with  pheno- 
crysts  of  feldspar  from  1  to  15  mm.  in  length,  and  an  occasional  one  of 
quartz  scattered  through  a  matrix  composed  of  narrow  altei-nating  bands  of 
almost  black  and  light-gray  material.  On  cross  fractures  of  the  rock 
the  phenocrysts  are  seen  to  •  be  drawn  out  in  the  direction  of  the  bands. 
Cleavage  takes  place  verj'-  readily  along  the  j^lanes  of  the  banding,  yielding- 
siTrfaces  covered  with  tiny  scales  of  black  biotite.  A  little  farther  from  the 
contact  the  light-colored  bands  are  thicker  and  more  distinct.  At  first 
glance  they  appear  to  be  uniformly  thick  for  long  distances,  but  a  more 
careful  inspection  shows  that  they  wedge  out  rapidly  and  are  replaced  by 
other  bands  of  the  same  character.     The  dark  bands  are  not  thicker  than 


468  THE  CRYSTAL  FALLS  IRON-BEARmG  DISTRICT. 

sheets  of  paper.  They  are  the  cross  sections  of  the  mica  coatings  on  the 
cleavage  planes. 

The  inspection  of  this  rock  in  the  hand  specimen  and  in  the  ledge 
leads  to  the  same  conclusion — that  it  is  an  intermediate  or  an  acid  lava,  a 
porphyrite,  or  a  porphyry  that  was  squeezed  until  it  became  schistose  and 
sheared  until  it  became  fissile. 

Under  the  microscope  the  feldspar  phenocrysts,  though  much  decom- 
posed and  filled  with  inclusions  of  quartz,  muscovite,  and  other  decomposi- 
tion products,  are  well  enough  preserved  to  exhibit  in  some  places  twinning 
striations.  The  greater  portion  of  the  phenocrysts  are  untwinned.  The 
twinned  material  borders  the  grains,  fills  in  cracks  between  Carlsbad  twins, 
and  is  irregidarly  distributed  through  the  untwinned  material,  occurring 
more  particularly  in  those  places  where  the  decomposition  of  the  original 
feldspar  is  most  complete.  The  twinned  feldspar  is  fresher  than  the 
untwinned  variety.  This  fact  and  the  manner  of  its  distribution  indicate  a 
secondary  origin  for  it.  The  quartz  phenocrysts  are  rare.  They  present 
their  usual  characteristics. 

The  groundmass  in  which  the  phenocrysts  lie  is  a  fine-grained  aggTe- 
gate  of  biotite,  quartz,  and  plagioclase.  The  biotite  is  a  greenish-brown 
variety.  It  occurs  in  large  plates  arranged  in  parallel  position  and  in 
small  flakes  occupying  the  same  parallel  position  and  lying  between  the 
quartz  and  the  plagioclase  grains.  The  banding  noticed  in  the  hand  speci- 
men is  due  to  the  arrangement  of  the  large  biotite  flakes  in  bands.  These 
are  separated  from  each  other  by  bands  of  quartz  and  plagioclase  that  are 
free  from  the  large  biotites,  though  they  contain  innumerable  small  flakes 
of  this  mineral.  Only  when  a  porphyritic  crystal  lies  in  the  way  of  tlie 
bands  do  these  depart  from  their  uniform  directions.  Here  they  bend 
around  the  phenocrysts,  leaving  on  both  sides  of  them  little  triangular  areas 
in  which  the  components  are  much  finer  grained  than  elsewhere  in  the  rock. 

The  light-colored  components  are  quartz  and  plagioclase.  These  min- 
erals are  in  small  grains  that  appear  to  be  intercrystallized  in  the  manner 
of  the  secondary  aggregate  that  constitutes  the  fine-grained  matrix  of  many 
greenstones,  of  the  aporhyolites,  and  of  other  rocks  that  have  sufi"ered 
intense  metamorphism.  The  quartzes  are  nearly  always  crossed  by  strain 
shadows  and  the  fresh  clear  plagioclase  by  interrupted  and  bent  twinning 
bars. 


BASEMENT  COMPLEX  OF  STURGEON  RIVEE  TONGUE.    469 

Here  and  there  in  the  midst  of  this  fine-grained  groundmass  are  noticed 
lenticuhxr  and  k)iig'  narrow  aggregates  composed  of  grains  of  plagiochise 
that  are  much  larger  than  the  grains  of  this  mineral  occurring-  in  the  sur- 
rounding matrix.  They  look  as  though  they  might  be  the  crushed  remains 
of  what  were  originally  plagioclase  phenocrysts. 

Thus  the  microscopic  study  of  these  rocks  tends  to  confirm  the  results 
of  their  field  study.  They  were  probably  porphyritic  lavas  or  intercalated 
flows  that  have  sufl'ered  alteration  as  the  result  of  intense  pressure  and 
movement.  Their  present  composition  suggests  that  they  were  oi'Iginally 
quartz-porphyries  or  perhaps  andesitic  porphyrites.  Whatever  their  original 
nature,  their  origin  is  different  from  that  of  the  biotite-schists  of  the  Mar- 
quette district.-' 

THE    INTRUSIVE    ROCKS. 

The  intrusives  in  the  schists  and  gneissoid  granites  of  the  Basement 
Complex  are  granites,  identical  with  the  gneissoid  granites  above  described, 
and  greenstones.  The  former  cut  only  the  schists.  They  are  probably 
apophyses  from  the  larger  granite  masses.  The  greenstones  cut  the  schists 
and  the  granites.  They  are  similar  in  all  respects  to  the  greenstones  in  the 
sedimentary  series,  and  thus  are  the  youngest  rocks  in  the  district,  with 
the  exception  of  the  horizontal  Paleozoic  sandstones  and  limestones  that 
cap  some  of  the  higher  hills. 

The  greenstones  are  all  more  or  less  altered  diabases.  In  some  the 
ophitic  structure  may  be  detected,  but  in  most  of  them  no  traces  of  their 
original  constituents  nor  of  their  structure  remain.  Nearly  all  are  more  or 
less  schistose.  The  only  evidence  that  the  most  schistose  phases  were  once 
massive  igneous  rocks  is  in  their  composition  and  their  occurrence  in  dike- 
like fissures.  As  the  schistosity  of  these  greenstones  increases,  the  amount 
of  their  altei'ation  also  increases;  there  is  a  greater  abundance  of  horn- 
blende present  in  them  and  a  greater  quantity  of  quartz,  until  in  the  most 
schistose  phases  the  rocks  are  now  typical  hornblende-schists. 

One  of  the  best  examples  of  these  greenstones  occurs  in  the  series  of 
ledges  extending  in  nearly  a  straight  line  for  6  miles  from  the  southern 
portion  of  sec.  13,  T.  42  N.,  R.  29  W.,  to  the  northeast  corner  of  sec.  14, 
T.  42  N.,  R.  28  W.     Except  in  its  eastern  ledges  the  rock  constitutes  bold, 

'  Mou.  U.  S.  Geol.  Survey,  Vol.  XXVIII,  pp.  200-203. 


470  THE  CRYSTAL  FALLS  lEON-BEARING  DISTRICT. 

rounded,  bare  knobs  with  ahnost  perpendicular  sides,  usually  situated  in 
the  midst  of  swamps.  The  main  mass  of  the  knobs  is  a  rather  fine-grained, 
slightly  schistose,  gray  rock  exhibiting  the  diabasic  structure  on  weathered 
surfaces.  On  the  south  sides  of  the  knobs  the  rock  is  much  denser,  and  in 
most  cases  is  much  more  highly  schistose  than  the  main  rock  mass. 

Under  the  microscope  these  rock's  present  the  usual  features  of  schistose 
dike  greenstones.  They  consist  almost  exclusively  of  hornblende,  plagio- 
clase,  and  quartz.  The  hornblende,  which  is  the  common  yellowish-green 
variety,  occurs  in  long  plates  and  in  columnar  crystals,  some  of  which  are 
idiomorphic  in  cross  section,  and  also  in  slender  needles  penetrating  the 
quartz  and  feldspar.  These  two  minerals  form  an  aggregate  between  the 
larger  hornblendes.  The  feldspar  is  mainly  a  calcium-soda  plagioclase, 
though  a  small  quantity  of  albite  may  also  be  present.  It  occurs  as  irreg- 
ular grains  embedded  in  a  mosaic  composed  of  rounded  grains  of  the  same 
feldspar  and  of  quartz,  and  appears  to  be  a  new  crystallization  subsequent 
to  that  of  the  greater  portion  of  the  plagioclase.  At  any  rate,  a  single 
large  grain  often  fills  the  interstices  between  numbers  of  the  mosaic  grains 
and  extinguishes  uniformly  over  large  areas.  The  magnetite  in  the  rock  is 
titaniferous.  It  occurs  in  little  crystals  and  in  small  irregular  grains  that 
are  often  surrounded  by  a  granular  zone  of  leucoxene. 

This  rock  may  serve  as  a  type  of  nearly  all  the  other  dike  greenstones 
in  the  district  under  discussion.  Some  may  be  more  schistose  than  this  one, 
while  a  few  ma}^  be  more  massive,  but  in  general  characteristics  they  are  all 
similar.  The  more  schistose  rocks  diff'er  from  the  less  schistose  varieties 
simply  in  the  possession  of  a  greater  amount  of  quartz  and  a  greater  quantity 
of  what  appears  to  be  newly  formed  feldspar.  Their  greater  schistosity  is 
due  to  the  more  uniform  elongation  of  their  components. 

The  fine-grained  greenstones  found  on  the  edges  of  the  coarser-grained 
ones,  and  occasionally  as  independent  dikes,  are  weathered  diabases  of  the 
normal  type. 

COMPARISON  OF  THE  STURGEON  RIVER  AND  THE  MARQUETTE  CRYSTALLINE 

SERIES. 

The  Basement  Complex  in  this  area  is  essentially  like  that  in  the  Mar- 
quette district,  except  that  the  altered  tuffs  so  abundant  in  the  northern  area 
are  absent  from  that  now  under  discussion.     The  biotite-schists  of  the  two 


ALGONKIAN  KOCKS  OF  STURGEON  EIVBR  TONGUE.  471 

areas  seem  also  to  be  different  in  origin,  althoug-h  this  can  not  be  stated 
with  certainty,  since  the  origin  of  the  Marquette  schists  is  not  so  clear  as  is 
that  of  the  Sturgeon  River  schists.  There  is  enough  similarity  between  the 
crystalline  series  in  the  two  areas  to  leave  no  doubt  as  to  their  practical 
identity.  If  the  Marquette  Basement  Complex  is  Archean,  the  crystalline 
series  underlying  the  conglomerates  in  the  Sturgeon  River  tongue  is  also 
Archean. 

THE  ALGONKIAX  TROUGH. 

The  sedimentary  rocks  comprised  within  the  Sturgeon  River  Algonkian 
tongue  may  be  separated  into  a  conglomerate  series  and  a  dolomite  series. 
The  conglomerate  series  consists  of  schistose  conglomerates,  arkoses,  qnartz- 
ites,  slates,  and  certain  sericitic  schists  that  are  squeezed  arkoses.  The 
dolomite  series  embraces  crystalline  dolomites,  a  few  thin  beds  of  quartz- 
ite,  a  few  breccias  and  conglomerates,  and  some  slates. 

It  is  possible  that  a  third  series,  composed  essentially  of  slates,  also 
exists  in  the  district,  but  if  so  it  is  not  advisable  to  separate  it  from  the 
dolomite  series,  since  its  exposures  are  very  few  in  number,  and  the  slates 
which  comprise  its  main  mass  are  so  nearly  like  the  slates  belonging  in  the 
dolomite  series  that  they  can  with  difficulty  be  distinguished  from  these. 

Associated  with  the  sedimentary  rocks  are  great  inasses  of  basic 
igneous  ones.  Some  of  these  are  unquestionably  intrusive  masses,  as 
shown  by  their  relations  to  the  conglomerates,  while  others  appear  to  be 
interleaved  sheets.  A  very  few,  apparently  bedded  greenstones,  on  close 
examination  seem  to  be  composed  of  intermingled  sedimentary  and  igneous 
material.     These  may  be  altered  tuffs. 

Nearly  all  the  sedimentary  as  well  as  the  igneous  rocks  embraced  in 
the  trough  are  schistose,  and  thus  are  sharply  distinguished  from  the  brown 
Potsdam  sandstones  and  the  Silurian  limestones  that  here  and  there  lie 
approximately  horizontal  on  their  upturned  edges.  The  squeezing  of  the 
pre-Potsdam  formations  has  been  so  intense  that  both  conglomerates  and 
dolomites  have  been  forced  into  closely  appressed  folds,  which  in  the  con- 
glomerates are  for  the  most  part  apparently  isoclinal.  The  strike  of  the 
latter  rocks  is  nearly  east  and  west,  and  their  dip  nearly  perpendicular, 
except  in  one  or  two  cases.  The  dolomites  are  less  closely  folded  than  the 
conglomerates.     Their  dips  are  mucli  less   steep,    and  their  strike  varies 


472  THE  CRYSTAL  FALLS  IROJS[-BEAEING  DISTRICT. 

considerably,  except  in  the  narrow  eastern  portion  of  the  tongue,  where 
it  is  approximately  parallel  to  that  of  the  conglomerate,  i.  e.,  a  few 
degrees  north  of  east. 

RELATIONS  BETWEEN  THE  CONGLOMERATE  AND  THE  DOLOMITE  SERIES 
AND  CORRELATION  WITH  THE  FELCH  MOUNTAIN  FRAGMENTALS. 

The  relations  of  the  conglomerates  to  the  dolomites  are  best  shown 
by  the  distribution  of  their  respective  outcrops,  as  members  of  the  two 
series  are  nowhere  in  contact.  In  the  central  portion  of  the  tongue  the 
conglomerate  outcrops  are  limited  to  the  district  between  the  central  granites 
and  the  southern  area  of  the  Basement  Complex.  The  dolomites,  on  the 
other  hand,  are  limited  to  the  country  north  of  the  central  granite.  Its 
outcrops  are  found  scattered  over  the  northern  tier  of  sections  in  T.  42  N., 
Rs.  28  W.  and  29  W.,  and  the  southern  tier  of  sections  in  T.  43  N.,  Rs.  28  W. 
and  29  W.  Between  them  and  the  granite  to  the  north  is  a  belt  of  country 
devoid  of  exposures.  It  is  heavily  drift  covered,  consisting  of  sand  plains 
and  sand  hills,  from  beneath  which  no  ledges  of  any  kind  protrude.  This 
barren  belt  measures  about  a  half  mile  in  width,  in  sec.  2,  T.  42  N.,  R.  28  W., 
gradually  increasing  in  width  till  it  reaches  the  center  of  sec.  1  in  T.  -tS  N., 
R.  29  W.,  where  it  opens  out  into  the  large  Pleistocene  area  whose  southeast 
edge  is  shown  on  the  map  (PI.  LI).  In  the  eastern  portion  of  the  district 
the  northern  granites  and  the  conglomei'ates  approach  each  other,  and  the 
dolomite  belt  becomes  very  narrow,  finally  disappearing  toward  the  east 
side  of  T.  42  N.,  R.  28  W. 

The  relative  distribution  of  the  conglomerate  and  dolomite  ledges, 
when  considered  with  reference  to  the  triangular  outline  of  the  area 
embraced  between  the  northern  and  the  southern  granite-schist  complexes, 
suggests  that  the  two  formations  constitute  a  western-pitching  syncline  with 
the  dolomite  in  the  center  and  the  conglomerates  with  their  associated  beds 
on  the  two  flanks.  The  conglomei'ates  comprising  the  southern  flank  are 
well  exposed,  but  those  of  the  northern  flank  are  not  seen.  They  are 
believed  to  underlie  the  glacial  deposits  in  the  barren  strip  of  country 
bordering  the  northern  granites.  The  conglomerates,  according  to  this 
view,  are  older  than  the  dolomites. 

Toward  the  center  of  the  dolomite  area,  in  the  north  half  of  sec.  6, 
T.  42  N.,  R.  28  W.,  and  at  a  few  places  farther  west,  there  are  ferruginous 


ALGONKIAN  EOGKS  OF  STURGEON  ElVBR  TONGUE.  473 

beds  in  the  dolomite  series.  If  these  represent  the  upper  portion  of  the 
dolomite  formation,  as  is  the  case  with  similar  rocks  in  the  Felch  Mountain 
range,  it  is  clear  that  as  we  approach  the  center  of  the  Sturgeon  River 
tongue  the  rock  beds  met  with  are  younger  than  those  on  its  borders.  This 
is  in  line  with  the  supposition  that  the  Sturgeon  River  tongue  is  a  westward- 
pitching  syncline. 

The  belief  that  the  conglomerates  are  beneath  the  dolomites  in  the 
Sturgeon  River  area  is  further  strengthened  by  the  fact  that  the  jirincipal 
conglomerate  in  the  Felch  Mountain  range  is  benea.th  a  dolomite  which  is 
identical  in  character  with  the  Sturgeon  River  dolomite.  This  conglomerate 
is  regarded  as  the  base  of  the  Lower  Menominee  series  in  this  district,  with 
the  dolomite  above  it,  known  as  the  Randville  dolomite,  immediately  suc- 
ceeding it.  If  the  conglomerates  and  dolomites  in  the  two  districts  are  the 
same,  the  Sturgeon  River  rocks  are  Lower  Menominee. 

RELATIONS  BETWEEN  THE  DOLOMITES  AND  CONGLOMERATES  AND  THE 

OVERLYING  SANDSTONES. 

At  several  places  the  conglomerates-  and  dolomites  are  overlain  hy 
well-defined  Lake  Superior  sandstone.  The  sandstone  usually  caps  hills, 
on  the  lower  slopes  of  which  ledges  of  the  underlying  rocks  appear.  The 
contacts  between  the  overlying  sandstone  and  the  underlying  rocks  are 
rarely  seen,  but  the  fact  that  the  former  are  always  horizontal,  while  the 
latter  are  always  very  steeply  inclined,  leaves  no  doubt  that  there  is  a 
strong  unconformity  between  them. 

THE  CONGLOMERATE  FORMATION. 

The  conglomerate  formation  comprises  very  much  squeezed  granitic 
conglomerates,  arkoses,  sericite-schists,  quartzites,  a  few  beds  of  banded 
rocks  believed  to  consist  largely  of  tuffaceous  material  (see  pp.  486-487), 
and  occasional  beds  of  slates.  Nearly  all  the  members  of  the  series  are 
schistose,  the  arkoses  in  some  cases  passing  into  very  well  characterized 
sericite-schists.  Occasionally  the  arkoses  show  obscure  traces  of  ripple 
marking,  and  more  frequently  very  well  defined  cross  bedding. 

All  the  rocks  of  this  formation  strike  in  a  nearly  uniform  direction,  N. 
76°-84°  E.,  and  dip  almost  vertically.  In  one  or  two  instances  observed 
the  dip  is  as  low  as  65°,  but  in  most  cases  it  varies  between  85°  N.  and 
85°  S.     The  strike  of  the  schistosity  is  approximately  parallel  to  the  strike 


474  THE  CEYSTAL  PALLS  IRON-BE ARIKG  DISTRICT. 

of  the  bedding,  as  is  also  the  direction  of  the  elongation  of  the  pebbles  so 
abundant  in  the  conglomeratic  layers. 

From  the  slight  changes  iii  dip  observed  in  the  beds,  as  well  as  the 
great  width  of  the  formation  in  some  places,  it  is  evident  that  folding  must 
exist.  It  is  probable  that  in  the  wider  portions  of  the  area  occupied  by 
these  rocks  there  are  present  two  or  more  folds,  so  closely  appressed  that 
the  beds  on  the  opposite  limbs  can  not  be  correlated.  Hence  they  appear 
as  members  of  a  consecutive  series  of  conformable  members  with  a  nearly 
uniform  dip  throughout.  In  the  narrower  portions  of  the  area  it  can  not 
be  told  whether  more  than  one  fold  is  loresent  or  not.  In  any  event,  the 
folding  is  practically  isoclinal. 

The  ledges  of  the  conglomerates  and  their  associated  beds  occur  in  the 
southern  portion  of  the  Sturgeon  River  tongue  throughout  its  entire  extent. 
No  exposures  have  been  found  north  of  the  granite  areas  in  the  central  and 
western  portions  of  the  tongue. 

IMPOKTANT   EXPOSURES. 

The  arkoses,  the  sericite-schists,  and  the  conglomeratic  phases  of  the 
series  can  be  best  studied  at  the  dam  of  the  Sturgeon  River  near  the  north- 
west corner  of  sec.  17,  T.  42  N.,  R.  28  W.  Here  they  form  a  continuous 
ledge  of  well-bedded  layers  striking  N.  83°  E.,  and  dipping  85°  S.,  which 
measures  at  least  250  yards  in  width  and  400  yards  in  length.  (See  PI.  LII.) 
The  conglomerates  are  pink  in  color.  They  contain  immense  numbers  of 
white  quartz  pebbles  and  bowlders,  fewer  and  smaller  ones  of  pink  granite, 
and  many  fragments  of  red  feldspar  in  a  matrix  composed  of  moderately 
coarse  granite  debris.  All  the  fragments  and  pebbles  in  these  rocks,  as  well 
as  then-  matrix,  show  plainly  the  eflPects  of  pressure  (PL  LIII.)  The  matrix  of 
all  specimens  is  more  or  less  schistose,  and  the  coarse  sand  grains  embedded 
in  it  are  in  many  cases  elongated  in  the  direction  of  the  schistosity.  Most  of 
the  pebbles  and  bowlders  in  the  conglomerate  are  also  flat  and  parallel  to 
the  schistose  plane.  How  far  these  phenomena  are  due  to  mashing,  to  rota- 
tion into  parallel  positions  during  flattening,  and  to  original  sedimentation, 
respectively,  can  not  be  determined  in  most  cases,  since  the  schistosity  of 
the  rock  and  the  elongation  of  the  pebbles  are  both  approximately  parallel 
to  the  bedding — i  e.,  the  pebbles  are  nearlj^  in  the  positions  assumed  by 
unequidimensional    pebbles    in   a  well-bedded    conglomerate.      In  a  few 


us  GEOLOGICAL   SURVEY 


MONOGRAPH  XXXVl  PL  III 


n  \ 


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mW^    ^      '^ 


Mm>^^'f 


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JULIUS  BiEN  aco.N.r 


MAP  OF  EXPOSURES 

IN  SEC. 7  AND  IN  PORTIONS  OF  SECS.  8, 17,  AND  18,  T.  42  N.,P.  28  W.,  M  ICH  I  GAN 

VICINITY  OF  DAM  ON  EAST  BRANCH  OF  STURGEON  RIVER 

I  A.  Arkose 
SI.  Slate 
Cj.    Quartzite  INTRUSIVE 

C.    Conglomerate 
G.T.  Greenstonetiiffaceous 


,,j  G.  Greenstone 

G . S . Greenstone  Schist 


ALGONKIAN  EOCKS  OP  STURGEON  EIVER  TONGUE.  475 

instances  the  scliistosity  may  be  seen  to  meet  the  bedding  at  a  very  acute 
angle.  In  this  case  the  pebbles  are  usually  arranged  with  their  longer  axes 
parallel  to  the  schistosity,  though  there  are  always  present  a  large  number 
that  lie  parallel  to  the  bedding  planes. 

In  the  least  schistose  phases  of  the  rocks  the  pebbles  are  nearly  round 
and  the  matrix  possesses  a  well-defined  fragmental  texture,  but  in  those 
beds  in  which  the  schistosity  is  more  pronounced  the  matrix  is  seiicitic  and 
the  pebbles  are  lenticular.  The  most  completely  schistose  jihases  resemble 
augen-gneisses.  In  these  the  matrix  is  an  almost  typical  sericite-schist. 
The  quartz  pebbles  have  been  crushed  and  flattened  into  long  narrow 
stringers  or  plates  of  quartz,  some  of  which  are  continuous  for  long  distances 
(6  or  7  inches),  while  others  are  broken  into  separate  parts,  which  when 
rounded  on  their  edges  yield  quartz  lerses  like  the  "augen"  of  so  many 
augen-gneisses.-' 

The  nonconglomeratic  beds  interstratified  with  the  conglomerates  are 
usually  more  completely  schistose  than  the  latter.  The  least  schistose  beds 
are  arkoses.  These  often  show  ripple  marking  and  current  bedding.  As 
the  schistosity  increases,  the  quantity  of  sericite  present  also  increases,  until 
in  the  most  highly  schistose  phases  sericite-schists  result. 

Some  of  the  arkoses,  as  well  as  some  of  the  finer-grained  conglomerates, 
in  addition  to  being  schistose,  are  also  foliated — i.  e.,  they  are  built  up  of 
plates  or  leaves,  along  the  planes  between  which  they  split  very  easily. 
When  this  is  the  case,  the  cleavage  surfaces  are  covered  by  small  scales  of 
silvery  mica.  The  foliation  is  so  pronounced  in  many  cases  that  the  rocks 
are  almost  fissile. 

Besides  these  rocks  there  are  present  near  the  dam  great  ledges  of 
coarse  and  fine  grained  greenstone  (see  PL  LII),  whose  relations  to  the 
sedimentary  beds  at  first  glance  appear  to  be  those  of  interleaved  flows. 
Upon  close  inspection  some  of  these  masses  disclose  intrusive  features. 
Although  they  almost  invariably  follow  the  bedding  of  the  fragmental 
rocks,  some  of  the  greenstones  can  be  seen  to  cut  across  the  layers  in  such 
a  manner  as  to  leave  no  doubt  of  their  intrusive  character. 

'  The  best  examples  of  these  extremely  schistose  conglomerates  are  not  found  in  the  exposures 
referred  to  above,  but  they  are  well  developed  along  the  line  between  sees.  11  and  12,  T.  42  N.,  K.  29  W. 
Here  the  width  of  the  series  is  but  one-fourth  mile,  whereas  the  total  width  of  these  rocks  and  tbeir 
associated  greenstones  near  the  dam  measures  a  full  mile. 


476  THE  CEYSTAL  FALLS  IRON-BEARING  DISTRICT. 

On  the  old  road  leading  to  the  dam  the  conglomerates  and  arkoses  are 
intruded  by  an  altered  diabase  in  a  most  complex  way.  To  the  north  of 
the  road  is  the  great  mass  of  the  greenstone,  within  which  are  considerable 
areas  of  the  conglomerate.  Within  the  belt  of  conglomerate,  on  the  other 
hand,  are  several  bands  of  the  eruptive  rock  which  roughly  follow  the  bed- 
ding of  the  sedimentary  one,  but  which  cut  across  it  in  a  minor  way. 

At  the  contact  of  the  main  mass  of  greenstone  and  the  conglomerate 
are  numerous  interlaminations  of  the  two  rocks,  the  greenstone  having 
intruded  the  conglomerate  along-  its  bedding  planes.  At  one  place  a  dozen' 
alternations  of  the  two  were  noted  within  a  foot.  Moreover,  for  some 
distance  from  the  greenstone  the  conglomerate  appears  to  be  impregnated 
with  material  from  the  intrusive,  so  that  it  has  taken  on  a  greenish  tinge. 
This  impregnation  in  one  instance  has  gone  on  so  far  as  to  produce  what  is 
apparently  a  greenstone  matrix  containing  separate  pebbles  from  the  con- 
glomerate, the  groundmass  of  the  conglomerate  having  apparently  been 
absorbed.  The  greenstone  adjacent  to  the  conglomerate  is  traversed  by 
narrow  pegmatite  veins  in  various  directions,  some  of  the  lai'gest  being  not 
more  than  2  inches  in  width.  There  is  no  evidence  of  a  granitic  intrusion, 
the  pegmatites  appearing  clearly  to  be  the  result  of  an  interaction  between 
the  basic  igneous  rock  and  the  more  acid  fragmental  one.  At  one  place 
along  the  contact  there  is  a  belt  of  very  coarse  hornblendic  material  that  is 
cut  through  and  tlirough  by  the  pegmatite  veins. 

East  and  west  of  the  dam  for  some  distance  are  other  ledges  of  con- 
glomerate. They,  however,  as  a  rule,  present  no  features  different  from 
those  exhibited  by  the  great  ledge  described  above.  In  all,  especially  in 
those  occurring  in  sees.  9  and  10,  T.  42  N.,  R.  28  W.,  the  interbanding  of 
conglomeratic  and  nonconglomeratic  layers  is  beautifully  shown. 

Near  the  north  quarter  post  of  sec.  11,  T.  42  N.,  R.  28  W.,  the  arkoses 
have  a  purple  rather  than  a  pink  tinge.  On  cross  fractures  they  are  seen  to 
be  spangled  with  glistening  black  needles  and  plates  of  hornblende,  which 
lie  with  their  long  axes  in  all  azimuths.  The  little  crystals  appear  to  be 
more  abundant  in  some  layers  than  in  others. 

The  best  exposures  of  quartzite  are  found  near  the  north  quarter  post 
of  sec.  11,  T.  42  N.,  R.  28  W.,  and  at  1,300  paces  W.,  150  N.,  of  the  south- 
east corner  of  sec.  7,  T.  42  N.,  R.  29  W.  The  rocks  are  black.  They  occur 
in  beds  varying  in  thickness  from  a  few  inches  to  several  feet. 


ALGONKIAI^  EOCKS  OF  STURGEON  EIVER  TONGUE.  477 

PETKOGRAPHICAL   DBSCEIPTIONS. 

As  might  naturiilly  be  expected,  the  least  schistose  of  the  arkoses  and 
conglomerates  exhil^it  the  fewest  evidences  of  alteration  in  the  thin  section. 
In  addition  to  the  pebbles  in  the  conglomerates,  these  rocks  consist  of 
ronnded  and  angular  grains  of  quartz,  microcline,  orthoclase,  and  of  various 
plagioclases,  and  a  few  of  microperthite,  embedded  in  a  finer-grained 
aggregate  of  the  same  minerals,  tiny  flakes  of  gi-een  biotite  and  of  color- 
less mnscovite  or  sericite,  a  few  plates  of  chlorite,  particles  and  crystals  of 
magnetite,  and  little  nests  and  isolated  grains  of  epidote,  with  occasionally 
some  calcite.  Many  of  the  feldspar  grains  are  altered  into  sericitic  products, 
colored  red  by  small  particles  of  various  iron  oxides  and  by  red  earthy 
substances. 

The  composition  and  microstructure  of  the  schistose  arkoses  and  of 
the  schistose  matrices  of  the  conglomerates  vary  greatly  in  different  speci- 
mens, being  determined  largely  by  the  original  composition  of  the  different 
beds  and  the  amount  of  squeezing  to  which  they  have  been  subjected.  No 
attempt  will  be  made  here  to  describe  in  detail  all  the  changes  suffered 
by  these  rocks;  a  simple  statement  of  the  tendency  of  these  changes  will 
be  given. 

The  quartz  pebbles  in  the  moderately  schistose  conglomerates  show 
plainly  that  they  have  been  under  great  stresses.  The  smaller  ones  all 
exhibit  undulatory  extinction.  The  larger  ones  are  sometimes  peripherally 
granulated,  and  sometimes  etched  or  con-oded  on  their  edges,  as  though 
they  had  suffered  partial  solution.  By  this  process  small  portions  of  the 
original  particles  have  been  separated  from  them,  and  the  dissolved  silica 
has  been  redeposited  among  the  grains  of  the  surrounding  matrix  as  sec- 
ondary quartz.  In  their  interiors  many  of  the  larger  pebbles  have  been 
changed  to  a  mosaic  of  differently  oriented  parts,  which  interlock  so  per- 
fectly that  they  appear  to  have  crystallized  together. 

The  groundmass  in  which  the  pebbles  lie  is,  in  a  few  cases,  a  frag- 
mental  aggregate  of  quartz  and  several  feldspars,  with  the  addition  of  seri- 
cite and  other  crystallized  components.  In  most  cases,  and  in  all  in  which 
schistosity  is  marked,  no  fragmental  structure  is  noticeable.  The  ground- 
mass  is  an  interlocking  mosaic  of  fairly  large  quartz  grains  that  appear  to 
have  crystallized  in  situ,  between  which  are  smaller  grains  of  the  same 


478  THE  CRYSTAL  FALLS  lEON-BBARING  DISTRICT. 

character,  large  and  small  spicules  and  plates  of  sericite,  crystals  of  magne- 
tite, and  a  few  needles  of  chlorite  and  other  secondary  substances.  Between 
these,  again,  is  often  a  cement  of  what  seems  to  be  secondary  quartz.  The 
schistosity  of  the  specimens  is  due  to  the  arrangement  of  the  sericite  in 
approximately  parallel  positions,  and  to  the  elongation  of  the  quartz  grains 
in  the  same  direction.  The  pink  color  of  the  rocks  is  produced  by  red 
earthy  substances  in  the  feldspars  and  in  their  decomposition  products. 

In  the  most  schistose  phases  of  the  conglomerates  the  quartz  pebbles 
have  been  mashed  into  plates,  several  of  which  join,  end  to  end,  forming 
sheets,  which  in  the  thin  section  appear  as  long  narrow  lines  of  variously 
oriented  quartz  grains,  each  of  which  is  crossed  by  strain  shadows. 

The  larger  quartz  grains  in  their  matrices  are  broken  into  parts,  and 
these  parts  are  differently  oriented  with  respect  to  one  another.  Other 
grains  seem  to  have  entirely  recrystallized,  for  they  are  now  made  up  exclu- 
sively of  the  same  kind  of  interlocking  quartzes  as  are  present  in  the  fine 
portions  of  the  groundmass  in  which  the  coarse  quartz  grains  are  embedded. 
In  the  groundmass  of  these  rocks  sericite  is  very  abundant,  and  feldspar  is 
rare.  From  the  proportions  of  these  minerals  present  it  would  appear  that  the 
former  has  been  derived  largely  from  the  latter.  Biotite  is  also  present  in 
many  specimens  as  small  green  flakes,  but  this  mineral  is  not  widely  spread. 

The  conclusion  from  the  study  of  the  thin  sections  of  the  schistose 
conglomerates  is  that  there  has  been  a  crystallization  of  new  substances, 
principally  quartz,  sericite,  biotite,  and  magnetite,  from  the  materials  of  the 
original  granitic  sediments.  Perhaps  a  portion  of  the  crystaUization  was 
the  result  of  alteration  of  the  original  components  before  squeezing  took 
place.  The  larger  portion,  however,  was  accomplished  under  the  influence 
of  pressure.  The  result  of  the  mashing  and  recrystallization  is  a  schist, 
which  between  crossed  nicols  has  the  aspect  of  a  typical  crystalline  schist, 
but  which  in  natural  light  exhibits  its  conglomeratic  nature  in  the  presence 
of  the  large  quartz  lenses,  with  the  outlines  of  flattened  pebbles,  in  a  fine- 
grained groundmass. 

The  pink  arkoses  differ  from  the  conglomerates  simply  in  the  absence 
from  them  of  the  pebbles.  The  schistose  varieties  are  similar  in  every 
respect  to  the  schistose  groundmass  of  the  squeezed  conglomerates.  Both 
in  the  hand  specimen  and  in  the  thin  section  the  schistose  arkoses  exhibit 
striking  resemblances  to  jnuscovitic  gneisses. 


ALGONKIAN  EOCKS  OF  STUKGEON  ElVEK  TONGUE.  479 

The  purple  arkoses  differ  from  the  pink  ones  just  described  in  contain- 
ing chlorite  and  hornblende,  and  in  addition  some  apparently  newly  formed 
feldspar,  notably  a  feldspar  with  the  microcline  twinning.  As  a  rule,  these 
rocks  are  more  feldspathic  than  the  matrices  of  the  conglomerates,  and  they 
contain  much  less  quartz.  The  larger  grains  of  both  quartz  and  feldspar 
are  corroded  as  if  partially  dissolved.  They  have  lost  their  smooth,  rounded 
contours  of  sand  grains,  and  now  possess  irregular  jagged  ones,  which, 
however,  are  not  due  to  secondary  enlargements. 

The  characteristic  components  of  these  rocks  are  the  chlorite  and  the 
hornblende.  The  former  mineral  is  present  in  plates  intermingled  with 
grains  of  epidote,  while  the  hornblende  is  in  dark-green  or  light-green 
plates,  and  in  acicular  or  columnar  crystals  that  are  idiomorphic  in  cross 
section.  The  crystals  are  distributed  indiscriminately  through  the  rocks, 
with  their  longer  axes  lying  in  all  azimuths.  They  were  evidently  formed 
after  the  squeezing  that  made  the  rock  schistose.  The  plates,  moreover, 
include  within  themselves  such  great  numbers  of  the  other  components  of 
the  rock  that  their  parts  often  appear  to  be  independent.  Under  crossed 
nicols,  however,  many  of  these  apparently  independent  plates  are  discov- 
ered to  polarize  together.  No  evidence  is  present  in  any  of  the  sections  as 
to  the  source  of  the  material  that  gave  rise  to  the  hornblende.  The  fact, 
however,  that  all  of  the  horublendic  rocks  are  banded,  that  some  layers  are 
rich  in  amphibole  while  others  are  completely  devoid  of  this  mineral,  sug- 
gests the  notion  that  the  hornblendic  schistose  arkoses  consist  partly  of 
sedimentary  and  partly  of  tuffaceous  materials.  As  we  shall  see  later,  this 
origin  is  ascribed  with  more  confidence  to  some  very  peculiar  rocks  to  be 
discussed  later. 

Crushing  effects  are  noticed  in  some  of  the  hornblendic  arkoses,  but 
their  present  condition  appears  to  be  due  more  to  chemical  changes  pro- 
duced in  them  than  by  mechanical  action.  The  chemical  changes  were  no 
doubt  superinduced  by  the  mashing,  but  this  can  only  be  inferred  from  the 
fact  that  they  are  more  pronounced  in  the  schistose  phases  of  the  rocks  than 
in  those  phases  in  which  the  schistosity  is  poorly  developed. 

THE    DOLOMITE    FORMATION. 

The  dolomite  formation  comprises,  as  has  been  stated,  both  dolomitic 
limestones  and  calcareous  slates,  and   occasionally  quartzites,  sandstones, 


480  THE  CEYSTAL  FALLS  IRON-BEAEING  DISTRICT. 

and  conglomeratic  and  brecciated  beds.  As  a  rule,  exposures  are  small  and 
scattered.  Their  distiibution  lias  already  been  described.  All  ledges 
observed  may  be  seen  by  reference  to  the  map  (PI.  LI). 

IMPORTANT   EXPOSURES. 

Good  exposures  of  the  dolomites  occur  in  the  NW.  J  sec.  6,  T.  42  N., 
R.  28  W.  The  ledge  nearest  the  northwest  corner  of  the  section  is  a  hard 
flesh-colored  dolomitic  marble,  containing  here  and  there  little  quartz 
grains.  This  is  cut  by  joints',  and  is  traversed  by  small  chert  bands.  The 
bedding  is  more  or  less  contorted,  but  its  general  strike  is  N.  45°  E.,  and 
its  dip  is  45°  NW.  About  one-fourth  mile  east  of  this  ledge  is  a  small,  bare 
knoll,  composed  of  interlaminated  pink  marbles,  conglomerates,  red  sand- 
stones, and  red  slates,  varying  in  thickness  from  a  few  inches  to  a  foot  or 
more.  The  conglomerate  consists  of  marble  pebbles  and  slate  and  chert 
fragments  in  a  calcareous  quartzitic  matrix.  The  strikes  and  dips  are  uni- 
foi-m  throughout  the  ledge,  the  former  being  nearly  east  and  west  and  the 
latter  45°  S.  The  difference  in  dip  of  the  beds  of  these  two  exposures 
indicates  plainly  the  presence  in  this  place  of  a  little  westward-pitching 
anticline. 

Other  prominent  exposures  of  the  dolomite  series  are  in  the  NW.  | 
sec.  1,  T.  42  N.,  R.  29  W.,  and  in  the  SE.  4  sec.  35,  T.  43  N.,  R.  29  W.  In 
the  first-named  locality  is  a  high,  bare  knob,  and  a  cluster  of  small  ledges, 
in  which  dolomites,  conglomerates,  and  slates  are  all  well  exposed.  The 
dolomites,  for  the  greater  part,  are  massive  pink  marbles  crossed  by  joint 
planes.  In  places  the  rocks  take  on  a  greenish-yellow  tinge,  and  become 
schistose.  At  1,500  paces  N.,  1,930  W.,  of  the  southeast  corner  of  sec. 
1,  T.  42  N.,  R.  29  W.,  the  dolomite  forms  a  well-defined  bed,  striking 
N.  45°  E.  and  dipping  70°  SE.  Above  this,  to  the  southeast,  is  a  bed  of 
coarse-grained  granitic  sandstone  or  quartzite,  which  in  turn  is  over-lain 
by  beds  of  gray  quartzite  alternating  with  thin  slates  and  fine-grained 
conglomerates.  Farther  south  is  a  ridge  of  well-bedded,  fine-grained 
quartzite  and  bluish-gray  slate,  the  individual  layers  being  usually  less 
than  one-half  inch  in  thickness.  This  rock  grades  into  a  gray  schistose 
dolomite,  and  the  whole  quartzite-slate  series  strikes  N.  75°  E.  and  dips 
63°  S.  The  exposures  in  section  35  are  almost  pure  marbles,  in  which  no 
traces  of  bedding  have  been  detected. 


ALGONKIAN  BOOKS  OF  STURGEON  EIVER  TONGCTB.  481 

I'ETliOGUAPHlCAL   DESCUIPTION. 

Ill  thill  section  tlie  iniirl:)les  appear  as  very  close-gTaiued  aggregates 
of  calcite  aud  dolomite,  usually  untwinned,  but  occasionally  twinned  in 
the  ordinary  manner  of  these  minerals.  Here  and  there  among  the  car- 
bonates are  rounded  quartz  grains,  but  the  greater  portion  of  this  min- 
eral appears  to  have  crystallized  in  situ  between  the  calcite  and  dolomite 
individuals. 

All  the  marbles  are  of  the  same  general  character.  They  diifer  only 
in  the  quantity  of  silica  present  and  in  the  presence  or  absence  of  the  tiny 
dust  grains  producing  the  color.  The  schistose  varieties  owe  their  schistos- 
ity  to  the  elongation  of  their  components. 

The  quartzites  and  slates  interbedded  with  the  marbles  possess  no 
unusual  characters.  They  are  similar  to  the  corresjjonding  rocks  inter- 
stratified  with  the  Marquette  dolomites.  The  conglomerates  interstratified 
with  the  dolomites,  slates,  and  quartzites  are  of  two  kinds.  One  is  com- 
posed of  marble  and  slate  fragments  cemented  by  quartzite,  and  the  other 
of  small  granite  pebbles  embedded  in  granite  sand.  The  latter  are  evi- 
dently composed  of  the  detritus  of  the  granites  underlying  the  dolomite 
series,  while  the  marble-bearing  conglomerates,  or  perhaps  more  ^jroperly 
breccias,  are  interformational  beds  conformable  with  the  beds  below  them, 
and  also  with  those  above.  They  are  similar  in  every  respect  to  the  inter- 
bedded breccias  in  the  Kona  dolomites  on  the  Marquette  range. 

SLATES   AND   SANDSTONES   ON   THE   STURGEON   RIVER. 

The  rocks  in  the  SW.  ^  sec.  34,  where  the  road  to  Sagola  crosses  the 
Sturgeon  River,  are  placed  in  the  dolomite  formation,  although  they  differ 
somewhat  from  that  portion  of  the  series  described.  These  rocks  are  white 
calcareous  sandstones,  that  look  very  much  like  the  Potsdam  sandstone 
where  it  overlies  limestones,  and  a  light-green  slate,  which  near  joint 
planes  and  other  cracks  has  a  light  purple  color.  According  to  Dr.  J.  M. 
Clements,  who  visited  the  spot,  the  slate  overlies  the  sandstone.  "The 
river,"  he  writes  in  his  notebook,  "gives  a  section  through  these  rocks,  and 
makes  the  strike  seem  to  be  N.  35°  W.,  dip  50°  N.  It  appears  to  me,  how- 
ever, that  the  true  strike  is  about  N.  85°  E.,  and  dip  40°  S."  If  these  rocks 
belong  to  the  marble  series,  they  constitute  its  upper  part.  The  slate 
closely  resembles  some  of  the  slates  in  tlie  Kona  dolomite  formation  of  the 

MON  XXX.VI 31 


482  THE  CRYSTAL  FALLS  lEON-BBAEING  DISTRICT. 

Marquette  range.     It  is  a  very  fine  grained  rock  composed  of  very  small 
splinters  of  quartz,  flakes  of  sericite,  and  a  few  of  chlorite. 

THE  IGNEOUS  ROCKS. 

The  igneous  rocks  associated  with  the  sedimentary  beds  in  the  Sturgeon 
River  tongue  are  all  greenstones  in  composition.  Many  of  them  are  unques- 
tionably intrusive;  a  few  ma}'  be  tuffaceous. 

The  intrusive  greenstones  do  not  differ  essentially  from  those  cutting 
the  Basement  Complex.  Some  of  them  are  in  the  form  of  small  bosses. 
Others  are  clearly  dikes,  though  for  the  most  part  these  dikes  follow  the 
bedding  of  the  sedimentary  rocks.  Still  others  may  be  intrusive  sheets. 
The  rocks  regarded  as  possibly  tufPaceous  are  distinctly  banded.  Some  are 
made  up  of  alternate  bands  of  dark  and  light  shades.  The  darker  bands 
consist  principally  of  a  schistose  greenstone,  and  the  lighter  ones  principally 
of  arkose  or  granitic  sandstone.  These  rocks  are  well  bedded,  apparently 
constituting  a  definite  portion  of  the  conglomerate  series  near  its  lower 
horizon.^ 

THE   INTRUSIVE   GREENSTONES. 

The  intrusive  greenstones  are  usually  fairly  massive  rocks,  with  a  dark 
bluish-green  color  and  a  moderately  tine  grained  texture.  On  their  edges 
they  often  pass  into  schistose  phases,  presenting  the  structure  and  appear- 
ance of  chlorite-schists.  A  very  typical  schist  of  this  character  occurs  on 
the  southern  edge  of  the  great  greenstone  mass  1,525  to  1,600  paces  north 
and  300  to  400  west  of  the  southeast  corner  of  sec.  18,  T.  42  N.,  R.  28  W. 
In  the  hand  specimen  the  rock  appears  to  be  a  well-characterized  chlorite- 
schist,  spangled  with  plates  of  a  light-colored  muscovite  measuring  1.5  to 
2  mm.  in  diameter. 

The  intrusive  character  of  some  of  the  greenstones  is  clearly  shown  by 
the  fact  that  they  occur  immediately  on  the  strike  of  the  conglomerate  bands, 
and  often  cutting  across  them,  as  is  the  case  at  300  paces  east  of  the  north- 
west corner  of  sec.  17,  T.  42  N.,  R.  28  W.  (see  PL  LII),  and  at  400  paces 
south,  100  west,  of  this  same  corner. 

PETROGRAPHICAL  DESCRIPTION. 

The  greenstones  intrusive  in  the  Algonkian  sediments  are  not  essen- 
tially different  from  those  cutting  the  members  of  the  Basement  Complex. 

'  See  Van  Rise's  Notebook  184,  pp.  21-23. 


IGNEOUS  BOOKS  OF  STUKGEON  EIVER  TONGUE.  483 

They  differ  from  the  latter  in  containing,  as  a  rule,  less  quartz  and  a  very 
nmch  greater  abundance  of  epidote.  The  epidote  is  all  secondary,  as  is  also 
the  quartz,  so  that  the  only  noticeable  difference  between  the  two  sets  of 
greenstones  is  dependent  upon  differences  in  the  nature  of  their  alteration, 
which  in  turn  are  probably  the  results  of  differences  in  environment.  Both 
sets  of  greenstones  have  been  squeezed,  but  those  in  the  Basement  Complex 
are  associated  with  crystalline  schists,  while  those  in  the  Algonkian  series 
are  associated  with  fragmental  beds. 

In  addition  to  hornblende,  plagioclase,  epidote,  and  a  little  quartz, 
almost  all  the  later  greenstones  contain  biotite,  small  crystals  of  magnetite, 
and  irregular  grains  of  ilmenite  or  of  a  titaniferous  magnetite.  Their 
structure  is  schistose  through  the  arrangement  of  the  larger  hornblendes 
and  biotites  and  the  elongation  of  the  feldspar  grains  in  approximately 
parallel  directions.  As  a  rule,  their  thin  sections  present  no  unusual  features. 
They  all  show  dirty  green  hornblende  plates,  greenish-brown  biotite  flakes, 
magnetite  crystals,  etc.,  embedded  in  a  mass  of  irregular  grains  of  decom- 
posed plagioclase,  the  principal  decomposition  product  of  the  feldspar  being 
in  almost  all  cases  epidote. 

Often  the  proportion  of  epidote  present  is  very  great.  It  occurs  as 
colorless  crystals  and  grains  scattered  through  the  hornblende,  and  as  light- 
yellow  plates  and  grains  embedded  in  the  mass  of  altered  plagioclase.  In 
the  rock  at  500  paces  east,  125  north,  of  the  southwest  corner  of  sec.  8,  T.  42 
N.,  R.  28  W.  (PI.  LII),  the  replacement  of  the  plagioclase  by  epidote  has  pro- 
ceeded so  far  that  no  trace  of  the  feldspar  can  be  discovered.  In  the  hand 
specimen  the  rock  is  seen  to  be  a  massive  mixture  of  black  ghstening  horn- 
blende crystals  in  a  yellowish-green  groundmass  possessing  a  sugary  texture. 
In  the  thin  section  the  hornblende  is  present  as  bluish-green  plates  that  are 
often  idiomorphic  in  cross  section.  The  groundmass  in  which  they  lie  is 
composed  of  epidote  and  quartz.  The  epidote  is  in  large  yellowish-green 
irregularly-outlined  plates,  including  particles  of  magnetite  and  small 
rounded  quartz  grains.  Most  of  the  quartz  is  in  isolated  grains  between  the 
epidote  plates  and  in  little  nests  of  interlocking  grains.  Small  magnetite 
granules  are  scattered  everywhere  throughout  the  section,  through  all  of  the 
components  indiscriminately 

The  coarser  greenstones  show  plainly  in  the  hand  specimen  the  ophitic 
structure,  even  where  the  rocks  are  schistose.     In  the  section  this  structure 


484  THE  GEYSTAL  FALLS  lEON-BEARING  DISTEICT. 

is  often  obscured  by  the  abundance  of  decomposition  products.  Under  low 
powers  of  the  microscope,  however,  it  can  nearly  always  be  detected.  In 
a  few  of  the  finer-grained  varieties,  phenocrysts  of  plagioclase  are  occasion- 
ally met  with.  They  are  clouded  by  inclusions  of  biotite  flakes  and  shreds 
of  hornblende  and  by  tiny  ^^articles  of  a  kaolinitic  or  sericitic  mineral. 
From  their  composition  and  structure,  it  is  clearly  evident  that  the  intrusive 
greenstones,  whether  massive  or  schistose,  are  altered  phases  of  diabase  or 
of  diabase-porphyrite. 

The  dark-green  chlorite-schist  referred  to  as  occurring  in  the  edge  of 
one  of  the  greenstone  masses  is  a  chloritic  biotite-schist  spangled  with  large 
flakes  of  a  light-colored  mica.  The  rock  consists  of  biotite,  chlorite,  musco- 
vite,  quartz,  and  rutile.  The  liiotite  is  in  broad  thin  plates,  arranged 
approximately  parallel,  and  embedded  in  a  mass  of  chlorite,  the  greater 
portion  of  which  is  a  greenish-brown  variety  that  looks  as  though  it  may 
have  been  derived  from  hornblende.  A  smaller  ^sortion  of  the  chlorite  is  in 
light-green  plates,  like  the  chlorite  so  frequently  found  in  chloidte-schist. 
The  quartz  is  in  small  rounded  grains  exhibiting  strain  shadows,  scattered 
here  and  there  through  the  chlorite  and  between  the  biotite  plates.  It  is 
much  more  abundant  in  soiiie  portions  of  the  rock  than  in  others,  forming 
bands  rich  in  quartz,  between  others  in  which  very  little  of  this  mineral  is 
present.  The  rutile  is  in  large  quantity.  It  constitutes  large  greenish- 
yellow  grains.  Some  of  these  are  rounded  forms,  others  are  prismatic 
crystals  measuring  0.08  mm.  to  0.12  mm.  in  length,  while  still  others  are 
clearly  defined  elbow  twins.  They  occur  everywhere  throughout  the  slide, 
but  are  rare  in  the  quartz.  They  are  most  abundant  in  the  chlorite  and  in 
the  large  plates  of  light-colored  mica  that  have  been  mentioned  as  character- 
istic features  of  the  hand  specimens.  These  have  all  the  properties  of  mus- 
covite.  They  lie  indiscriminately  among  the  other  com2Donents,  irrespective 
of  the  schistosity  of  the  rock,  and  contain  very  few  inclusions,  with  the 
exception  of  the  rutile  grains.  The  lines  of  biotite,  to  the  arrangement  of 
which  the  rock  owes  its  schistosity,  do  not  bend  around  the  muscovite  as 
they  do  around  the  eyes  in  an  augen-gneiss,  but  they  continue  their  courses 
up  to  the  edge  of  the  muscovite  grain,  and  there  abruptly  stop.  From  these 
facts  it  is  clear  that  the  muscovites  have  originated  since  the  rock  containing 
them  was  rendered  schistose.  As  in  the  case  of  many  other  secondary 
minerals,  it  appears  that  these  were  produced  from  the  components  of  the 


IGNEOUS  ROCKa  OF  STURGEON  RIVER  TONGUE.  485 

schist  by  a  process  which  resuhed  in  the  absorption  of  all  of  them  except 
rutile.  The  process  may  have  been  connected  with  contact  action,  but  no 
evidence  in  favor  of  this  supposition  has  been  obtained. 

There  are  a  few  other  types  of  greenstone  occasionally  met  with 
among  the  dike  and  other  intrusive  forms  of  the  district,  but  they  do  not 
diifer  in  any  marked  degree  from  those  described,  except  that  some  are 
quite  schistose.  One  or  two  of  these  contain  oval  aggregates  of  epidote, 
plagioclase,  and  quartz,  that  may  represent  inclusions  of  foreign  rocks. 
They  are  now,  however,  so  much  altered  that  it  is  difficult  to  determine 
their  character  with  any  degree  of  certainty. 

The  rock  of  one  or  two  other  exposures  in  the  area  underlain  mainly 
by  the  conglomerates  deserves  mention  before  the  banded  greenstones  are 
discussed.  The  rock  referred  to  is  a  heavy,  lustrous,  black  schist  that 
resembles  in  many  respects  a  hornblende-schist.  On  fresh  fractures  across 
the  schistosity  parallel  lines,  darker  than  the  main  mass  of  the  rock,  may 
be  easily  detected.  These  are  the  edges  of  cleavage  planes,  whose  surfaces 
are  coated  with  brassy  yellow  mica  plates.  In  thin  section  these  rocks 
differ  very  little  from  the  schistose  greenstones  referred  to  above.  They 
consist  of  a  heterogeneous  schistose  mass  of  green  hornblende,  cloudy 
plagioclase,  quartz,  epidote,  chlorite,  and  magnetite.  Biotite  flakes  are  met 
with  occasionally,  but  they  are  by  no  means  common,  except  on  the  cleav- 
age surfaces.  Rocks  of  this  class  have  not  only  been  made  schistose  by 
squeezing,  but  they  have  also  suffered  shearing  along  what  are  now  the 
cleavage  planes.  They  are  almost  identical  in  microscopic  and  macroscopic 
features  with  the  hornblende-schists  in  the  Basement  Complex. 

THE   BANDED   GREENSTONES. 

Distincth'  banded  rocks,  composed  partly  of  basic  material  with  the 
composition  of  greenstone,  form  a  well-defined  hillock  in  sec.  17,  T.  42  N., 
R.  28  W.,  about  250  paces  north  of  the  west  quarter  post  of  this  section, 
and  a  group  of  outcrops  on  the  east  bank  of  the  Sturgeon  River,  imme- 
diately west  of  this  point. 

The  rocks  in  question  are  banded  in  mediumly  coarse-grained  dark 
bands,  containing  large  quantities  of  green  hornblende,  and  in  fine-grained 
lighter  ones,  that  resemble  in  the  hand  specimen  bluish-black  quartzites  or 
cherts.     In  some  bands  there  are  large  lenticules  of  white  quartz,  that  show 


486  THE  CRYSTAL  FALLS  IRON-BEARING  DISTRICT. 

plainly  on  weathered  surfaces,  like  the  flattened  pebbles  in  a  squeezed  con- 
glomerate or  the  drawn-out  parts  of  quai'tzose  layers  in  a  mashed  bedded 
rock.  These  bands,  though  not  very  well  defined,  run  continuously  for 
long  distances,  and  strike  and  dip  conformably  with  the  conglomerate  beds 
exposed  200  paces  to  the  north. 


PETROGRAPIIICAL   DESCRIPTION. 


In  the  thin  section  the  lighter-colored  layers  of  these  rocks  are  seen 
to  be  composed  of  very  irregularly  outlined  and  rounded  quartz  grains, 
cemented  by  a  mass  of  finer  quartzes  and  small  grains  of  zoisite,  little 
clumps  of  chlorite,  some  decomposed  feldspar,  and  particles  of  magnetite. 
Occasionally  a  plate  of  yellowish  epidote  occurs  in  the  midst  of  this  aggre- 
gate, and  scattered  here  and  there  through  it  are  large  plates  of  green  horn- 
blende with  the  cellular  structure  so  common  to  secondary  minerals.     These 
hornblendes  lie  irregularly  in  the  slide,  and  include  grains  of  all  the  other 
components.     The  quartz  grains  are  small  and  are  independently  oriented, 
but  frequently  little  groups  of  them,  with  the  outlines  of  sand  grains,  are 
met  with.     There  is  Httle  evidence  of  schistosity  in  these  layers,  but  they 
exhibit  a  banding  produced  by  the  alternation  of  coarser  and  finer  constit- 
uents.    In  the  darker  layers  the  proportion  of  hornblende  is  much  greater 
than  it  is  in  the  hghter  ones.     Indeed,  some  bands  consist  almost  exclu- 
sively of  large  cellular  plates  and  radial  aggregates  of  plates  of  this  mineral, 
only  the  small  interstitial  spaces  between  the  large  amphiboles  being  filled 
with  an  aggregate  of  quartz-zoisite,  small  hornblende  needles,  and  magnet- 
ite.    In  some  sections  biotite  is  also  present.     It  occurs  most  abundantly 
in  the  quartz-zoisite  aggregate,  filling  the  interstitial  spaces  between  the 
amphiboles,  but  is  present  also  as  inclusions  in  this  latter  mineral.     Some 
of  the  biotite  in  the  hornblende  appears  to  grade  into  its  host,  and  certain 
portions  of  the  amphibole  possesses  the  brown  color  of  the  mica,  with  the 
optical  properties  of  the  hornblende.     The  large  amphiboles  are  evidently 
the  youngest  components  in  the  rocks,  though  they  were  plainly  produced 
before  the  schistosity.     In  those  layers  in  which  the  schistosity  is  strongly 
marked  this  structure  is  produced  mainly  by  the  parallel  arrangement  of 
the  biotite  and  the  small  amphibole  needles  and  plates  in  the  quartzose 
aggregate.     The  larger  cellular  hornblendes  lie  across  the  schistose  planes, 
and  when  they  do  so,  the  lines  of  biotite   and  of  small  amphiboles  pass 


IGNEOUS  ROCKS  OF  STUEGEON  EIVER  TONGUE.  487 

around  tlieiu  exactly  as  tliey  would  do  were  the  large  hornblendes  present 
before  the  rock  was  squeezed.  Sometimes  the  amphibole  masses  that  form. 
so  large  a  proportion  of  the  schistose  bands  are  single  crystals,  sometimes 
they  are  fragments  of  crystals,  and  at  other  times  they  are  groups  of 
radiating  crystals.  The  magnetite  is  very  much  more  abundant  in  the 
hornblendes  than  in  the  surrounding  quartz  aggregate,  sometimes  being 
confined  exclusively  to  this  mineral,  as  though  it  were  one  of  the  products 
(the  hornblende  being  the  other)  resulting  from  the  decomposition  of  some 
original  constituent,  probably  augite.  Little  particles  of  hematite,  on  the 
otlier  hand,  are  abundantly  disseminated  through  the  quartzose  aggregate, 
and  are  practically  absent  from  the  hornblende.  Much  of  it  appears  to 
have  been  derived  from  magnetite. 

The  evidence  derived  from  the  microscopic  study  of  sections  of  these 
banded  rocks,  so  far  as  it  relates  to  their  crigin,  is  disappointing.  The 
quartzose  layers  are,  in  all  pi'obability,  sedimentary.  The  hornblendic 
layers,  however,  differ  from  these  so  much  in  composition  that  their  material 
must  have  had  a  different  source.  It  is  possible  that  the  quartzose  layers 
represent  sediments  derived  from  the  granitic  portions  of  the  Basement 
Comjjlex,  while  the  hornblendic  layers  represent  sediments  derived  from 
the  basic  portions  of  the  Basement  Complex;  or,  it  may  be  that  the  acid 
layers  have  the  origin  ascribed  to  them,  while  the  basic  ones  are  mixed 
sediments  and  basic  tuffs.  The  sections  of  the  dark  layers  of  these  rocks 
resemble  so  strongly  the  sections  of  the  basic  laj^ers  in  the  Clarksburg,  series 
of  mixed  tuffs  and  sediments  in  the  Marquette  district  that  the  writer  is 
inclined  to  regard  the  rocks  as  composed  partly  of  tuffaceous  material.  On 
the  other  hand,  the  banded  rocks  occur  so  close  to  the  boundary  between 
the  sedimentary  area  and  the  Basement  Complex,  which  near  this  boundary 
is  composed  mainly  of  basic  schists,  that  it  would  seem  but  natural  that 
they  should  contain  large  qiiantities  of  basic  material  derived  from  these 
schists.  The  original  structure  of  the  layers  has  been  so  completely 
destroyed  by  mashing  that  it  can  not  give  any  evidence  as  to  the  nature 
of  the  beds.  We  are  therefore  compelled  to  rely  entirely  upon  their  com- 
position to  aid  us  in  discovering  their  origin.  This  indicates  simply  that 
much  of  their  material  was  derived  either  from  volcanic  ashes  or  from  the 
debris  washed  from  the  basic  portions  of  the  Basement  Complex. 


INDEX 


A.  Page. 

Aa  structure,  sketch  of 120, 121, 122, 123. 124 

(See  Ellipsoidal  structure.) 

Aci  Castello,  basalt  from 121 

Aci  Trezza,  basalt  from 121 

Acid  intrusives,  described 45-49, 190-198, 426 

age  of 49 

distribution  of 190 

in  A  rchean 45-49 

in  Felch  Mountain  Kange 426 

in  Hemlock  formation 77 

in  "Upper  Huronian 164 

Acid  lavas,  of  Hemlock  formation,  described 80-94 

banding  of 91,92,93,94 

micropegmatitic  texture  in 89 

pressure  effects  in 87, 88 

schistose,  described 87-94 

Acid  py roclastics,  described 94, 95 

Acid  volcanics  of  Hemlock  formation,  described 80-95 

Actiuolite  from  hornblende 215 

of  adinole .' 209 

of  cblorite-schist 442 

of  metabasalt 105, 133 

of  picrite-porphyry 214,  215 

of  py  roclastics = 147 

of  slate 205,209 

of  spilosite 206 

orientation  of 105 

(See  Amx^hibole.) 

Actinolite- schist  from  gray  wacke 57 

of  pyroclastics 147 

Adaraello  tonalite 230 

Adams,  F.  D.,  on  analysis  of  slates  and  granites 58 

on  pyroxene  zone  about  olivine 256 

Adinole,  analyses  of 208 

compared  with  analyses  of  clay  slate  and  spilo- 
site    210 

in  Mansfield  formation 64 

described 208-209 

Ajibik  quartzite 451 

correlation  of xxv,  xxvi 

relations  to  Groveland  formation xxi,449,456 

Albitefrom  feldspar 151,201 

of  metabasalt 99 

of  spilosite,  plate  of 302 

Algonkian,  contact  "veith  Archean,  effect  on  topog- 
raphy    386 

deposition  of 456,  457 

distribution  of 331,427 

folding  of 427,428 

of  Felch  Mountain  Range,  distribution  of 384 

succession  in 384-385 

structure  of 384-385 

intrusives  in,  described 426 


Page. 

Algonkian  of  Marquette  district,  distribution  of 452-453 

of  Sturgeon  River  tongue,  described 458-437 

comparison  with  Algonkian  of  Felch  Moun- 
tain tongue 462 

folding  of 471-t72 

igneous  rocks  of 482-487 

pressure  effects  iu 471 

relations  to  Archean 461-463 

relations  to  Lower  Marquette 462 

relations  to  Archean xvil,  331,  399,  427,  458 

relations  to  drainage 334,  335 

relations  to  Paleozoic  formations 331,  383 

relations  to  quartz  porphyry 439 

(See  Huronian,  Upper  Huronian,   Lower  Hu- 
ronian.) 

Allanite  in  acid  lavas 89 

included  in  epidote-zoisite 444 

Allen,  Andrews,  referred  to 22 

Alteration  of  audesine ,. 224 

of  basic  volcanics 152 

of  biotite 43,  393 

of  bronzite,  plate  of 306 

of  calcite 146 

of  diabase 469 

of  ellipsoidal  basalt 292 

of  feldspar 42, 170, 171, 192,  201.  224, 228,  478 

plate  of 288 

of  glauconite 422 

of  grit 168-169 

of  hornblende. 234,235,237 

of  metabasalt 1 17 

described  126-135 

of  picrite-porphyry 213 

of  slate 14 

of  tutfs 141 

Amasa  (town) 12,143,162 

Amasa  area,  ore  deposits  of 177 

succession  in 177-178 

(See  Hemlock  mine.) 

American  Black  Slate  Co.,  analysis  of  slate  from 61 

Araphibole  of  greenstone 486,  487 

of  metabasalt 127 

of  mica-schist - 415 

of  picrite-porphyry 214 

orientation  of 127,214,405,486 

(See  Actinolite,  Hornblende,  TremoUte.) 

Amphibole-peridotite,  described 253-254 

crystallization  of 257 

gradation  to  olivine-gabbro 254-260 

gradation  to  wehrlite 254-260 

Amphibole-schist  of  Sturgeon  River  Archean,  de- 
scribed    465-467 

Amphiboliie,  analysis  of 397 

489 


490 


INDEX. 


Page. 

Arapliibolite  of  Arcliean,  described 395-397 

from  basic  volcaiiics 152 

intruding  tnica-scbist 392 

Aniygdaloidal  texture  in  metabasalt. . . .  102, 113, 122, 123. 442 

described 124^126 

plate  of 280,282,284,290 

cause  of  distribution  of,  in  basic  lavas 95 

in  eruptive  breccia 136 

in  Hemlock  scbist - ■^^S 

in  pyroelastics 138, 14C 

Amygdules  of  calcite,  plate  of 282 

of  clilorite,  plate  of 284 

of  feldspar,  plate  of 284 

of  quartz,  plate  of 284 

order  of  deposition  of 125 

pressure  eftects  in 126, 128 

Analyses  of  adinoles 208 

of  iimpbibolite ^ 397 

of  clay  slate 59,61 

of  clay  slate,  spilosite,  and  adinole,  comparison  of         21 0 

of  dolomite 409,435 

of  ;:;neiss 391 

of  granite 389 

of  bornblende 242 

of  hornblendegabbro 263-264 

of  iron  ore,  by  Brooks,  referred  to 19 

of  iron  ore 181 

of  Mansfield  mine 69 

of  Mansfield  slate 59,61 

comparison  witb  contact  products 209-211 

of  metabasalt 103,106.107 

of  mica-diorite 231,263-264 

of  niica-scbist ^9^ 

ofnorite 245,263,264 

ofperidotite 259,263,264 

of  picrite-porphyry 219 

of  spilosite 207 

Auatase  included  in  bornblende 236 

of  gabbro  and  norite  - 236 

of  metabasalt 129 

{See  Octabedrite.) 

■  Andesine,  alteration  of 224 

altering  tomuscovite 224 

altering  to  epidote-zoisite 224 

of  diorite 224 

of  gabbro  and  norite 233 

of  metabasalt 104 

twinnina;  of 104 

Andesite,  of  Hemlock  formation 107 

Anticline  determined  by  magnetic  observations-  366, 372,  373 

described - 370-371 

Antoine    dolomite   of    Menominee    district    corre- 
lated     XXV,  XXVI 

Apatite  included  in  biotite 217,234,239 

in  feld  spar 234 

in  quartz 194,419,420 

in  spbene 244 

of  acid  lavas 91 

of  biotite -granite 192 

of  gabbro  and  uorite 239 

of  metabasalt 99 

of  periodite 252 

of  picrite-porphyry 216 

of  outlining  feld  spra-  crystals 239 

Apbanitic  texture  in  metabasalt 98, 99, 212 

Apobasalt,  use  of  term 96,98 


Page. 

274 
87 
276 
276 
276 


Aporbyolite  "witb  perlitic  parting,  plate  of 

Aporhyolite-porpby ry  described 

breccia,  plate  of 

perlitic  parting  of.  plate  of 

pressure  effects  in,  plate  of 

use  of  term 80 

Aragon  mine 69 

Arcbean,  described xviri,  38-49, 385-397,  428-430,  463^71 

acid  dikes  in,  described 45-49 

age  of 39 

basic  dikes  in,  described 46-49 

biotite-granite  of,  described 40-43 

iUkes  in,  described 45-49 

distribution  of 38,  39,  331,  427 

ellipse 26,  38,  333,  427 

erosion  of xxiv,  39 

folding  of xxiii 

intrusives  in xviu,  38,  45-49 

metamorpbism  of x viii,  xxiii 

of  Crystal  Falls  district  correlated  witb  Arcbean 

of  Marquette  district xxv,  xxvi 

of  Felcb  Mountain  range,  described 385-397 

distribution  of 385 

topography  of 386,  387 

of  Marquette  district  correlated  T,7itb  Archean 

of  Menominee  district xxv,  xxvi 

correlated  witb  Archean  of  Sturgeon  River 

tongue 470-471 

distribution  of 452-453 

of  Micbigamme  Mountain  and  Fence  River  areas, 

described 428-430 

of  Sturgeon  River  tongue,  described 463-471 

comparison  with  Archean  of  Marquette  dis- 
trict    470-471 

relations  to  Algonkian 459,  460,  461-463 

origin  of 39 

relations  to  Algonkian xvii, 

331,  378,  379,  386,  399,  427,  458, 460.  461-463 

relations  to  Cambrian 26,  331 

relations  to  drainage 334,335 

relations  to  Groveland  formation 416,424 

relations  to  Huronian.     (See  Relations  to  Algon- 
kian.) 

relations  to  Lo^er  Huronian xix,  55 

relations  to  Mansfield  formation 424 

relation.s  to  overlying  formations 39 

relations  to  Paleozoic  rocks 26,  331 

relations  to  quartz-porphyry 429 

relations  to  Randville  dolomite xix,  51.  53, 55,  407 

relations  to  Silurian  rocks 26 

relations  to  Sturgeon  quartzite xis,  398, 401 

relations  to  Upper  Huronian xxii 

topography  of 38,39,333 

Arenaceous  slate  group  of  Rominger 164 

Argentine  Republic,  gabbros  of 255, 256 

Arkose  of  Sturgeon  River  conglomerate  formation 

described 478-479 

pressure  etfects  in 479 

Armenia  mine,  description  of  ore  body  at 183 

location  of,  178;  op 186 

table  of  shipments  from,  op 186 

Ash  beds  described 142-143 

Azoic  system 16, 17,  375 

of  Sturgeon  Rivertongne 460 

Augite  altering  to  hornblende 100 

altering  to  pilite 211 


INDEX. 


491 


Augito  altering  to  uralito , lOi,  2U1, 212 

cU'avajroof 237 

I'ryatallizatiou  of 257 

of  amphiboU'-peridotito 255 

of  gabhroautl  norito 237 

of  horubleiide-gabbro 243 

of  inetabaaalt 100,104,211 

of  iiietiidolerite 200,201 

of  peridotite 250 

of  tuffs 138 

iu  dolomite 436 

included  iu  hornblende 237,  250,  255, 257 

iuc'lnding  biotito 250 

including  hornblende 250,255 

including  hyperathene 255 

zonal  intergrowth  with  hornblende,  plate  of 320 

zonal  intergrowth  with  olivine  and  hornblende.  255-260 

Augite-andesite  of  Heinloct  formation 108 

Ausweichungs-clivage.     (See  Pyroxene) 440 

B. 

Bad  Water  village 374 

Balsam  village 143 

Banding  of  acid  lavas 87,88,01,92 

described 93,94 

of  acid  pyroclastics 95 

of  Archean  granite 49 

of  ash  bed 143 

of  biotite  and  chlorite 225 

of  biotite-granite 43-44 

of  biotite-schist 467,468 

of  Bone  Lake  crystalline  schist 149 

of  chert  of  Mansfield  mine 69 

of  gneiss 390 

of  granite 45,49 

of  greenstone 485-487 

of  Groveland  formation 417-416 

of  hornblende-schist 465,  40G 

of  Mansfield  schist 413 

of  mica-gneiss 197 

of  quartzite 401 

of  Handville  dolomite 409 

of  rhyolite-porphyry '     81 

of  spilosite-desraosite,  plate  of ,. 306 

of  Sturgeon  formation 431 

of  tuff 138 

of  volcanic  conglomerate 144 

Barrois,  Charles,  referred  to 44 

Basalt.     (See  Metabasalt.) 

Bascora,  F.,  on  devjtritied  lavas 80,  87,  96 

Base  level.     (See  Peneplain.) 

Basement   Complex  of  Stnrgeon   Kiver  tongue  de- 
scribed (see  Archean) 463-471 

Basic  intrusives  described 198-212 

age  of 48,  49 

correlation  of 189 

effect  on  topography,  sketch  of 46 

in  A rchean 49 

in  Felch  Mountain  range 446 

in  Hemlock  formation 77 

described 204 

in  Mansfield  slate  described 203-204 

in  other  intrusives  described  204 

in  Upper  Huronian 164,211 

described 204 

metamorphism  of  Mansfield  slate  described 204-211 


Page. 

Basic  intrusives,  schistosity  of 47-48 

transfer  of  material  to  slate 211 

Basic  lavas  described 95-135 

Basic  volcanics,  alteration  of 152 

described 95-148 

Ba83ett,V.H.,  indebtedness  to 106,264 

Bayley,  "W.  S.,  indebtedness  to 12 

on  Clarksburg  formation 132 

on  metamorphism  of  basic  volcanics 152 

on  schistose  pyroclastics 148 

referred  to xv.  22 

Eebb,  E.  C.,  indebtedness  to xvi.  12 

Becke,  F.,  on  granodiorite 231 

on  relations  of  pyroxene  and  amphibole 258 

on  tonalite 230-231 

on  tonalite-porphyrite 229 

referred  to 1 05 

Bedding  of  Upper  Huronian 167 

Benson,  Vt.,  analysis  slate  from 61 

Bessemer  ore  of  Mansfield  mine 68 

of  Mansfield  slate 27 

Biotite,  alteration  of 43, 393 

altering  to  calcite 192,225 

to  chlorite-  43,  47, 192, 193,  215,  216,  225,  228,  248,  403,  425 

to  epidote 43 

to  epidote-zoisite 225 

to  rutile 43,192,202,225 

to  sagcnite 192,193 

to  sphene 192,225 

banding  with  chlorite 225 

crystallization  of 258 

from  feldspar 42,82,89,90,92,150,170,192,201 

included  in  augito 250 

included  iu  feldspar 151, 171 

included  in  hornblende 251,  260,  261 

included  in  muscovite 298 

included  in  quartz 47,171,394,403,466 

included  in  plagioclase 193,484 

including  apatite 217,234,239 

including  epidote 225,226 

including  ilmenite 216 

including  iron  oxide 252 

including  sagenite 403 

including  sphene 225,  404 

including  zircon 234 

iutergrowu  wifh  muscovite 393, 414 

of  acid  lavas 89,91 

of  amphibolite 296 

of  amphibole-peridotite 254,  258 

of  amphibole-schist 466 

of  araygdules 124 

of  basic  dikes 47 

of  biotite-granite 42,  43, 192, 193 

of  biotite-schist 468,484 

of  Bone  Lake  crystalline  schist 150 

of  diorite 

of  gabbro  and  norite 

of  granite , 

of  graywacke 

of  greenstone    

of  Hemlock  schist 

of  metadolerite 

of  metamorphosed  Mansfield  slate 

of  mica-diorite,  plate  of 

of  mica-schist 

of  muscovite-biotite-gneiss,  plate  of 


225 
234 
198 
170 
4S6 
444 
202 
205 
308 
392. 393 
298 


492 


INDEX. 


Biotite  of  mnscovite-'biotite- granite 193 

of  peridoiite 252,257,261 

of  picrite-porphyry 215 

of  pyroclastica 147 

of  sedimentaries  developed  by  intrusion 195 

of  sedimentary  inclusions  in  granite 197 

of  spilosite 206  ; 

orientation  of 393,425,468,486  , 

paralle]  growth, -witb,  muscovite 170  | 

penetrating  feldspar 4=14  | 

pressure  effects  in 43,  248  j 

relations  of  orientation  to  hornblende 486  . 

relations  of  orientation  to  nmscovite 484  ^ 

replacing  feldspar - 193   ' 

Biotite-gneiss  of  Arcliean 429 

Biotite-granite  described 40-43, 191-193 

banding  of 43,44 

plate  of 308 

micropegmatitic  structure  in,  described 192-193 

schistosity  of 44 

Biotite-scbist  of  Algonkian  of  Sturgeon  Kiver  tongue  484 

of  Arcbean  of  Sturgeon  Kiver  tongue 467-469 

of  Hemlock  formation 442 

Birkinbine,  John,  on  ore  shipments 186 

Blaney  mine.     {See  Hope  mine.) 

Blocklava.    (SeeEllipsoidalstructure,  Aastructure  ) 

Bog  iron  ore  of  Upper  Huronian 182 

Bone  Lake  described 35 

referred  to 15^ 

Bone  Lake  crystalline  schists  described 148-152 

Bonney,  T.  G-.,on  alteration  of  olivine 218 

on  ellipsoidal  structure 118-119 

Botryoidal  ore 180 

Brackett,  11.  N.,  on  ultrabasic  intrusives 220 

Brauuer,  J.  C  ,  on  ultrabasic  intrusives 220 

Breccia,  eruptive,  of  Hemlock  formation,  described.  135-136 

Tolcanic,  use  of  terra 137 

Breeciation  of  Groveland  formation 418 

of  metabasalt 11''^ 

Brittany  granite  compared  with  Crystal  Falls  granite  44 

Brogger,  W.  C,  on  diorite  and  gabbro  families 242 

on  monzonito  group 232 

on  rock  analyses 105 

on  use  of  term  diorite 222 

Bronzite  altering  to  serpentine 238 

plate  of... 306 

altering  to  talc 238 

plate  of 306 

included  in  hornblende 250 

plate  of 306 

including  ilmenite 238 

including  rutile 238 

in  zonal  intergrowth  with  hornblende,  plate  of. .  318 

of  broDzite-norite 244 

plate  of 318 

alteration  of,  plate  of 306 

of  bronzite-norite-porphy ry 246 

of  gabbro  and  norite 238 

of  peridotite 250 

Bronzite-norite  described 244-247 

plate  of 318 

analysis  of 245 

crystallization  of  minerals 2(32 

intruding  hornblende-gabbro 243,  249,  265 

Bronzite -norite -porphyry 246 

plate  of 320 


Paga 

Bronzite-norite-porpbyry  altering  to  serpentine 246 

intruding  hornblende-gabbro 249 

Brooks,  A.  H.,  referred  to 22 

Brooks,  T.  B.,  on  composition  of  iron  ore 181 

on  correlation  of  Menominee  rocks 19 

on  correlation  of  Upper  Huronian 164 

on  Felch  Mountain  range 376,  377.  378,  379 

on  iron-bearing  rocks  of  Michigan 16-17 

on  magnetic  observations 24,  337 

on  Menominee  district 19 

on  Mesuard  series 452 

on  Paint  Kiver  district 11 

on  Sturgeon  Kiver  tongue 460-461 

on  Upper  Huronian 172, 173 

referred  to xv,  21 

Brooks,  Kominger,  and  Pumpelly,  map  of  Upper  Pen- 
insula of  Michigan 18 

Brown,  E  F.,  on  analysis  of  iron  ore 69 

Brule  Kiver  described 31 

referred  to 13, 15, 161 

Building  stones  of  Hemlock  formation  described. . .  153, 154 
Burt,  William  A.,  map  of  part  of  Upper  Peninsula  .  15 

on  Crystal  Falls  rocks 13 

on  Felch  Mountain  range 375 

on  Sturgeon  Kiver  tongue 460-161 

referred  to - 16,21 

C. 

Calciferous  limestone,  relations  to  Potsdam 383 

Calcification  of  chlorite 132 

of  feldspar 131 

of  metabasalt 117,130,132,133,134 

Calcite,  alteration  of 146 

altering  to  limonite - 153 

developed  by  dynamic  action 432 

from  biotite 192,225 

from  feldspar 82,90,131,132 

from  hornblende 203,  21 5 

of  acid  lavas 89,93 

of  amygdules 124 

plate  of 282 

of  basic  dikes 47 

of  biotite-granite 192 

of  ellipsoidal  metabasalt,  plate  of 292 

of  Groveland  formation 420 

j          of  marble 481 

of  metabasalt 100,101,117,127.128,129,132 

j                  plate  of 290 

I          of  metadolerite 203 

j          of  peridotite 252 

;          of  py roclastics 146, 147 

of  tuff 142 

orientation  of 132 

pseudomorphs  after  feldspar 132 

replaced  by  iron  carbonate 133 

replacing  chlorite 132 

replacing  feldspar 131 

Caledonia  mine.     (See  Mansfield  mine.) 

Calumet  and  Heckla  mine 399 

Cambrian  sandstone 29 

deposition  of xxiv 

erosion  of xxiv 

relations  to  Algonkian 331,473 

relations  to  Arcbean xxiv,  26, 331 

relations  to  Ke weenawan 162 

relations  to  intrusives 188 


INDEX. 


493 


Page. 
Camliriaii  sniulstonc,  relatioiiH   to  Upper  Hurouiaii     xxiv, 

155.161,162 

Carbon  of  Mansflold  slate 60 

Carbonate  ilevcloped  by  dynamic  action 432 

Carbonation  of  metabasalt 117, 130, 132, 133, 134 

Carboniferous  clays,  analyses  of 59 

CataL-lastic  structure  in  feldspar 44 

in  granite 194 

in  quartz 41,  44 

Cellular  texture  in  hornblende 486 

Channing,  J.  Part,  figure  by 65 

on  Mansfield  mine 66 

referred  to 22 

Cbanning  (town) : 175 

Chert  fragments  in  conglomerate 64 

of  MansfieUl  formation  described 62 

of  Upper  Huronian 166 

pressure  efl"ect3  in 177 

relations  to  ore  deposits 182 

road  material 154 

Cherty  carbonate  from  organic  matter 184 

in  conglomerate 64 

Chester,  F.B.,  (?)  on  gabbros 247 

Chicago,  Milwaukee,  and  St.  Paul  Eailway 95, 143, 175 

Chicago  and  Northwestern  Railway 156, 175,  423 

Chlorite  altering  to  epidote 132 

banding  with  biotite 225 

calcification  of 132 

from  biotite 43,  47, 192, 193,  215,  216,  225,  248,  403,  425 

from  feldspar 82,99,111,131,201 

from  hornblende 100,203,214,215,237 

included  in  quartz 403,  419,  420 

including  epidote 248 

including  ilmenite 146,  216 

including  iron  ore 218 

including  sagenite 403 

including  titanite 404 

of  adinole 208  ! 

of  altered  slate 209  I 

of  amygdules 124,125  i 

plateof 280,284  \ 

of  arkose 479 

of  basic  dikes 47  , 

of  biotite-granite 192, 193   . 

of  biotite-schist 484  ; 

of  Bone  Late  schist 151 

of  dolomite 410  i 

of  graywacke 170 

of  metabasalt 98,  99, 

101, 117, 118, 127, 128, 129, 131, 132, 133, 134 

of  mica-schist 196 

of  phy Uite 440 

of  picrite-porphyry 213,  217,  218 

of  pyroclastics 145 

of  spilosite 206 

plate  of 302,  304 

of  spilosite-desmosite,  plate  of 306 

of  slate 205,  209 

of  tuff 141^  142 

of  volcanic  conglomerate 143,144, 145 

orientation  of us,  127, 133, 146 

pseudomorphs  after  biotite 217, 228 

after  garnet 403 

replaced  by  calcite 132 

Chlorite-schiat  from  basic  volcanica 152 

from  gray wacte 57 


Page. 
Cblorite-scbist  intrusive  in  .^Mgonkian  of  Sturgeon 

River 432 

of  Hemlock  formation 442 

of  Upper  Huronian ig6,  174 

Cbloritization  of  metabasalt 117 

Chrustscbofr,  C.  von,  on  relations  of  pyroxene  and 

amjihibole 258 

Claire  mine,  location  of 178,  op.  186 

table  of  shipments  from op.  186 

Claire  Mining  Company.     {See  Claire  mine.) 

Clarke,  F.  "W.,  analysis  by 61 

Clarksburg  volcanics 132,165 

comparison  with  banded  greenstones  of  Sturgeon 

Kiver  tongue 487 

replacing  Michigamme  and  Ishpeming   forma- 
tions             XXVI 

Clastic  volcanic,  plate  of 284 

Clay  slates,  analyses  of 59,  61 

analyses  of,  compared  with  altered  clay  slates  ..  209,211 

composition  of 58,60 

from  granite 58 

of  Mansfield  formation  described 57-62 

origin  of 5g 

relations  to  ferruginous  chert 63 

relations  to  ore  bodies 63 

relations  to  siderite-slate 63 

Cleavage  of  .7cid  lavas 88-89 

of  augite 250 

of  biotite-schist 467 

of  Hemlock  schist , 443,445 

of  hornblende ? ^51 

of  phyllite 439,  440 

of  pyroxene 237 

Clements,    J.    Morgan,    on     ellipsoidal    structure, 

referred  to US 

on  Hemlock  volcanics 446 

on  Sturgeon  River  sandstone 481 

on  volcanics  of  Crystal  Falls  district 20 

referred  to xx vi,  437,  447 

Cole,  G.  A.  J.,  on  ellipsoidal  structure 118, 119 

Columbia  mine,  description  of  ore  bodies 182 

location  of 179,  op.  186 

table  of  shipments  from op.  1 86 

referred  to 161 

Commonwealth 385 

iron  ores  at xxvi 

Compass,  dial,  use  of 24,  341 ,  342, 344 

Concentration  of  ore  in  synclinal  troughs  (see  Iron 

ore  deposits,  origin  of) 183, 184 

Conglomerate  altering  to  sericite-schist 475 

basalt  of  Upper  Huronian 163 

of  Hemlock  formation 76,  152,153 

*    intruded  by  diabase 476 

of  Mansfield  ore  deposit 63-64, 68 

intruded  by  greenstone 475-470 

of  Sturgeon  River xviir,  xxtv,  461,462,  481 

described 472-479 

of  Upper  Huronian xxn,  166 

pressure  effects  in 474,  xviii 

volcanic,  described 143-145 

plate  of 284 

use  of  term 136 

Copper,  absence  of,  in  Huronian  volcanics 125 

Corrigan,  McKinney  Sc  Co.     (*S'ee  Crystal  Falls  mine.) 
Cortlandt  series,  comparison  with  Crystal  Falls  in- 

trusives 222 


494 


INDEX. 


Page. 
CoutcMchiiig 380 

Credner,  H.,  on  Felch  Mountain  range 376 

on  Menominee  district 377 

on  oritrin  of  iron  ore 71 

Cretaceous  subaideDce xxiv 

Cross,  Whitman,  on  metadolerite 97 

Crystal  Falls  area,  ore  deposits  of 178 

comparison  with  output  of  Menominee  mines...  186 

comparison  with  output  of  region 186 

discovery  of  ore  deposits 175 

Crystal  Falls  district,  drainage  of 31-36 

elevations  in 30,  31,  332 

folding  of 26 

geographical  limits  of 25 

physiography 13,29-37,329-335 

relations  to  Marquette  district 11,25,329 

relations  to  Menominee  district 11,25,329 

structure  and  stratigraphy  of 25-29 

Crystal  Falls  mine,  analysis  of  ore  from 181 

location  of 178,  op.  186 

table  of  shipments  of op.  186 

referred  to 161 

Crystal  Falls    series,   correlation   with   Marquette 

series xxv,  xxvi 

correlation  with  Menominee  series xxv,  xxvi 

metamorphisra  of xxiv,  xxv,  sxvi 

Crystal  Falls  syncline xxni,  26, 178 

'  described 158-161 

Cry  stall  ine  schist  of  Bone  Lake  described 148-152 

of  Upper  Huronian* 166, 167, 171, 172 

Crystallization  of  minerals  of  basic  rock  described . .  257-259 
of  minerals  of  intrusive  series 262 

Culver,  G.E., referred  to 22 

Current  bedding  in  quartzite 53 

D. 

Dakyns.  J.  R.,on  plutonic  rocks 222 

Dalmer,  K..  on  ellipsoidal  structure 118, 119 

Dana.  J.  D.,  ou  ellipsoidal  structure... 120,121,124 

sketch  by 120 

on  metadolerite 96 

on  origin  of  volcanics 78 

referred  to 95 

Darton,  N.  H.,  ou  ultrabasic  intrusive^ 219-220 

referred  to 95 

Dathe,  E.,  on  ellipsoidal  structure 118, 119 

Deer  River 38,  75,88,  333 

described 31,32-35,334 

development  of 32-35 

topography  of  valley 29 

Delphic  mine,  location  of op.  186 

table  of  shipments  from op.  186 

Desmosite  gradation  to  spilosite,  plate  of 306 

of  Mansfield  foi-mation 64 

described 207 

De  Soto  Mining  Co.  {se(?  Mansfield  mine) ---  65 

Devitrification  of  aporhyolite 87 

of  metabasalt 102,103,126 

of  tuffs 138 

Dewitt,  N.  Y.,picrite-porphyry  at 219 

Diabase,  alteration  of 469 

altering  to  greenstone 466-484 

altering  to  borublende-schist 466 

intruding  conglomerate 476 

intrudiu^  granite 429 

intruding  Felch  Mountain  series 426 


Page. 

Diabase  intruding  Sturgeon  River  series 469 

(^SeeMetadiabase.) 

Dial  compass,  use  of 24,344 

described 341-342 

Diamond-drill  work  at  Hemlock  mine,  figure  of 177 

in8ec.20,  T.45N.,R.33  W 176 

Differentiation  of  magma 265 

Dike,  associated  with  ore  deposits  at  Paint  River 

mine 183 

in  Monitor  mine 183 

in  Paint  River  mine 183 

acid,  in  Archean,  described 46-49 

basic,  in  Archean,  described 46-49 

effect  on  topography,  sketch  of 46 

(See  Basic  dikes,  Acid  dikes.) 

Diller,  J.  S.,on  ultrabasic  intrusives 220 

referred  to 95 

Diopside  of  gabbro  and  norite 238,  212 

Diorite,  comparison  with  granodiorite 231 

crystallization  of  minerals  of 262 

intruded  by  diorite-porphyry 265 

intruded  by  granite 194 

intruded  by  hornblende-gabbro 265 

of  intrusive  series  described 222-232 

use  of  term 22.223 

Diorite-porphyry  intruding  diorite 265 

intruding  hornblende-gabbro 265 

Diorite-schist  associated  with  ore  deposits 183 

Diorite.     See  Motadiorite. 

Dip  needle,  use  of 24.  344 

described 342-343 

Doane  exploration 447, 449 

Dolerite,  contact  with  granite 192 

dikes  associated  with  ore  deposits 18.1 

grading  into  ba.salt 200 

including  sedimentary  rocks 20:J 

intruded  by  granite 194, 204 

intruding  Archean 48 

intruding  Hemlock  formation 77 

intruding  pyroclastics 147 

intrusive,  endomorphic  effects  of 211 

metamorpbism  of  Mansfield  slate,  described 204,  211 

relations  to  intrusives  of  other  districts 189 

relations  to  picrite' porphyry 212 

use  of  term 96 

{See  Metadolerite,  Basic  intrusives.) 

Dolomite  described 408-411,  431-437,  479-482 

analyses  of 409, 435 

containing  foreign  minerals 436 

metamorphisra  of 432 

of  Michigamme  Mountain  and  Fence  River  areas, 

described 431-437 

of  Randville  formation,  described 408-411 

of  Sturgeon  River  tongue,  described 479-482 

distribution  of 1 - 472 

relations  to  conglomerate -* 471 

relations  to  Felch  Mountain  fragmentals  . . .  472-473 

relations  to  Lake  Superior  sandstone 473 

orientation  of 410 

(See  Randville  dolomite.) 

Drainage  of  Crystal  Falls  district 31-36,334-335 

Drift.     (See  Pleistocene.) 

Dunn  Iron  Mining  Company.     {See  Dunn  Mine.) 

Dunn  mine,  analysis  of  ore  from 181 

depth  of 185 

description  of  ore  bodies 182 


INDEX. 


495 


I'a-r. 

Dunn  luiiif,  liK-aliuii  of 179,  op.  180 

table  of  shipment H  from op.  186 

Dynamic  iictiou.     {Sec  Pressure  effects.) 


Economic      ])iodiict8    of    Hemlock     formation    de- 
scribed   153-154 

Elevations  of  Crystal  FiiUs  district 30,31,332 

Elldaleii,  S-i\-eaeu   LUlletlinta  of 92 

Ellipsoidal  structure  m  metabasalt  described 112-124 

figure  of 112,113,114 

plate  o( 116,292,298 

Ells,  R.  "\Y.,  on  ellipsoidal  structure 118. 119 

Enlargement  of  feldspar 144 

of  quartz 57,85,404-405 

Enstatite  of  gabbro  and  uorite 238 

Epibasalt,  use  of  term 98 

Epidiorite,  use  of  term...^ 97,222 

Epidolerite.  use  of  term 97 

Epidoto  from  biotite 225 

/rom  cblorite 132 

from  feldspar 82,92,111.169,170,201,248,483 

from  hornblende 100,203 

included  in  biotite 225,220 

248 

151 

47 

444 

47 

445 

47 

- 443 

151 

442 

226 


included  ni  chlorite 

included  in  feldspar 

included  in  bornblende 

'   included  in  ilraenite 

included  in  quartz 

including  zoisite 

of  basic  dikes 

of  biotite-scbist 

of  Bone  Lake  schist 

of  chlorite-schist 

of  diorite 

of  greenstone 483,  485,  486 

of  metabasalt 101, 102, 117, 118, 127, 134 

of  py roclastics 147 

of  spilosite 206 

plate  of 302 

of  tuff 141 

of  variolite Ill 

of  volcanic  conglomerate 143 

Epidote-scbist  from  basic  volcanic 152 

of  Hemlock  formation 442 

Epidote-zoisite — 

from  feldspar 42,99,127,192,224 

from  bornblende 237 

including  allanite 444 

including  spbene 225 

of  metabasalt 99, 131, 132 

of  araygdules 124 

zonal  structure  in 101 

Epjdotizatiou  of  metabasalt 117 

Eriksen,  E.  T .,  referred  to 22 

Erosion  of  Archean xxiv,  39 

of  Cambrian u xxiv 

of  Huronian sxi,  xsii,  xxiv 

of  Pleistocene xsiv,  29 

Eruptive  breccia  formed  by  intrusion 195 

of  Hemlock  formation  described 3  35, 136 

Escanaba  Piver  described 335 

Eutaxitic  structure  in  metabasalt 103 

Exploration  for  iron  ore 12,  73,  330 

in  Mansfield  slate 73 

of  Mansfield  ore  deposit 67 


Page. 

Fairchild,  C.N. .indebtedness  to 331 

Fairbanks,  H.  "W.,  on  ellipsoidal  structure 118 

on  gabbros 240 

on  ultrabasic  rocks 247 

Fairbanks  mine.     (See  Lincoln  mine.) 

False  bedding  in  TTjipcr  Huronian 168 

Felch  Mountain  range  described 374-426 

elevation  of 1 331 

geographical  position  of 374 

literature  on 375-383 

relations  to  Sturgeon  River  rocks 462,  472, 473 

Feldspar,  alteration  of 42, 127, 170, 1 92,  201,  224,  478 

plate  of 288 

altering  to  albite 151,  201 

altering  to  biotite 42,  82,  89,  90,  92, 150, 170, 192,  201 

altering  to  calcite 82,90, 131, 132 

altering  to  cblorite 82,  99,  111,  131, 201 

altering  to  epidote 92,  111,  161, 170,  201,  248.  483 

altering  to  epidote-zoisite 42,  99, 127, 192,  224 

altering  to  feldspar Ill,  131, 151, 169, 170, 171,  201,  248 

altering  to  iron  oxide 42, 92 

altering  to  mica 169, 170 

altering  to  muscovite  . . .  42,  82,  90,  92, 192,  201,  224,  228,  248 

plate  of 286 

altering  to  paragonite 41, 42 

altering  to  quartz 42, 

99,  52,  111,  131, 151, 169, 170, 171,  201,  248 

altering  to  sericite 52,  89,  99, 101,  111,  127, 131,  477 

altering  to  zoisite 201 

plate  of 286 

cataclastic  structure  in 44 

enlargement  of 144 

included  in  dolomite _ 436 

included  in  bornblende 202 

plate  of 312 

included  in  microcline 294 

included  in  uralite 202 

including  apatite 234 

including  biotite 151, 171 

including  epidote 151 

including  bornbleiide 151 

including  iron  oxide _ 151,  234 

including  muscovite 468 

including  quartz 191.468 

including  rutile 234 

of  acid  lavas 90 

of  ampbibole-peridotite 255 

of  amygdulea 124 

plate  of .  284 

of  basalt,  plate  of 282 

of  biotite-granite 41,42, 191, 193 

of  biotite-scbist 467,  468 

of  Bone  Lake  schist 150, 151 

of  bronzite-norite-ijorpbyry 246 

of  diorite 223-225 

of  dolomite 436 

of  gabbro 234,  241,  242,  248 

of  granite 198,  388.  464 

of  graywacke 56, 169 

of  greenstone 470 

of  Groveland  formation 420 

of  metabasalt 99, 102, 104, 127, 129, 211 

of  metadolerite 200, 201 

of  micadiorite 227 

of  mica-schist 414 


496 


INDEX. 


Page. 

Feldspar  of  muacovite-biotite-gneiss,  plate  of 298 

of  peridotite - 252,257,260,261 

of  phyllite *« 

of  pjroclastics 146, 147 

of  quartz-niica-diorite-porphyry,  plate  of 310 

cf  rhyolite-porphyry 81,  82,  84,  85 

pressureeffects,  plate  of 

of  sedimentary  inclusions  in  granite 

of  slate,  altered ' *•■ 

of  spilosite -• 206 

145 

255 

Ill 


278 
197 
205 


414 
57 


of  tuff 

of  ultrabaaic  roots 

of  variolite 

of  TOlcanic  conglomerate 144,145 

of  volcanic  sand,  plate  of 296 

orientation  of - 84,90,171 

outlined  by  apatite '39 

penetrated  by  biotite -  -  -  • 

by  tourmaline 

penetrating  hornblende,  plate  of 322 

perthitic 41,42 

pressure  eflects  in 42,  90,  92,  169,  254,  388,  464 

plate  of 276 

replaced  by  biotite 1^3 

replaced  by  calcite 131 

twinning  of 41,42,89,191,201,224,233,468 

zonal  jntergrowtb  witb  borublende 256 

plate  of 322 

zonal  intergrowtb  witb  pyroxene,  plate  of 322 

zonal  structure  in 197,233 

Fence  River  described 31,35,334,335 

referred  to XIX,  38,  39,  50,  51, 152,  329,  333,  408,  427, 430 

Fence  Eiver  area  described 427-450 

168 

62 

63 

22 

417 


Page. 
Folding  of  Upper  Huronian,  relations  to  intrusivea.  189-190 

time  of 161,188 

Foliation  of  araphibole 395 

of  gneiss 430 

of  granite 387 

of  Sturgeon  formation 402 

Forbes,  indebtedness  to 331 

Ford  River,  described 334,  335 

Forsythe,  E.  J.,  analyses  by 435 

Foster,  J.  "W.,  and  Whitney,  J.  D.,  on  Felcb  Mountain 

range 375 

on  Crystal  Falls  district - 16 

map  of  Lake  Superior  land  district 17 

referred  to 21 

Fouque,  F.,  on  ellipsoidal  basalt 120, 121, 124 

Fox,  H.,  on  ellipsoidal  structure 118 

Frankenstein,  wekrlite  from 219 

Friction  breccia  of  Hemlock  formation 136 

Fumar(dc  action  on  sedimentaries 57 

Fundamental  Complex.     (,Sce  Arcbean.) 

Futterer,  Karl,  on  pressure  eflects  in  quartz 90 

on  pressure  effects  in  feldspar 91 


Ferruginous  cbert 

of  Mansfield  formation  described. 

relations  to  clay  slate 

Finlay,  J.  E.,  referred  to 

Flag  ore  of  Groveland  formation 


Floodwood  road 

Florence,  iron  ores  at xxVi 


175 
87 

278 
45 

244 


discovery  of  ore  deposits  at 

Plow  structure  in  aporhyolite-porphyry 

plate  of 

in  granite 

in  bornblende-gabbro 

inmetabasalt 99,102,105 

plate  of 280 

in  tufl's 138 

f^&m  Banding.) 
Folding  determined  by  magnetic  observations,  de- 
scribed    306-372 

of  Algonkian xxill,  163,427,428 

of  Sturgeon  Paver 471,472,474 

of  Arcbean - ^-^'" 

of  Crystal  Falls  district 26 

of  graywacke,  caused  by  intrusion 195 

of  Groveland  formation 448 

of  Hemlock  formation 441 

of  Mansfield  formation 438,  439 

of  Marquette  district 26, 452 

of  Menominee  district  •. 26 

of  Eandville  dolomite 432-434 

of  Sturgeon  quartzite -  399-400 

of  Sturgeon  Eiver  series 471,472,474 

of  Upper  Huronian  described 158-162 

sketch  of 


179 


Gabbro  described 

altering  to  greenstone-schist 

crystallization  of  minerals  of 

intruded  by  granite 

intruding  bornblende-gabbro 

pressure  effects  in 

relations  to  peridotite 

Garnet  of  actinolite-scbist 

of  graywacke 

of  Mansfield  schist 

of  mica-schist 

pseudomorphs  after  chlorite 

Geikie,  A.,  on  ellipsoidal  structure 

on  Tertiary  basalt 

Geographical  limits  of  Crystal  Falls  district 

of  Felcb  Mountain  range 

Glass  in  metabasalt 

in  tuff 

{See  Devitrification.) 

Glauconite,  alteration  of 

of  Groveland  formation 

of  Mesabi  iron-bearing  formation 

Glidden  exploration 

Globular  basalt.    (&m  Ellipsoidal  structure  in  meta- 
basalt.) 

Gneiss,  analyses  of 

developed  by  intrusion 

of  Arcbean  xviii 

described 

recrystallizatiou  of 

Gneissoid  biotite-granite  described 

Gneissoid    granite,   included  in    granite-porphyry, 

sketch  of 

of  Sturgeon  Eiver  tongue  described 

pressure  effects  in ■ 

Granite  altering  to  clay  slate 

altering  to  pbyllite 

analyses  of 

gradation  into  quartzite 

banding  of 

composition  of 

contact  with  dolerite 


233-249 
466  . 
262 
194 
265 

247,  248 
261 
450 
167 
413 
415 
403 

114, 118 
75 
25 
274 
99 
138 

422 
XX 

422 
161,  163 


391 

198 

43 

390,391 

390,  391 

43,44 

45 

463,  464 

44 

58 

58 

389 

51,52 

45 

58 

192 


INDEX. 


497 


Page. 

Granite,  contact  -with  sodimeiUaries 194-198,300 

pliito  of 298 

ioUationof 287 

iiicliuUug  sedimentary  fragments 195 

iutnuied  by  diabase 429 

intrudi;ig  iron-bearing  formation 370,381,426 

intruding  dolerite 204 

intruding  Folcli  Mountain  range 426 

intruding  gneiss 381 

intruding  granite 429 

intruding  Sturgeon  formation 426 

intruding  Sturgeon  Kiver  series 459 

iloribiban   (Brittany)   compared    witli    Crystal 

Falls  granite - 44 

of  Arcbean xviii,38,45,49,428,429 

described 40-13,387-389 

of  Micbigarame  River 16 

of  Sturgeon  River 459,464 

of  Sweden 44 

porpbyritic 40,45 

jjressure  effects  in 44, 194,464 

relations  to  quartzite 376 

relations  to  otber  intrusives 194 

(See  Biotite-granite,  Muscovite-biotite-granite, 
G-rauitite,  Gneissoid  granite.) 

Granite-porphyry,  sketch  of 45 

Granitite 226 

described 40-43 

Granitic  texture  in  diorite 223 

Granodiorite,  comparison  with  diorite    of   Crystal 

Falls 231 

Granopbyric  texture 85 

Granular  texture  in  hornblende-gabbro 240, 244 

plate  of - 316 

in  perlodotite 250 

Granulation  of  feldspar 42,169 

plate  of 276,316 

of  quartz 51,90,169 

Graywacke  altering  to  actinolite-scbist 57 

altering  to  chlorite-schist 57, 166 

altering  to  miea-scbist 57, 166 

brecciated  by  intrusion 195 

intruded  by  granite 194 

plate  of 298 

magnetitic  of  Upper  Huronian 176 

of  Mansfield  slate 56,57 

of  Sturgeon  formation 431 

of  Upper  Huronian 166, 167, 168, 169-174, 176 

of  sec.l9,T.46X.,R.32  W 27 

pressure  etfects  in 170 

recrystallizatiou  of 195, 198 

plate  of 298 

Great  slate  formation  of  Menominee  district sxv,  xxvi 

Great  "Western  mine,  description  of  ore  bodies  at  . . .         182 

depth  of 185 

fispuring  of  rocks 185 

location  of 178,  Op.  186 

table  of  shipments  from Op.  186 

Green  Bay,  streams  tributary  to 31.  334 

Greenstone 14 

from  diabase 484 

from  diabase-porpbj'rite 484 

in  conglomerate  of  Hemlock  formation 442 

intrusive  in  Algonkian  of  Sturgeon  River  tongue  482-485 

intrusive  in  Arcbean 38 

intrusive  in  conglomerate .' 475,  476 

MON   XXXVI — 32 


Greenstone  of  Mansfield  mine... 

topograjihy  of 

Greenstone-schist  from  diabase  . 

from  gabbro 


Pago. 

63 

333 

466 

466 

of  Arcbean  of  Sturgeon  River 465, 467 

Gregory,  G.  W.,on  ellipsoidal  structure 118, 119 

Grifdth  Sc  Nathaniel  quarries,  analysis  of  slate  from.  61 

Grit  altering  to  chlorite ]68, 169 

altering  to  muscovite 108, 169 

Groveland  formation  described sx,  415-423,  446-450 

correlation  of  (see  Relations) xxv,  xxvi 

distribution  of 415,416,446-448 

folding  of 448 

intruded  by  granite 426 

magnetic  observations  in 447, 448 

origin  of 423 

pressure  effects  in 449 

quartzites,  resemblance  to  Mesabi  chert 422-423 

relations  to  Arebean 416, 424 

relations  to  Ajibik  quartzite xxr,  449, 456 

relations  to  Hemlock  formation xx,  xxi,  xxvi,  447 

relations  to  Mansfield  formation 411,413,438,447 

relations  to  Negaunee  formation xx,  449, 455 

relations  to  Eandville  formation xx 

relations  to  Upper  Huronian xxii,  425 

thickness  of xvii,  448 

topogranby  of 415,  416, 446-448 

Groveland  mine 413, 415 

Grijnerite- schist  of  Groveland  formation 418 

Giimbel, ,  on  epidiorite 97 


H. 

Halleflinta,  of  Elfdalen,  Sweden 

Hampton  Village,  N.  T.,  analysis  of  slate  from  . 
Harriman.F.  J.,  referred  to 

Hartz  Mountain,  analysis  of  spilosite  from 

Hatch,  F.,  on  limburgite  . 


92 
61 
22 
207 
221 

Hawaii,  volcanics  of 75, 120 

Ha-wes.  G.  "W.,  on  metadolerite 96 

Hedstrtim,  H.,  on  micropoikilitic  texture 83-84 

Hematite  deposits  of  Upper  Huronian  described...  180,182 

of  Groveland  formation 417, 418, 419, 420, 450 

of  Mansfield  ore  deposits  described 69-70 

of  Paint  River  district 18 

Hematite  from  basic  volcanics 152 

from  siderite 168 

included  in  biotite 252 

included  in  quartz 419 

of  am y  gdules 1 25 

of  Bone  Lake  schists 151 

of  clay  slate 57 

of  quartzite 450 

orientation  of 418 

Hemlock  formation  described xx,  73-154, 440-446 

acid  volcanics  of,  described 80-95 

classification  of 79, 80 

correlation  of xxv,  xxvi 

distribution  of 27,73,74,440-441 

economic  products  from 153, 154 

folding  of 441 

metamorphism  of xxiv,  446 

of    Micbigamme   Mountain    and   Fence    River 

areas  described 440-446 

origin  of 27,78 

pressure  effects  in xxiv,  75 

pyroclastics  of,  described 135-148 


498 


INDEX. 


Page. 

Hemlock  formation,  pryoclastics  of,  plate  of 140 

relations  to  lutmsiyes 77,204 

relations  to  Mansfield  formation xs,  xsi,  xxvi,  64,  76 

relations  to  Groveland  formation xx,  xxi,  xxvi,  447 

ralations  to  Randville  dolomite xx,  75 

relations  to  Upper  Hurouian 77 

rtiyolite-porphyry  of,  described 81-87 

sedimentaries  of,  described 152, 153 

thickness  of ' XVII ,  27,  74,  75,  441 

topograpby  of 73,  74,  440, 441 

Hemlock  mine 157, 1 77 

analysis  of  ore  from 181 

diamond  drill  work  at,  figure  of 177 

exploitation  of  ore  deposits  at -  175 

location  of Op.  186 

table  of  shipments  from Op.  186 

Hemlock  Kiver  described 31 

Hesse-Darmstadt,  wehrlite  from 219 

Hillebrand,  "W.  F.,  analvsis  by 61 

Hobbs,  Wm.  H.,  on  luetamorpbism  of  schists 130 

HoUister  mine,  location  of 178,  Op.  186 

Holy oke  formation 20 

Hoosau  schists  of  western  Massachusetts 394 

Hope  mine,  location  of 178,  Op.  186 

table  of  shipments  from Op.  186  | 

Horizontal  needle,  deflection  of,  figure  of 345  j 

Hornblende,  alteration  of 234-235 

altering  to  actinolite 215 

altering  to  calcite 203,215 

altering  to  chlorite 100,  203,  214,  215,  237 

altering  to  epidote 100,203,237 

altering  to  magnetite 215 

crystallization  of 257,258,259 

from  augite 100,201 

grading  into  hornblende 241,245,251 

plate  of 312 

included  in  augite 255 

included  in  feldspar 151, 484 

included  in  quartz 466 

including  anatase 236 

including  augite 250,255,257 

including  biotite 251,260,261 

including  brouzite 250 

plate  of 306 

including  epidote 47 

including  feldspar 235,241 

plate  of , 312 

including  ilmenite 236,246 

including  iron  oxide 47,215,  251,487 

including  malacolite 238 

including'  olivine 251, 257, 260 

including  pyroxene 237,  241,  245,  251,  260 

plate  of - 3 12 

including  quartz 47 

including  rutile 236,239,248 

including  spinel 236 

inclusions  in 240, 245,  248, 260 

plate  of 318 

intergrown  with  augite,  plate  of 320 

intergro wn  with  augite  and  olivine '  255-260 

intergrown  with  bronzite,  plate  of 318 

intergrown  with  feldspar 256 

plate  of 322 

intergrown  with  pyroxene 251, 260 

plate.of 320 

intergrown  with  spinel 256 


Page. 

Hornblende  of  amphibolite 396 

of  amphibole-peridotite 253, 258 

of  arkose 479 

of  basic  dike 47 

of  biotite-granite 192 

of  Bone  Lake  schists 150 

of  bronzite-norite-porphyry 246 

of  diorite 226 

ofgabbro    234,237,248 

of  greenstone 470,483,486 

of  hornblende-gabbro 240,  241, 243, 245 

plate  of 312 

of  hornblende-gneiss 174, 465 

of  Hemlock  schist 445 

of  metabasalt 100 

of  metadolerite 200,202,203 

ofperidotile 251,252,257,260 

of  picrite-porphyry 215 

of  tuff 141,142,145 

of  volcanic  conglomerate 145 

of  volcanic  sand,  plate  of 296 

orientation  of 174,214,215,395,396,465 

penetrated  by  feldspar,  plate  of 322 

pbenocrysts  in  Bone  Lake  schists 150 

pressure  effects  in 237 

relations  of  orientation  to  biotite 486 

zonal  structure  in 2-26, 234, 235,  236,  256 

{See  Hornblende  intergrowth.) 

Hornblende-gabbro  described 240-244 

plate  of 312,314,316,318 

analyses  of 242,263-264 

crystallization  of  minerals  of 262 

intruded  by  bronzite-norite 243,  249,265 

intruded  by  diorite 265 

intruded  by  gabbro 265 

intruded  by  peridotite 246,  260,  265 

Hornblende-gneiss  of  Arcbean  described 395-397 

intrusive  in  Upper  Huronian 167, 173, 174, 175 

pressure  effects  in 175 

Hornblende-schists  from  diabase 466 

from  gabbro 466 

intrusive  in  Sturgeon  River  Algonkian 485 

of  Arcbean  of  Sturgeon  River 465-467 

of  Upper  Huronian 173 

Hornblende-slate  described 14 

distribution  of 13-14 

Hubbard,  Bela,  on  Sturgeon  River  tongue 459,461 

Hulst,  N.  P.,  on  composition  of  ore  deposits 69 

Huron  Mining  Co.     {See  Columbia  mine.) 

Huronian  rocks,  correlation  of xsv,  xxvi 

distribution  of,  plate  of 160 

erosion  of xxiv 

folding  of XXII,  25, 163 

metaraorphism  of xxiii 

relations  to  Arcbean 39, 378, 379,  380, 410 

relations  to  Cambrian xxi v,  28 

succession xxv,  xxvi 

unconformity  within xxvii 

(See  Algonkian,  Upper  Huronian,  Lower  Huronian.) 

Hutcbings.W.Maynard,  analyses  by 59 

on  composition  of  clay 60 

on  spilosites 206 

on  transfer  of  material  from  basic  intrusive 211 

Hyalopilitic  texture  in  metabasalt 98, 99, 104 

Hyperstbene  included  in  augite 255 

Hypidiomorphic  texture  in  gabbro 233 


INDEX. 


499 


Page. 

75 
105 

84 
265 

95 


Iceland,  volcanica  of 

Iddiug-s.  J.  P.,  on  niagmatic  differentiation 

ou  micropoikilitic  texture 

on  origin  of  igneous  rocks 

referred  to , 

lUinoia  Steel  Co.    {See  Tonngstown  mine.) 

Ilmenito  altering  to  rutile * 202, 239 

altering  to  sphene 239 

included  in  biotite 1 216 

included  in  bronzite 238 

included  in  chlorite 216 

included  in  horubleude 236, 246 

including  epidote '  444 

ini-luding  quartz 444 

of  basic  dikes 47 

of  Bone  Lake  schist 151 

of  chlorite  schist ' 442 

of  gabbro  and  uorite 236,238,239 

of  Hemlock  schist 444 

of  metadolerite 202 

of  picrite-porphy ry 216 

of  sedimentary  inclusions  in  granite 197 

Instruments,  magnetic,  use  of,  described 341-344 

Interrange  exploration 439, 448 

Intersertal  texture  in  tuetabasalt 98, 99, 104 

Intruaives  described 187-265, 469-470, 482-485 

age  of 188,189 

correlation  of 189 

effect  on  magnetic  observations 371,372 

in  Archean xviii,  38,45-49 

in  Felch  Moantain  range 426 

in  Hemlock  formation 77 

in  Huronian xxm,  190 

in  Mansfield  slate 63,64,204^211,413 

in  Sturgeon  Kiver  tongue,  described 469-470,  482-485 

in  Upper  Huronian 164, 174, 175, 190 

influence  on  topography 54, 333 

metamorphism  of  Mansfield  slate,  described-  .204^211, 413 

relations  to  Cambrian 188 

use  of  term 187 

IntrusiTe  series,  related,  described 221-265 

composition  of 263-265 

crystallization  of  minerals  of 262 

relative  age  of  rocks  of 265 

textures  of 262 

Intrusives,  unrelated,  described 190-221 

Iron-bearing  formation  described sx,  415-423, 446-450 

correlation  of 20, 330 

intruded  by  granite 381 

magnetic  observations  in 338, 339 

near  Crystal  Falls,  succession  in 179,180 

of  Felcb  Mountain  range 378 

described 415-423  ' 

distribution 377 

Eominger  on 381-383 

of  Penokee  district,  located  by  magnetic  work .  -  24 

of  Upper  Huronian 168 

relations  to  marble 376 

relations  to  volcanics  of  Penokee  series ,    sxi 

(See  Groveland  formation,  Iron-ore  deposits.) 
Iron  carbonate.     {See  Siderite.) 

Iron  County H 

Iron  Mountain 385 

slates  of,  correlation  witb  Mansfield  schist 413 

Iron  ore,  analyses  of : 69 

by  Brooks,  re  I'erred  to 19 


Page. 

Iron  ore,  composition  of. isi 

from  siderite 70.  71, 184 

from  eruptive  rock 334 

origin  of 70, 71, 184 

Credner  on 71 

Irving  on 70,71,13.0 

Spurr  on 422 

Van  Hise  on 39, 70, 71, 130, 168 

Iron  ore  deposits  associated  with  intruaives 183 

comparison   -with  ore  deposits  of  adjacent  dis- 
tricts   180-181 

concentration  of,  at  Mansfield  mine 72 

conditions  favorable  for,  "Van  Hise  on 72 

discovered  at  Crystal  JFalls 175 

discovered  at  Florence 175 

explorations  for 12,330 

of  Amasa  area 175, 177 

of  Armenia  mine,  description  of 183 

of  Crystal  Falls  area 178,186 

production  of  ore  from,  table  of op.  186 

of  Crystal  Falls  district,   compared  with  Me- 
nominee iron  ores 19 

table  of  production  from op.  186 

of  Columbia  mine  described 182 

of  Dunn  mine  described 182 

of  Great  Western  mine  described 182 

of  Felch  Mountain  range,  Burt  on 375 

of  Hemlock  mine 177 

of  Mansfield  formation 27,62 

described 65-73 

of  Mansfield  mine 63 

described 67-70 

of  Paint  Kiver  district.  Brooks  on 18 

of  Paint  Hiver  mine ]83 

of  Pesbakumme  Falls 375 

of  sec.  20,  T.  45  K.,  K.  33  "W 176 

of  S6c.34,T.46N.,R.33'W 176 

of  Ux^per  Huronian 28, 166 

described 175-186 

distribution  of,  described 176-180 

history  of 175 

methods  of  mining  in,  described 184-185 

origin  of,  described 183-184 

prospecting  in 185 

relations  to  adjacent  rocks,  described 182-183 

size  of,  described 184 

sketch  of  occurrence  of 182 

table  of  shipments  from op.  186 

relations  to  claj'  slate 63 

search  in  Bone  Lake  schists 151 

shipments  from  Crystal  Falls  area,  table  of op.  186 

Iron  oxide  altering  to  sphene 212 

from  feldspar 42, 92 

included  in  chlorite 218 

included  in  feldspar 151, 234 

included  in  hornblende 47,  215,  251 

included  in  quartz 419 

included  in  siderite 133 

of  acid  lavas 89, 91 

of  amygdules 124 

of  biotite-granite 192 

of  Bone  Lake  schists 151 

of  conglomerate 64 

of  dolomite 53 

of  gabbro  and  norite 239 

of  Groveland  formation 417, 419,  449 


500 


INDEX. 


Page. 
1,99,117,127,131 
202 


Iron  oxide  of  metabaaalt 

of  raetadolerite 

of  musco^ite-biotite-gneisB,  plate  of 

of  peridotite 

of  picrite-porphyry 314,216, 

of  variolites 

veins  in  Groveland  formation 

Iron  pyrites  of  basic  dikes 

of  peridotite "^^-^ 

Iron  Star  Co.     {See  Great  "Western  mine.) 

Iron  Star  mine.     {See  Great  "Western  mine.) 

Ishpeming  formation,  correlation  of xxv,  XXVI 

Irving,  K,  D.,  in  concretionary  structure  in  ferrugi- 
nous cliert *22 

on  Felcli  Mountain  range 379-380 

on  magnetic  mapping ■  -    24,  336 

on  mica-schist ^^^ 

on  origin  of  iron  ore 70,71, 130 

Ivrea,  Italy,  norite  from,  analysis  of 244, 245 

Italy,  noritea  from 235 


Jacob's  statf,  use  of,  described 342 

Janesville.N.Y.,  analysis  of  slate  from 261 

Johnston-Lavis,  H.  J.,  on  resorption  by  intrusives. .  227 

Judd.J.AV.,  on  basalts 104 

Julien,  A.  A.,  on  lithology  of  Crystal  Falls  district .  21 

on  Upper  Huronian 173-174 


K:ayser,E.,  analyses  by 207-208 

22 
394 
221 
220 
219 

95 
162 


Page. 
Lake  Superior  Survey,  reconnaissance  by 329 

topography  by 22 

Lamont  mine,  location  of 178,  Op.  186 

table  of  shipments  from Op.  186 

[See  Monitor  mine.) 
Lamont  Mining  Company.     {See  Lamont  Mine.) 

Land  Survey,  United  States 23, 343 

Lane,  A.  C,  referred  to 21 

Lang,  H.  Otto,  referred  to 105 

Larsson,  Per,  on  composition  of  iron  ore 69 

Laurentian.     {See  Arcbean.) 

Lavas,  basic,  described 95-135 

Lawson,  A.  C,  on  ellipsoidal  structure 118-119 

on  Laurentian  and  Coutcbiching 380 

Lean  ore  in  Upper  Huronian 182 

Lee  Peck  mine,  location  of 178,  Op.  186 

table  of  shipments  from Op.  186 


Kelley,  F.  T.,  referred  to 

Kemp,  F.  J.,  analysis  of  mica-schist 

ou  limburgite 

on  ultrabasic  intrusives 

on  picrite-porphyry 

referred  to  

Keweenawan  series  relations  to  Cambrian  sandstone . 

Kona  dolomite,  correlation  of xxv,  xxvi 

Krakatao,  tuff  from 142 

L. 

La  Platte  River,  olivine  gabbro  from 255 

Labradorite  of  gabbro  find  norite 233 

of  bornblende-gabbro 241 

of  metabasalt 104,105 

twinning  of 104 

Lakes  of  Crystal  Falls  district,  origin  of 32 

Bone,  described 35 

referred  to 156 

Huron,  Huronian  rocks  of  north  shore  of xvii 

Michigan,  streams  tributary  to 31 

Superior,  streams  tributary  to 31 

Light,  described 34 

Liver,  described - 34 

Mary 161 

Squaw 335 

SuuDog 335 

Tobin 164 

Trout 335 

Lake  Superior  land  diatrict,  map  of,  by  Foster  and 

Whitney 17 

Lake  Superior  sandstone  described 28 

unconformity  with  Huronian 28 

{See  Potsdam  sandstone.) 

Lake  .Superior  Survey 183 


Leith,  C.  K. ,  indebtedness  to 

Leucoxene  of  greenstone 

of  metabasalt 

of  metadolerite 

surrounding  magnetite 

Lewis,  H.  C,  on  saxonite-porpbyry 

ou  alteration  of  olivine 

Lewis,  indebtedness  to 

Life  in  Mansfield  slate,  presence  of  carbon. 
Light  Lake  described 


12 
470 
100 
202 
470 
220 
218 
331 
60 
34 

Limburgite,  porpby ritic  intrusive,  described 212-221 

Lime  rock  of  Mansfield  mine 63, 67,  69, 70 

Limestone  of  Felch  Mountain  range,  Rominger  on..  383 

of  hemlock  formation 153 

{See  Dolomite,  Randville  dolomite.) 

Limonite  deposits  of  Upper  Huronian 180  . 

from  calcite 153 

from  siderite 1 68 

of  Groveland  formation 420 

Lincoln  mine 179 

analysis  of  ore  from 181 

location  of 178,  Op.  186 

table  of  shipments  from Op.  186 

Lincoln  Mining  Co.     {See  Lincoln  mine.) 

Lindgren,  "Waldemar,  on  granodiorite 231 

Liver  Lake  described 34 

Long  Portage 174 

Long  Portage  series 173 

Lessen,  K.,  on  altered  clay  slates 206,209 

Lower  Huronian  series  described xvii, 

xviil,  50-154,  398-423,  430-450, 471-481 

erosion  of XXI,  xxii 

magnetic  rocks  of,  described 338-341 

relations  to  Arcbean xix,  55 

relations  to  intrusives 187, 190 

relations  to  Upper  Huronian xvii, 

XXir,  158, 160, 161, 162, 163, 176,  424 

auccessiou  in xxv,  xx vi,  50 

thickness  of xvii 

Lower  Marquette  aeries,  distribution  of 453-455 

relations  to  Crystal  Falls  series xxv,  xxvi 

relations  to  Menominee  series xxv,  xxvi,  451-457 

relations  to  Sturgeon  River  series 462 

ore  deposits,  comparison  with  Crystal  Falls  ore 

deposits 

Lower   Menominee,   relations    to    conglomerate    of 

Sturgeon  River  tongue 

relations  to  Lower  Marquette 451-457 

Luster  mottling  in  raetadolerite 100 


181 


473 


INDEX. 


501 


M. 

Page. 

Magnotio  instruuieuts,  «3e  of,  described 341-344 

Ma.i;netic  ubaervations 24,77 

described 336-372 

efiect  of  iutniaivos  on,  described 371-372 

iu  Grnveland  format  ion 377,  415,  416,  447,  448 

in  Negaunee  forniiition 453,455 

in  melabasalt 134 

in  Sturgeon  Hi ver  tongue 460 

in  Upper  Hurouian 175, 176 

described 156-157 

enfolding 366-373 

Magnetic  rocks  described 338-341 

depth  of,  determination  of 354-356 

Magnetism  iu  magnetic  rocks,  distribution  of 339-341 

466 

152 

215 

217 

168 

394 

252 

436 

487 

394 

466 

151 


Magnetite  altering  to  sphene 

from  basic  volcanics 

from  hornblende 

from  olivine 

from  siderite 

included  in  microcliue 

included  in  biotite 

included  in  dolomite 

included  in  hornblende 

included  in  quartz 

of  amphibole-schist 

of  Bone  Lake  schist 

of  graywacke' 170,176 

of  greenstone 470, 487 

of  iron-bearing  formation 338, 417, 419, 421 

of  metabasalt 100 

of  metadolerite 202 

of  peridotite 261 

of  picrite-porphyry 218 

of  pyroclastics 147 

of  sedimentary  inclusions  in  granite 197 

of  volcanic  conglomerate 143 

surrounded  by  leucosene 466, 470 

surrounding  amygdaloidal  cavities 102 

titaniferous 47 

Malacolite  included  in  hornblende 238 

of  gabbro  and  norite 238 

Manhattan  mine,  location  of op.  186 

table  of  shipments  from op.  186 

Manganese  ore  of  Upx>er  Huronian 181, 182 

Mansfield  (to-mi) 12,54,130,178,203,204 

Mansfield  formation  described xx,  54^73, 411-415, 437-440 

adinole  of,  described 208-209 

Bessemer  ore  in   27 

clay  slate,  described 57-62 

analysis  of 59,  61 

compared  with  analyses  of  contact  product.  209-211 

compared  with  clay 60 

compared  with  New  York  slate 61 

compared  with  Vermont  slate 61 

chert  of  described 62 

correlation  of xxv,  xx\a,  413 

desmosite  of 207 

distribution  of 27, 54, 438 

exploration  in 73 

folding  of 438, 439 

gray  wacke  of,  described 56-57 

iron  ores  of 27, 62 

described 65-73 

metamorphosed  by  intrusives 204^211, 413 

of  Felch  Mountain  range  described 411-415 


Page. 
Mansfield  formation  of  Michigamme  Mountain  and 

Fence  lii ver  areas  described 437-440 

phyllite  of,  described 57-G2 

possible  continuation  of 55 

relations  to  adjacent  formations gg  64 

relations  to  Archean 424 

relations  to  Groveland  formation 411,  413, 438, 447 

relations  to  Hemlock  formation xx,  xxi,  xxvi,  64, 76 

relations  to  intrusives 63,  64,  203,  204 

relations  to  Randville  formation xx,  55, 4  U ,  434,  438 

relations  to  slate  of  Michigamme  Mountain 56 

relations  to  Upper  Huronian xxii 

siderite-slate  of,  described 62 

spilosites  of,  descri  bed 206-207 

structure  of g^ 

thickness  of xvn,  64, 65,  412, 438, 439 

toijography  of 54^433 

Mansfield  mine 27  54  411 

described 65-70 

analyses  of  ore  from 59 

cross  section  of g3 

concentration  of  ore  at 72 

figure  of  cracks  in Q5 

location  of op.  186 

table  of  shipments  from op.  186 

Mansfield  ore  deposit,  exploration  of 67 

concentration  of 70 

origin  of 70 

relations  to  surrounding  beds 68 

Marble  of  Randville  formation 434, 435 

described 408-311 

of  Sturgeon  River  dolomite 480, 481 

relations  to  iron-ore  formation 376 

relations  to  quartzite 376 

{See  Dolomite,  Randville  dolomite.) 

Marquette  district,  correlation  of  formations  of XXA', 

XXVI,  189,  330,  451-457,  470-471 

folding  of 26, 452 

Clarksburg  formation  of iy2 

iron-beai'ing  formation  of,  connection  with  Crys- 
tal Falls  iron-bearing  formation.. 330 

Martin-Garcia,  olivine-gabbro  from 255 

Martite  of  Groveland  formation 417-419 

Mary  Lake jei 

Mastodon  Iron  Co.    {See  Mastodon  mine.) 

Mastodon  mine,  analysis  of  ore  from 181 

caving  system  at 185 

location  of 179,  op.  186 

method  of  mining  at 184,185 

table  of  shipments  from op.  186 

Matrix  of  ellipsoidal  metabasalt 122, 132,133,135 

described 114-118 

plate  of 116,  284,  298 

Matthews,  E.  B.,  indebtedness  to 331 

referred  to 22 

Maurer,  E.  R.,  referred  to 22 

McCutcheon'a  Lake 91 

McKim,  J.  A.,  referred  to 22 

McNair,  F.  W.,  referred  to 22 

Melaphyres 98 

Menominee  district,  correlation  of  formations  of xxv, 

XXVI,  330 

folding  of 26 

relations  to  Crystal  Falls  district 11,  25 

structure  of 378 

iron  ore  of,  compared  with  Crystal  Falls  iron 
ore '. 19,181,186 


502 


INDEX. 


Page. 

Menominee  Eiver 175,  335, 374, 375,  460 

described 32,  334 

ilenomiDce  series  of  Sturgeon  Pdver 462 

Merriam,  "W".  N.,  indebtedness  to 12 

on  dike  at  Glidden  exxjloration 183 

magnetic  lines  traced  by 12 

referred  to xv,xxvi,22 

sketcli  of  folding  of  Upper  Huronian 179 

Merrill,  G.P.,  on  ultrabasic  intrusives 220 

Mesnard  quartzite  of  Marquette  district 452 

correlation  of xx v,  xxvi 

Mesabi  cbert,  resemblance  to  Groveland  quartzite.  -  422, 423 

Mesabi  iron-bearing  formation ,  origin  of 422 

Mesqua-cum-a-cum-sepe  (riTer) 15 

Metabasalt  described 98-135 

alteration  of 126-135 

aniygdaloidal,  plate  of 280,282,284,290 

analyses  of 103,106,107 

carbonation  of 132, 133;  134 

conglomerate  of  Upper  Huronian 163 

devitrification  of 102,103,126 

ellipsoidal,  described 112-124 

figure  of 112,113,114 

plate  of 116,292 

matrix  of,  described 114-118 

plate  of 284,.298 

eut-axitic  texture  in 103 

flo'w  structure  in,  plate  of 280 

fragments  in  volcanic  sand,  plate  of 296 

grading  into  dolerite 2O0 

intrusives  described 211-212 

perlitic  parting  in,  plate  of 294 

road  material IS'i 

eilicification  of 133, 134 

spberulitic 98, 108 

texture  of,  plate  of 280, 286, 288, 290, 294 

use  of  term 96,97,98 

variolitic,  described 108-111 

plate  of 110 

wea^tbering  of 134-135 

Metadiabase,  use  of  term 96,  97 

Metadolerite,  described 199-204 

analysis  of 203 

distribution  of ■ 199 

relations  to  Hemlock  volcanics 204 

relations  to  Mansfield  slates 203, 204 

relations  to  other  intrusives 204 

relations  to  Upper  Huronian 204 

use  of  term 96, 97 

Metamorpbism  of  Arcbean x viii 

of  Crystal  Falls  series xxni,  xxiv,  xxv,  xxvi 

of  Hemlock  formation xxiv,44G 

of  quartzite 425-426 

of  Upper  Huronian 28,425-426 

{See  Alteration.) 

Method  of  location  of  ledges 23 

Methods  of  mining 184, 185 

Methods  of  work  of  Lake  Superior  Division 22, 23,  24 

Metropolitan  mine,  granite  dike  at 381 

Mica,  from  feldspar 169,170 

included  in  microcline 394 

of  acid  lavas 89,  91 

of  biotite-granite 43-192 

of  granite 198,388,464 

of  Hemlock  schist 444 

of  bornblende-gabbro 243 


Page. 

Mica  of  bornblende-schist 465,  485 

of  Mansfield  slate 205 

oi"  mica-schist 414 

of  quartzite 425 

orientation  of 43, 44, 89,  91, 198, 387, 388 

Mica-diorite  226 

analysis  of 231,263,264 

comparison  with  monzonite 232 

opbitio  texture  in,  plate  of 308 

Mica-gneiss,  developed  by  intrusion 195, 197 

from  basic  volcanics 152 

of  graywacke 170 

of  Upper  Huronian 166,167,171,172,173 

pressure  efi'ects  in 171 

Miea-scbist  described 392-395, 423-426 

analyses  of 394 

developed  by  intrusion 195, 196 

from  basic  volcanics 152 

intruded  by  amphibolite 392 

intruded  by  pegmatite 392 

of  Arcbean,  described 392-395 

of  Pelch  Mountain  range,  described 423, 426 

of  graywacke 170 

of  Mansfield  schist 413 

of  Upper  Huronian 166. 167, 171, 172, 173, 174, 425 

pressure  efi'ects  in 171 

relations  to  Kaudville  dolomite 392 

relations  to  Sturgeon  quartzite 392 

Mica-slate  described 15 

distribution  of 14, 15 

of  Mansfield  formation 439 

Mica-titanite  incl  uded  in  d  olomite 436 

Michel-Levy  on  method  of  feldspar  measurement. . .  104, 224 

on  ophitic  texture 200 

on  texture  of  rhyolite-porphyry 81, 85, 105 

Michigamme  dam 55 

formation,  correlation  of xxv,  xxvi,  28, 165 

replaced  by  Clarksburg  volcanics xxvi 

Michigamme  jasper,  magnetic  observations  in 339 

Michigamme  Mountain xx,  56,  446, 447, 448 

analyses  of  dolomites  from 435 

elevation  of 333 

slate  of - 56 

Michigamme  Mountain  area  described 427-450 

Michigamme  Piver 16, 

21,  54,  65,  156,  161,  163,  164,  166.  167,173, 
174, 187, 190, 194, 203,  204,  205,  240,  241,  243, 
245,  249,  253,  329,  330,  331,  333,  411,  432,  438 

described 31,334,335 

course  of 54, 55 

changing  channel  of 56 

Michigan  Exploring  Co.    (See  Michigan  mine.) 

Michigan  mine 157 

location  of op.  186 

table  of  shipments  from —    op.  186 

Microcline,  including  feldspar 394 

including  magnetite 394 

including  mica ^ 394 

including  quartz 393, 403 

of  biotite-granite 41,42 

of  diorite 225 

of  mica-schist 394,414 

of  quartz-mica-diorite 228 

orientation  of 294 

Microgranitic  texture 83 

described 86,87 


INDEX. 


503 


Page. 

Microgranitic  toxturo  in  quartz-niica-diorite 228 

in  quartz-mioa-diorite-porphyry,  plate  ol' 310 

Microlitic  feldspar 99 

Micro-ophitic  texture  in  metabasalt .'-.  104,212 

Micropegiuatitic  texture  in  acid  lavas 89 

in  biotite-granite 191,192, 193 

indiorite 224 

in  gneisses 390 

in  metadolerite 201 

in  quartz-niica-diorite 227 

in  tonalite 230 

{See  Pegmatitic  texture.) 
Micropoikilitic    texture    in   rhyolite-porpbyry    de- 
scribed        83-86 

plate  of 270,272 

in  feldspar 171 

origin  of 85 

(See  Poikilitic  texture.) 

Milwaukee  and  Nortliern  Ewy 406 

Mines,  Aragon 09 

Armenia,  description  of  ore  body  at 183 

location  of 178,  op.  186 

table  of  shipments  from op.  186 

Blaney.     {See  Hope  mine.) 
Caledonia,     {See  Mansfield  mine.) 

Calumet  and  Hecla 399 

Claire,  location  of 178,  op.  180 

table  of  shipments  from op.  186 

Columbia,  descrijjtion  of  ore  bodies 182 

location  of 179,  op.  186 

table  of  shipments  from op.  1S6 

referred  to 161 

Crystal  Falls,  analysis  of  ore  from 181 

location  of 178,  op.  186 

table  of  shipments  from op.  186 

referred  to 161 

Delphic,  location  of op.  186 

table  of  shipments  from op.  186 

Dunn,  analysis  of  ore  from 181 

depth  of 185 

description  of  ore  bodies 182 

location  of 179,  op.  186 

table  of  shipments  from op.  186 

Fairbanks.     {See  Lincoln  mine.) 

Great  Western,  dessription  of  ore  bodies  at 182 

deptli  of 185 

fissuring  of  rocks 185 

location  of 178,  op.  186 

table  of  shipments  from op.  186 

Groveland 413,415 

Hemlock 157, 177 

analysis  of  ore  from ■...  181 

diamond  drill  woik  at,  figure  of 177 

exploitation  of  ore  deposits  at 175 

location  of op.  186 

table  of  shipments  from op.  186 

Hollister,  location  of 178,  op.  186 

Hope,  location  of 178,  op.  186 

table  of  shipments  from j op.  186 

Iron  Star.     {See  Great  "Western  mine.) 

Lament,  location  of 178,  op.  186 

table  of  shipments  from -    op.  186 

{See  Monitor  mine,) 

Lee  Peck,  location  of 178,  op.  186 

table  of  shipments  from op.  186 

Lincoln 179 


Page. 

Mines,  Lincoln,  analysis  of  ore,  from 181 

location  of 178,op.l86 

table  of  shipments  from op.  186 

Manhattan,  location  of op. 186 

table  of  shipments  from op.  186 

Mansfield 27,  54, 411 

described 65-70 

analyses  of  ore  from 69 

Cross  section  of G3 

concentration  of  ore  at 72 

figure  of  cracks  in 65 

location  of op.  186 

table  of  shipments  from op.  186 

Mastodon,  analysis  of  ore  from 181 

caving  system  at 185 

location  of 179,  op.  186 

method  of  mining  at 184, 185 

table  of  shipments  from op.  186 

Metropolitan,  granite  dike  at 381 

Michigan 157 

location  of op.  186 

table  of  shipments  from op.  186 

Monitor,  dike  in  {see  Lamont  mine) 183 

!North western 399,  407 

Paint  Kiver 183 

dike  in 183 

location  of 178,  op.  186 

ore  deposits  of 183 

table  of  shipments  from op.  186 

Pewabic 69 

Sheldon  &.  Schafer.     {See  Columbia  mine.) 
Smith.     (See  Armenia  mine.) 
ITnion.     {See  Columbia  mine.) 
"Wauneta.     {See  Hope  mine.) 

Toungstown 182, 186 

location  of 178,  op.  186 

table  of  shipments  from op.  186 

Mining,  depth  of 185 

Mining  methods 184-185 

Miarolitic  texture  in  tonalite 229 

Mixed  ore  in  Upper  Hurouian 182 

Monitor  mine,  dike  in  {see  Lamont  mine) 183 

Morbihan,  Brittany,  granite,  compared  witb  Crystal 

Falls  granite 44 

Muscovite,  development  of 484 

from  feldspar 42,  82,  90,  92, 192,  201,  224,  228,  248 

plate  of 286 

from  schistose  x>y roclastic 146 

from  staurolite 196 

included  in  feldspar 468 

included  in  quartz 171,  394,  403 

including  biotite 298 

iutergrown  with  biotite 393,  414 

of  acid  lavas 89 

of  basalt,  plate  of 290 

of  basic  dikes 47    > 

of  biotite-granite 192 

of  biotite-schist 484 

of  Bone  Lake  schist 151 

of  granite 464 

of  gray  wacke 170 

of  metabasalt 127,129 

of  mica-schist : 392, 393 

of  muscovite-biotite-granite 193 

plate  of 298 

of  pbyllito 440 


504 


INDEX. 


Page. 

Mascovite  of  sedimentarieBmetamorpliosed  by  intru- 
sion  195,197 

of  spilosite,  ijlate  of 30^ 

parallel  growth  with  biotite 170 

relations  of  orientation  to  biotite - 484 

Muscovite-biotite-gneiss,  plate  of 298 

of  Sturgeon  forn?ation 401 

Muscovite-biotite-granite ^^^ 

described 193,194 

Navitic  texture  in  raetabasalt 98, 99, 104 

Needle,  dip,  use  of 344 

described 342-343 

Keedle,  borizontal,  deflection  of,  figure  of 345 

Kegative  crystals  in  quartz 41 

Negaunee  iron-bearing  formation    451 

correlation  of XX,  xxv,  xxvi,  449,  455 

distribution  of ■ 453-453 

magnetic  observations  in 338,  339 

relations  to  Groveland  formation xx,  449, 455 

Ket  Kiver  described 31 

New  Haven ^^ 

Korite,  analyses  of 244,245,263-264 

described 233-249 

Nortb  iron  range.     (See  Felcli  Mountain  range.) 

Northeastern  area  described 451-457 

Northwestern  mine --•  399,407 

Norway  Portage 167,173,174,191,226 

Norway  Rapids -40 

Norway  slates,  correlation  with  Mansfield  schist 413 

0. 

Octahedrite,  included  in  hornblende 236 

of  gabbro  and  norite 236 

(See  Anatase.) 

Oligoclase  in  biotite-granite 41 ,  42 

Olivine,  altering  to  magnetite 217 

altering  to  pilite 211,217 

altering  to  serpentin  > 213, 217, 251, 253, 255 

altering  to  tremolite 218 

crystallization  of 257 

included  in  hornblende 251, 257, 260 

included  in  pyroxene 255,256 

including  spinel 252, 255 

of  amphibole-peridotite 255 

of  gabbro  and  norite 238,239 

of  metabasalt 104,211 

of  meladolerite 201 

of  peridotite 251 

of  picrite-porphyry 213, 217 

pseudomorphs  of 203, 213,  214 

zonal  intergrowth  -with  augite  and  hornblende..  255-260 

plate  of 320,322 

zonal  structure  in 258, 259 

01i\'ine- gabbro,  gradation  to  amphibole-peridotite  . .  254-260 

Oolitic  testurein  dolomite 435,437 

Ophitic  texture  in  diorite 223 

in  gabbro 233 

in  greenstone - 483 

in  hornblende-gabbro 240,  244 

in  metabasalt 212 

in  metadolerite 48,  200 

in  tonalite 230 

Organic  matter,  origin  of  cherty  carbonate 184 


Page. 

Orientation  of  actinolite 105 

of  amphibole 127,214,486 

of  biotite 393,425,468,486 

of  calcite 132 

of  chlorite 118,127,133,146 

of  dolomite 410 

of  feldspar 84,90,171,394,396 

of  hematite 418 

of  hornblende 174, 214, 215,  395,  396, 465 

of  mica 43,44,51,89,91,198,387-388,478 

of  phenocrysts  of  granite-porphyry 45 

of  quartz 51,84,118,132,133,171,402,404,475 

Ornamental  stones  of  Hemlock  formation  described.  153, 154 

Orthoclase,  alteration  of 224 

of  acid  lavas 89 

of  biotite-granite 191 

of  diorite 224,225 

of  granite 464 

of  m  asco vite-biotite-granite 3  94 

of  rhyolite-porphyry 182 

Ottrelite  of  chlorite-schist 442 

of  Hemlock  schist 445 

Oval  structure  in  Groveland  formation  described...  420-423 


Pahoehoe  structure  in  basalt  (see  Ellipsoidal  struc- 
ture)    119,120,121,123,124 

Paint  Eiver 11,21,161,164,167,174,179,187,190,194 

described 31 

Paint  River  district 18 

iron  ore  of 18 

named  by  Erooks 11 

Paint  River  Falls 18 

Paint  River  Iron  Company.    {See  Paint  River  mine.) 

Paint  River  mine 183 

dike  in 183 

location  of 178,  op.  186 

ore  deposits  of 183 

table  of  shipments  from op.  186 

Paint  rock  associated  with  ore  deposits 183 

of  Mansfield  ore  deposit 57,  68 

Palagonite  tuffs,  use  of  term 138 

Paleozoic  rocks,  deposition  of xsiv 

relations  to  lower  formations 26, 28,  376,  383 

(See  Potsdam  sandstone.) 

Paragonite  from  feldspar 41,  42 

Parallel  texture  of  hornblende-gabbro 244 

plate  of 314 

Patton,  H.  S.,  on  hornblende 251 

on  hornblende-picrite 254,255 

referred  to 21 

Peevie  Falls 167 

Pegmatite  in  Randville  formation 435 

intruding  mica-schist ., 392 

veins  in  greenstone 476 

Pegmatitic  texture  in  gneisses ■ 390 

Peneplain    of    Crystal    Falls    district,  relations    to 

Michigan  and  "Wisconsin  peneplain ]  3,  31 

Penokee  district,  correlation  of  series xxi,  189 

iron  formation  of,  located  by  magnetic  work 24 

ferruginous  carbonates  of 130 

Peridotite  described 249-262 

age  of 262 

analyses  of 259,263,264 

crystallization  of  minerals 262 

distribution  of 249,250 


INDEX. 


505 


Page. 

,     Poridotite,  ruliitious  of 249,250 

relations  to  g.ibbro 246,200,261,265 

(See  Welirlite.) 

Peril  tie  purtiug  iu  aporljyoiito 87 

I'l^itoof 274,276 

iu  Ij.tsalt  tuH'.  plate  of 294 

Pertliite  in  biotite-granlte 41,42 

Pesbakiiiiiine  Falls,  iron  ore  at 375 

Pesliakauinio  JRirer 14, 15 

Pewabie  mine 69 

Phenoorysts  of  granite-porpbyry,  orientation  of 45 

Phillips,  H.F,  indebtedness  to 331 

referred  to 22 

Phyllite,  of  Mansfield  formation 411,439 

descri  bed 57-62 

Pblogopite  of  dolomite 410 

Pbysiograpby  of  district 13,29-37,329-335 

Pichler,  A.,  on  alteration  of  staurolite 196 

Pickings,  Matber  &  Co.    {See  Hemlock  mine.) 

Picotite  of  picrite-porpbyry 217 

of  olivine 252 

Picrite-porpbyry  described 212-221 

Pilite  from  augite 211 

from  olivine 211,  217 

of  metabasalt 211 

of  metadolcrite 201 

of  picrite-porpbyry 218 

pseudpmorpbs  after  olivine 202 

pseudomorpbs  after  pyroxene 218 

Pilotasitic  texture  in  metabasalt 98, 99, 104, 212 

Pine.    {See  Timber.) 

Pinnite,  including  rutile 410 

of  dolomite 41Q 

PlagiocJase  included  in  hornblende 235, 241 

including  biotite I93  434 

including  hornblende 484 

of  acid  lavas 89 

of  ampbibolite 296 

of  aiupbibole-schist 4gg 

of  biotite-granite 41  42191 

of  biotite-scbist 4q8  459 

of  diorite 223  ''24 

of  gabbro  and  norito 233  234 

of  greenstone 483' 484 

of  bcrnblende-gabbro 240  241 

of  metabasalt '  jq-^ 

of  mica-diorite 227  23'' 

plate  of gQg 

of  mica-schist ^^^ 

of  muscovite-biotite-granite _         193 

of  pyroclastics -^^g 

of  rhyolite-porphyry g2 

of    tuffs jgg 

orientation  of gog 

pressure  effects  in 234 

zonal  structure  in 193-194 

Plagioclase-basalt  of  Hemlock  formation 108 

Poikilitio  texture  in  amphibole-peridotite 253 

in  contact  of  granite  and  sedimentary,  plate  of. .  300 

in  granite jgg 

in  hornblende 235  '>5i 

in  hornblendegabbro '241 

plate  of 019 

in  metadolcrite 200 

in  pyroxene '_'  253 

iu  quartz-mioa-diorite-porphyry,  plate  of 310 

inperidotite .' 250 

{See  Micropoikilitic  texture.) 


Point  Bonita,  California. 


40,45 
45 


Page. 

^    .   ^,,       .  99,108,113 

Point  Jlornto,  California,  gabbro  from 240 

Polar  magnetic  picrite-porpbyry 212,219 

Porphyritic  granite 

sketch  of 

I'orphyritic  limburgite  described 212-221 

Porphyritic  texture  in  gabbro 233  241 

plate  of 

in  graywacke 

iu  granite 

in  metabasalt 

in  picrite-porpbyry  - . . 

in  quartz-mica-diorite 

in  schistose  pyroclastics 


312 
167 
45 
127,129 
217 
228 
146 


383 


376 


87,1 


479 

48 

43,  248 

177 


{See    Ehyolite-porphyry,  '   Granite-porphyry, 
Aporhyolite-porpbyry.) 
Potsdam  sandstone,  relations    to  Calciferous    lime- 
stone  

relations  to  pre-Paleozoic  rocks,  xxiv,  26,  28,  331,  376,  473 

relations  to  Silurian 

Pressure  effects  in  acid  lavas 

in  Algoukian  of  Sturgeon  Paver  tongue 471 

in  amygdules 120,128 

iu  aporhyolite-porpbyry,  plate  of 276 

iu  arkose 

iu  basic  dikes 

in  biotite 

in  chert 

iu  conglomerate ^-^ 

iu  feldspar 42,  90,  92, 169,  234,  248,  388,  464 

plate  of 276^278 

in  gabbro 217' "48 

'°g™"e ''.'.'44, 194!  464 

iu  graywacke ■,  ^a 

iu  Groveland  formation 440 

iu  Hemlock  formation ^c 

in  hornblende ^gy 

i  n  hornblende-gabbro,  plate  of gig 

in  hornblende-gneiss i^c 

iu  mica-gneiss -.t^-, 

in  mica-schist ny, 

iu  quartz 41,51,53,57,81,82,91,93, 

133, 169,  225,  388,  402,  404,  426,  464,  468,  477,  478,  48-( 

Pl'i*e«f 268,  276^278 

in  quartzite jo 

in  Eandville  formation 435-437 

in  rhyolite-porphyry,  plate'  of 276  278 

in  sedimentaries,  caused  by  intrusion 195 

Pleistocene  deposits,  XXIV 29,33  75  332  333 

erosion  of  XXIV '       '   29 

relations  to  Huronian 155 

Prospecting  for  iron  ore jg= 

Platauia,  Gaetano,  on  ellipsoidal  structure 121  122 

Pseudo-conglomerate,  eruptive igg 

Pseudomorpbs  after  olivine 213  214  251  253 

after  pyroxene 214  218 

of  calcite,  after  feldspar 130 

of  chlorite,  after  biotite 217  228 

of  pilite,  serpentine,  and  magnetite,  after  pyrox- 
ene   218 

of  serpentine,  after  olivine 251  253 

Pumpelly ,  Raphael,  on  amygdules 125 

on  Felch  Mountain  series 380 

on  luster  mottling 200 

on  Menominee  iron  region 377 

on  Michigamme  Eiver 335 


506 


INDEX. 


Page. 

Purapelly,  RapliEel,  ou  pseudo-amygd  ules 203 

with  Broolis  and  Rominger,  map  of  Upper  Pe- 
ninsula of  Michigan .'.  18 

Pyroclastics,  acid,  described 94,  95, 135-148 

of  Hemlock  formation  described 135-148 

plate  of 140 

intruded  by  dolerite 147 

road  material 154 

Pyroxene  altering  touralite 48 

crystallization  of 258,259 

included  in  hornblende 237,  241,  245, 251, 260 

plate  of 313 

including  olivine 255,256 

including  serpentine 245 

lutergrown  with  hornblende 251 

of  amphibole-peridotite 258 

of  bronzite-norite-porphy ry 246 

of  gabbro 237,238,241,245 

of  metadolerite 201 

of  peridotite 250,260 

of  picrite-porphyry 213,218 

of  tuffs 138 

pseudomorphs 213,214.218 

zonal  iutergi  owth  with  feldspar,  plate  of 322 

zonal  intergrowth  with  hornblende 260 

plate  of * 320 

zonal  intergrowth  with  olivine,  plate  of 320,  322 


Quartz  and  magnetite,  oval  structure  in  Groveland 

formation 420-423 

augen  of  conglomerate 475 

cataclastic  structure  in 41 ,  44 

enlargement  of 57,85, 404, 405 

from  feldspar  . .  42,  52,  99,  111,  131, 151, 169, 170, 171,  201,  248 

from  granite 52 

globular  texture  in 86 

granulation  of 51,90 

included  in  feldspar 40,191,394,403,468 

included  in  hornblende 47 

included  in  ilmenite 444 

including  apatite 194,419,420 

including  biotite 47,171,394,403,466 

including  chlorite 403,  419,  420 

including  epidote 47 

including  hornblende 466 

including  iron  oxide 394, 419 

including  muscovite 171,  394,  403 

including  rutile 394, 420 

including  sericite 51 

including  tourmaline 420 

inclusions  in. .  41,  82, 191,  261,  388,  393,  396,  403,  404,  425,  466 

lenticules  in  greenstone 485,  486 

micropoikilitic  zones  about,  plate  of 270 

of  acid  lavas 89,91,93 

of  amphibolite 296,466 

of  amygdules 125,126 

plate  of 280,284 

of  basic  dikes 47 

of  biotite-schist 443,468,484 

of  Bone  Lake  schists 151 

of  conglomerate 477, 478 

of  diorite 225 

of  dolomite 410,436 

of  granite 191,193,194,198,388,464 

plate  of 298 


Page. 

Quartz  and  magnetite  of  gray  wacke 56, 57, 169, 431 

of  greenstone 470, 483, 486 

of  Groveland  formation 419, 421, 449, 450 

of  Hemlock  schist 444 

of  marble 481 

of  metabasalt 117, 118, 127, 129, 132, 133 

of  metadolerite 201 

of  mica-schiat 393,414 

of  peridotite 261 

of  phyllite '. 440 

of  pyroclastics 147 

of  q  uartz-mica-diorite-porphy ry ,  plate  of 310 

of  quartzite 52,  402,  404,  425,  450 

of  rhyolite-porphyry 81,  83,  84,  85,  86 

pressure  effects  in,  plate  of 278 

of  rock  intermediate  between  granite  and  quartz- 
ite   51 

of  sedimentary  inclusion  in  granite 197 

of  sericite-schist 443 

of  tuff 142 

orientation  of 51,  84, 118, 132, 133, 171,  402,  404,  475 

penetrated  by  tourmaline 57 

pbenocrysts,    aureoled,    of    rhyolite -porphyry, 

plate  of 272 

pressure  effects  in 41, 

51,  53,  57,81,  82,  90,  91,  93, 133, 159,  225, 

278,388,  402,  404,  426,  464,  468,  477,  484 

plate  of 268, 276 


rhomboehdral  parting  in 

veins  lu  greenstone 

Groveland  formation 

in  ore  deposits 

in  Eandville  formation 

Quartz-biotite-granite 

Quartz-diorite 

Quartz-mica-diorite 

Quartz-mica-diorite-poTphyry,  plate  of . 

Quartz-porphyry 

Quartz  rock  . 


82 

14 

449 

185 

435 

40 

230 

227,230 

310 

429 

134 

of  Mansfield  ore  deposit 63,  67,  69, 70 

Quartz-schist  of  Eandville  formation 52 

of  Sturgeon  formation 401 

of  Upper  Huronian 173 

Quartzite,  apparent  gradation  into  granite 39, 51, 52 

correlation  of xxv,  xxvi 

current  bedding  in 53 

metamorphismof 425,426 

of  Pelch  Mountain  Eange  described 398-405, 423^26 

of  Kaudville  formation 51-53 

of  Upper  Huronian xxii,  425 

pressure  effects  in 52 

relations  to  dolomite 51, 376 

relations  to  granite 376 

relations  to  ore, deposits 182 

{See  Sturgeon  quartzite.) 

E. 

Eailways ;  Chicago,  Milwaukee  and  St.  Paul 95, 143, 175 

Chicago  and  Northwestern 156,175,423 

Mil  waukee  and  Northern 406 

Eaisin,  C,  on  alteration  of  olivine 218 

on  ellipsoidal  structure 118 

Eandville  dolomite  described . .  xix,  27,  50-53, 406-411, 431-437 

analyses  of 409, 435 

correlation  of xxv,  sxvi 

distribution  of 26, 50. 406-408, 431, 432 


INDEX. 


507 


Page. 

Kanavillodolmiti'.  I'ohliugof 432-431 

iifFcluh  llouutaiu  tongue  described 406-411 

ofMichigammo  Mouutainaud  Fence  River  areas 

described 431-437 

jiressuro  eflfecta  in 435,  437 

quartzite  of 51-53 

relations  to  Archeau xix,  51, 53, 55, 407 

relations  to  Groveland  formation xx 

relations  to  Hemlock  formation xx,  75 

relations  to  intrusivea 434 

relations  to  Mansfield  foi'mation xx,  55,  411,  438 

relations  to  mica-schists 392 

relations  to  Sturgeon  quartzite..  xix,  51,407,  430,4:^1,434 

relations  to  Upper  Huronian 424 

relations  to  underlying  formations 53 

thickness  of xvii,  xix.  26,  53, 407,  408, 433 

topography  of 50,  406-408 

Kandville  station 406 

Range.    {See  Town.) 

Ransome,  F.  L.,  on  ellipsoidal  basalt .  113, 114.118, 119, 122, 123 

on  feldspar  sheaves 99 

on  ultrabasic  iDtrusives 220 

on  variolites 108 

Recomposed  granite  grading  into  quartzite 39 

of  Marquette  district 52 

Recryatallization  of  gneisses 390,  391 

ofgraywacke 195,198 

plate  of .• 298 

of  hornblende,  plate  of 316 

of  mica-gneiss 171 

of  quartz 404 

of  Sturgeon  river  conglomerate xxiv 

Reibungsbreccia  of  Hemlock  formation 136 

of  Upper  Huronian , 161,166,177 

Republic  (town) .■ 333,  334 

Republic  trough 26,  452, 453 

Resorption  rim  about  olivine 258 

Reyar,  E.,  referred  to 142 

Rhombohfedral  parting  in  quartz 82 

Rhyolite-porphyry 80, 190 

described 81-87 

banding  of 81 

plate  of 278 

microgranitic  texture  in  described 86, 87 

micropoikilitic  texture  in  described 83-86 

plate  of 270,272 

of  Hemlock  formation 80 

described 81-87 

phenocrysts  of,  plate  of 268 

pressure  effects  in,  plate  of 276,  278 

Richardson,  G.  B.,  analysis  by 408, 409 

Ridgway,  J.  L.,  indebtedness  to 13.  xvi 

Rieserferner,  tonalite  from 230, 231 

Ripple  marks  in  arkose 473 

Road  material  of  Hemlock  formation 154 

Roberts,  C.  T..  indebtedness  to 66, 182 

Romberg,  J.,  on  augite  of  gabbro 255 

on  olivine-gabbro j 256 

on  pyroxene  zone  about  olivine 256 

Rominger,  C-,  on  correlation  of  Menominee  rocks  ...  19 

on  correlation  of  dolerite  dikes 189 

on  correlation  of  Upper  Huronian 164 

on  Felch  Mountain  range 374,379,381 

on  Menominee  district 19 

on  iron-bearing  formation  near  Crystal  Falls 179, 180 

'  on  progressive  metamorphism 52 


Page. 

Komiuger,  C.on  Sturgeon  River  tongue 461 

on  Upper  Peninsula  of  Michigan 20 

referred  to xv 

with  Brooks  and  Pumpelly,  map  of  Upper  Penin- 
sula of  Michigan 18 

Kosenbusch,  H.,  analysis  by 166 

on  enstatite-porphyrite 247 

on  peridoti  to 249 

on  picrite-porphyry 220,221 

on  spilosite 206 

referred  to 193, 105 

Roth,  J., on  transfer  of  material  from  basic  intru- 

sives  to  slate 211 

on  weathering  of  granite 58 

Rothpletz,  A.,on  ellipsoidal  structure 118,119 

Rutile  from  biotite 43,  192,  202,  225 

from  ilraenite 202,  239 

from  titanic  iron 170, 192, 193,  230 

included  in  bronzite 238 

included  in  feldspar 234 

included  in  hornblende 236,  239,248 

included  in  pinnite 410 

included  in  quartz 394, 420 

of  acid  lavas 93 

of  biotite-granite 192,193 

of  biotite-schist 484 

of  Bone  Lake  schist 151 

of  gabbro 236,  239 

of  graywacke 56, 170 

of  nietabasalt 129 

of  metadolerite 202 

of  metamorphosed  Mansfield  slate 205 

twinning  of 205,  236 

S. 

Saalband  in  quartz-mica  diorite *       228 

Sagenite  from  biotite 192, 193 

included  in  biotite 403 

included  in  chlorite 403 

of  biotite-granite 192, 193 

Sand,  volcanic,  plate  of 296 

Sandstone  and  slates  of   Sturgeon  River  dolomite 

formation  described 481,482 

Sandstone,  Lake  Superior.    (5'ee  Potsdam  sandstone.) 

Sanford,  S.,  indebtedness  to 331 

Santorin,  basalt  from 1200 

Schafer,  R.  "W".,  on  basic  rocks 247 

Schistose  acid  lavas  described 87-94 

dikes  in  Archean 46-48 

pyroclastics  described 145-148 

Schistosity  of  acid  lavas 91 

of  basic  dikes 47,48 

of  gabbro 2i8 

plate  of 316 

of  granite 44, 194 

of  greenstone 470 

of  hornblende-gneiss 175 

of  hornblende-schist 465 

of  metabasalt 127 

of  mica-schist 114 

of  pyroclastics 147 

of  Sturgeon  River  conglomerate 474, 475 

Schists.    (See  Crystalline  schists.) 

Schlieren  in  metabasalt 98 

Scoriae  of  Hemlock  formation 70 

Section.     (See  Town.) 


508 


INDEX. 


Page. 

Sedimentariea,  contact  with,  granite,  described 194-198 

plate  of 300 

included  in  dolerite 203 

included  in  granite 195 

of  Hemlock  formation  described 152. 153 

of  Upper  Huronian  described 165-174 

volcanic,  described ^  136-145 

plate  of '.  296 

Sericite  from  feldspar 52, 89, 99,  lOi,  111,  127. 131, 477 

included  in  quartz 51 

of  acid  lavas 91 

of  nietabasalt 99,101,127 

of  Kandville  quartzite 52 

of    rock  intermediate    between    quartzite    and 

granite 51 

orientatiou  of 51, 478 

Sericite-scbist  from  conglomerate 475 

from  graywacke 57 

of  Hemlock  formation 443 

Serpentine  from  bronzite 238 

plate  of 306 

from  bronzite-norite-porpbyry 246 

from  olivine 213,217,251,255 

includ/sd  in  pyroxene 245 

intergro wn  witb  augite 253 

of  picrite-porpbyry 213,214,218 

pseudomorphs 214 

pseudomorpba  after  olivine 253 

pseudomorphs  after  pyroxene 218 

Sbeldon  &.  Scbafer  mine.     (See  Columbia  mine.) 

Sboldeis  exploration 447,449 

Siamo  slate 451 

correlation  of sxv,  xxvi 

Siderite  altering  to  hematite 168 

altering  to  limonite 168 

altering  to  magnetite 168 

including  iron  oxide 133 

interbanded  witb  clay  slate 168 

of  gray wacke 170 

of  Groveland  formation xx,  420 

of  metabasalt 117, 133 

of  siderite  slate 62 

origin  of 168 

replaced  by  silica 134 

replacing  calcite 133 

source  of  iron  ore 70, 71 ,  184 

Siderite-slate  of  Mansfield  formation  described 62 

of  Upper  Huronian 108 

relations  to  claj^  slate 63 

Sideritization  of  metabasalt 117 

Sidnaw  (town) 1'5 

Silicification  of  metabasalt 130, 133, 134 

of  siderite 134 

Silurian  rocks,  unconformity  with,  underlying  rocks .    26, 376 

Slates  described 1-1, 153, 169-174, 481, 482 

alteration  of 14, 166 

of  Mansfield  formation  described 204-211 

altering  to  chlorite-schist 166 

altering  to  mica-gneiss 166 

argillaceous 14 

fragments  in  Hemlock  tufls 76 

from  Benson,  Yt.,  analysis  of 61 

from  Hampton  village,  N.  Y.,  analysis  of 61 

from  Janesville,  N.  Y.,  analysis  of 61 

from  SoutbPoultney,Yt.,  analysis  of 61 

hornblende,  described 13, 14 


Page. 

Slates  of  Hemlock  formation 153 

of  Mansfield  formation,  described 57-02 

of  SturgeonRiverdolomiteformation, described.  481,482 

of  Upper  Huronian 166 

described 169-174 

relations  to  ore  deposits 68, 182 

transfer  of  material  from  basic  intrusives 211 

(See  Mansfield  formation.) 

Smyth,  H.  L.,  on  peneplain 31 

on  Randville  dolomite 27,  53 

referred  to xv.  xvl  22, 26, 50, 74, 152 

Smith,  G.  0.,  on  Archean  granite 38 

on  ellipsoidal  structure 118, 119 

Smith  mine.     {See  Armenia  mine.) 

Soapstone  associated  \\  ith  ore  deposits 68, 183 

of  Mansfield  formation 57 

Soil  of  Crystal  Falls  district 36,37 

Solar  compass,  use  of 24 

Sorby,  H.  C,  on  enlargement  of  quartz  grains 404 

South  iron  range.     (See  Menominee  range.) 
South  Mastodon  mine.    (See  Manhattan  mine.) 

South  Mountain,  Pa 124 

South  Poultney,  Yt.,  analysis  of  slate  from 161 

Sphenefrom  biotite 192,225 

from  ilraenite  — 239 

from  titanic  iron 144, 146, 170, 193,  212,  239 

included  in  biotite 225 

incl  uded  in  epidote-zoiaite 225 

including  apatite 244 

of  acid  lavas 91 

of  basic  dikes 47 

of  biotite-granite 192,  193 

of  gabbro 239 

of  gray  wacke 170 

of  metabasalt "  100 

of  metadolerite 202 

of  tuff 141 

of  volcanic  conglomerate 144 

pseudomorphs  after  magnetite 466 

surrounding  magnetite 466 

Spherulitic  texture  in  metabasalt  (<eeYariolite.).  98, 102, 108 

Spilite 98 

Spilosite  of  Mansfield  formation 64 

described 206,207 

plateof... 302,304 

analyses  of 207 

analysis  of  compared  with  analyses  of  clay  slate 

and  adinole 210 

gradation  to  desmosite,  plate  of 300 

included  in  dolerite 204 

Spinel  included  in  hornblende 236 

included  in  olivine 251,  252,  255 

intergrown  with  hornblende 256 

of  gabbro  and  norite 236 

of  peridotite 252 

Spurr,  J.  E,,  on  oval  areas  in  iron  formation 422 

Squaw  Lake 335 

Stalactitic  ore 180 

Staurolite  altering  to  muscovite 196 

of  gray  wacke 167 

of  mica-schist 195 

Steiger,  George,  analyses  by 59, 

61,  203,  208,  210, 242,  244,  245,  263,  264 

Stokes,  H.  K.,  analyses  by 103^ 

106,  207,  210, 219, 231,  259,  263,  264,  389,  391,  394,  397 
referred  to 260 


INDEX. 


509 


Page. 

Stratigraphy  of  Crystal  Falls  ilistrict 25-29 

Structuro  of  Crystal  Falls  district 25-29 

I'ln'oof 160 

of  Mausfiehl  formation 64 

Sturgeon  quartzite  dcscribod xviii,  398-405, 430-431 

correlation  of XXV  xxvi 

distribution  of 398-399 

folding  of 399,400 

intrtided  by  granite.. 428 

of  Fclcli  Mountain  range  described 398-405 

of  ilenomiuee  district,  correlated xxv,xxvi 

of    ilicbigamme   Monntain    and   Fence   River 

areas  described 130, 431 

relations  to  Archean xix,  398-401 

relations  to  mica-schist 392 

relations  to  Randville  formation xix,  407, 430,  431, 434 

relations  to  Upper  Huronian xxii 

thickness  of xvii,  x viii,  399-401, 431 

topography  of 398-399 

Sturgeon  Eiver 330,  376,  386,  401,  458,  485 

described 334,335 

Sturgeon  Eiver  dam 474 

Sturgeon  Eiver  tongue  described 458-487 

Algonkian  of,  described 471-482 

Basement  Complex  of,  described 463-471 

conglomerate  of xviii,  xxiv 

described 473-479 

literature  on 459-461 

relations  to  Felcb  Mountain  rocks 462 

relations  to  Lower  Marquette , 462 

Sun  Dog  Lake 335 

Sweden,  granite  of 44 

Syenite,  distribution  of I3 

Synclinal  troughs,  concentration  of  ore  in 183, 184 

Syncline,  Crystal  Falls,  described 158-161 

determined  by  magnetic  observations. . .  366-370,  372, 373 

of  Groveland  formation 416 

Syracuse,  N.  Y.,  plcrite-porpbyry  at 219 

T. 
Table  of  succession  in  Marquette,  Crystal  Falls,  and 

Menominee  districts xxv  xxvi 

Table  of  iron  ore  shipments  of  Crystal  Falls  area...  Op.  186 
Talc  from  bronzite 238 

Plateof 3Q5 

Teall,  J.  J.  H.,  onellipsoid.il  structure 118,123,124 

on  plutonic  rocks 000 

Test  pits  in  Mansfield  ore  deposits 67 

Timbei'  of  Crystal  Falls  district 13, 36  37 

Timbering  of  mine jg5 

Titanic  iron  altering  to  sphene 144, 146, 170  193, 239 

altering  to  rutile 170,193,239 

of  basic  dikes 47 


of  biotite.; 


included  in  biotite. 


■J<i^ite 192,193 


404 

included  in  chlorite 146  404 

included  in  quartz 146 

of  quartzite , 404 

Thonschiefernadeln 93  oqs 

Thiirach,  on  alteration  of  staurolite 

Tobin  Lake 

Tonalite 

comparison  with  Adamello  tonalite 

Topography  by  Lake  Superior  Survey 

by  U.  S.  Geological  Survey 

influenced  by  intrusive  rocks 


256 
420 
196 
298 


Page. 

Topography,  sketch  of 45 

of  Algonkian 333 

of  Archean 38,  39,  ,133,  386,  387 

of  Crystal  Falls  district 13,29-31,331-333 

of  Deer  Eiver  Valley 99 

of  greenstone 333 

of  Groveland  formation 415,416,446-438 

of  Hemlock  formation 73,74,440-441 

of  Huronian  rooks, plateof 150 

of  intrnsives 46,54,333 

of  Mansfield  formation 54  433  439 

of  Eandville  dolomite 50  406^08 

of  Sturgeon  quartzite 398-399 

of  Upper  Huronian 153-156 

Tornebohm, ,  referred  to 

Tourmaline  included  in  quartz 

of  mica-schist 

of  muscovite-biotite-gneiss,  plate  ct 

of  sedimentary  inclusions  in  granite 197 

penetrating  feldspar 57 

penetrating  quartz 57 

Town.41N.,E.29W !.';;"l4,375,460 

41N.,E.30W 375,377,460 

41  U".,  E.  30  W.,  section  2 355 

section  3 30- 

4iN.,E.3iw ^y^..^[.....[\[]    u 

41M".,E.  31  W.,  section  16 31 

section  25 3^^ 

41  JJ.,E.32\7 14 

41  N.,  E.  32  W,,  section  12 31 

section  33 igQ 

42N.,E.27W ;-"-'^'!;!":  458,460 

42  N.,  E.  37  "W.,  section  6 459 

^ectiom ^58^^59 

section  17 450 

section  18 453 

42N.,E.28  W 374,376,377,(158,460,472 

42  N.,  E.  28  "W.,  section  2 472 

section  3 4gQ 

section  4 4^0 

section  6 ^qq 

''™tion7 459,462,463 

section8 459,460,462,463 

section  9 ^-jq 

section  10 47^ 

section  II .~q 

section  14 4gQ 

sectioun 458,461,467,474,482,485 

section  18 458,482 

section  19 ^q^ 

section  21 377 

section  29 399 

section  31 399,407,412,413 

section  32...  374,377,378,381,385,399,412,416,423,424 

section  33 374 

377,  381,  386,  387,  399,  412,  416,  418,  423, 424,  426 

42  N.,  E.  29  -n" 14,  329,  374, 375, 376,  377,  458,  460, 472 

43  N.,  E.  29  W.,  section  1 458,480 

section  3 458,459 

section  7 475 

section  12 459,463 

section  13 459,469 

section  22 459 

section  23 459 

section  24 460 

section  26 454 


510 


INDEX. 


Page. 

Town.  42  N".,  E.  29  W.,  section  31 377, 

374,  378,  385,  398,  399,  406,  412,  415,  416,  460 

section  32 387,399,412,416 

section  33 386,407,412,416 

section  34 392,398,406,412,426 

section  35 392, 398,  399,  400,  406,  412,  426 

section  36 385,398,401,406,412 

42N.,R.30W 329  374,375,376,377,458,460 

42  N.,  E.  30  "W.,  section  12 459 

section  14 461 

section  25 411 

section  30 385 

section  34 374,  385,  398,  399,  411,  412,  41 5 

section  35 335,  399,  400,  406,  407,  412,  413,  415,  426 

section  36 335,  398,  407,  412,  415,  416,  418 

42  N.,  R.  31  W 14, 15,  16,  73,  199,  204,  329 

42N.,  R.  31  W.,  section  1 158 

section  2 158 

section  9 163, 1G6 

section  15 164, 190, 191,  211,  226,  240,  241 

section  16 167,211 

section  19 161,190,194 

section  20 164,190,191 

section  22 190, 191, 241,  249,  250,  253, 260 

section  28 164,241 

.  section  29 164, 190, 194,  241,  243,  245,  249,  253 

section  30 190,194 

section  32 167 

42K.,E.32W 14,15,18,156 


42  N.,  E.  32  W.,  section  1  . 

section  4 

section  12 

section  14 

section  17 

section  18 

section  19 

section  20 

section  21 

section  24 


199 

191 

158 

167 

74 

74 

74 

74 

164 

167 

section  28 164,229 

section  35 ." 167 

section  36 167 

42N".,  E.33W 18,156 

42N.,E.33  W.,sectionl3 18 

section  24 74 

42N.,E.28'W 329,460 

43N,,E.28'W" 375,459,460,472,458 

section  33 400 

43N.,E.29"W 458,459,460,472 

43  N.,  E.  29  "ST.,  section  1 472 

section  13 459 

section  24 459 

section  35 480 

43N.,E.30  \T 458,459,460 

43N.,E.31  W 76,203,204,329,427,447 

431f.,E.31"W.,section2 432 

section  3' 446 

section  5 438 

sectione 199 

section7 54,64,76,203,210 

sections 205,210 

section  10 438 

section  17 64,61,65,190,210 

section  18 54,  55 

section  19 223 

section  20 ; 65,223 


Town.  43  K.,  R.  31  "W.,  section  26  . 

Page. 
447 

247 

section  29 

54  64  190  205 

section  31 

164 

section  32 

161,163,164,199,203 

199,247 

section  34 

.     .                                      247 

43  N.,  R.32W 

18  74 

43  K.,  R.  32  W.,  section  1 

54 

199 

section  5 

141 

section  7 

199  204 

204 

section  9 

204 

section  11 

.            158 

158 

199 

section  20 

18  158 

164 

43  X.,  R.  33  ■S\'' 

18,74 

43N.,R.  35  W.,  section  ■   

158 

44N.,R.31  W 

38  334  427  428  432 

44  N.,R.  31  "W".,  section  3 

441 

section  4 

441 

section  10 

335 

section  15 

430,441,442 

434 

section  21 

429 

section  22 

427  441 

section  28 

432 

section  32 

section  33  ,..- 

432,434,438 

432  438  446 

44N.,R.32  W 

44  N.,  R.  32  "W.,  section  1 

38,199,334,427 

46, 53 

91  92 

section  9 

212 

55 

53 

section  18 

199  204 

212 

section  27 

212 

section  28 

..  .                            199  203 

95 

88 

441T.,R. 33  "W.,section  4  ...   . 

.                 X08 

44N.,R.42  W 

77 

45  !N".,  R.  30  W.,  section  5 

453 

429 

453 

section  9 

453 

453 

453 

45N.,R.  31"W. 

.  30,  38, 345, 427,  428,  431,  457 

45  N.  R.  31  "W".  section  6  . 

440 

441  447 

section  21 

335,441,447 

431 

section  28 

431,432,441 

45N.,E.32"W 

29,30,38,73,427 

45  IN".  R.  32  W.  section  1 

39 

51 

45N.,R.33  W 

73 

INDEX. 


511 


149 
149 
153 
453 
406 
1,427 
447 


Pago. 

Town.  45  N„K.  33  W.,aoction  9 157 

seotiou  15 149 

section  IC 149_  157 

3ection20 158,175,176 

section  22 

section  27 

section  34 

46  N.,  K.  30 'W.,  section  19 

section  40 

46N.,  K.31  W 13, 

46  N.,  R.  31  W.,  section  32 

46N..K.32W 38,148,155,156,427 

40N.,E.  32  W.,  section  16 149 

section  21 128,  334 

section  31 I49 

8ection36 73,  149 

46N.,  E.33  W 'l56 

46  N.,R.  saw.,  section  24 147,149,199 

section  27 204 

section  34 149,156,163,175,176 

47]S-.,E.30W 329,457 

47N'.,E.30  W.,  section  9 '. 334 

section  19 333 

section  30 333 

47N.,R.31W 13,329,332 

47N.,E.3i-\Y    ;,g(,tion  24 333 

section  25 333 

section  36 333 

47  N.,  E.  33 'NY.,  section  19 199,204 

48N.,E.31  W 

Tracliy tic  texture  in  pyroclastics 

Tremolite  developed  by  dynamic  action- . . 

from  olivine 

included  in  clilorite 

of  dolomite 


13 

147 

432 

217 

218 

..  408,410,436 

of  picrite-porpliyry 214,218 

(iSee  Amphibole.) 

Trout  Lalie 335 

Tuff,  basalt,  perlitio  parting  in,  plate  of 294 

breccia,  use  of  term I37 

conglomerate,  use  of  term 137 

of  Hemloclc  formation 64  75 

described 137-143 

thicl£ness  of 141 

origin  of 141-142 

palagonite,  use  of  term 133 

relations  to  asli  beds 143 

resemblance  to  Tertiary  and  Eecent  tuffs 142 

use  of  term 135 

Tuffogene  sediments  described 143-145 

Turner,  H.  TV.,  on  granodiorite 231 

Twinning  of  feldspar. ...  42, 104, 191,  201,  224,  228,  233,  261,  468 

of  muscovite 197 

of  pyroxene 250 

of  rutile 56,205,336 


Tlltrabasic    intrusives    in    Crystal    Falls    district 

described 212-221 

in  Upper  Huronian I64 

Unconformity.    (See    Eolations    under    particular 

formations  and  series.) 
Undnlatory    extinction    in    quartz    (see    Pressure 

effects) 41,  81,  82,  90, 133, 169,  225,  388,  464,  477 

Union  mine.    (See  Columbia  mine.) 

United  States  Land  Survey 343 


Page. 
United  States  surveyors  on  Sturgeon  River  tongue.  459-460 

United  States  Geological  Survey,  topography  by 22 

Upper    Cambrian.     {See    Potsdam,   Lake    Superior 
sandstone.) 

Upper  Huronian  described xviii,  x.\i,  27, 155-186, 423-420 

correlation  of 155,164,165 

distribution  of 155  155 

folding  of 'I58-I62il88 

sketch  of - _ 179 

relations  to  intrusives 189-190 

intrusives  in  ; 174,175,187 

magnetic  observations  in 156,157  339 

metamorphism  of 28,425  420 

of  Felch  Mountain  range  described 423-426 

of  Penokee  series ^xr 

ore  deposits  of 28 

described ^j.igg 

table  of  shipments  of Op.  180 


relations  to  Archeau  . 


relations  to  basic  intrusives .' 204,211,223 

relations  to  Cambrian  sandstone 155,161,162 

relations  to  grit 155 

relations  to  Groveland  formation xxi,  425 

relations  to  Hemlock  formation 77 

relations  to  intrusives ,. .,  164,190,204.211  223 

relations  to  Mansfield  formation xxir 

relations  to  Michigamme  formation 28, 105 

relations  to  Lovrer  Huronian xvii 

XXII,  158,  160, 161,  162,  163,  176,  424 

relations  to  Eandville  dolomite 424 

relations  to  Sturgeon  quartzite xxii 

relations  to  Upper  Marquette 155 

succession  in xxv  xxvi 

thickness  of 157 

topography  of ^ 155,136 

Upper  Marquette,  correlation  of xxv,  xxvi 

iron-bearing  series  referred  to 20 

iron  ore  deposits,  comparison  with  Crystal  Falls 

iron  ore  deposits 180,181 

relations  to  Upper  Huronian 155 

Uralite  from  augite 104  201  212 

from  pyroxene 43 

including  feldspar 202 

of  metadolerite 200 

of  volcanic  conglomerate 144 

V. 
Van  Hise,  C.  E.,  connection  with  Lake  Superior  Sur- 
rey   

indebtedness  to 

on  alteration  of  siderite 

on  concretionary  structure  in  ferruginous  chert. 

on  correlation  of  Crystal  Falls  ore  deposits 

on  Felch  Mountain  range 

on  formation  of  crystalline  schists 

on  Lake  Superior  stratigraphy 

on  Mesnard  area 

on  metamorphism  of  basic  rocks 

on  mica-schist 

on  Michigamme  formation 

on  ore  concentration 

on  ore  deposits  of  Armenia  mine 

on  ore  deposits  of  Hemlock  mine 

on  origin  of  siderite 

on  origin  of  iron  ore 39,70,71, 

on  sericite-schist  from  recomposed  granite 


21,22 

12 

168 

422 

20 

380 

172 

19 

452,  457 

152 

414 

165 

72 

183 

177 

168 

130, 168 

52 


512 


i:n^dex. 


Page. 
Van  Hise,  C.  E.,  on  eilicification  of  ore  formation  on         13-1 

Sturgeon  River  conglomerate 461 

Van  Horn,  F.  R.,onnorites 235,247 

Van  "Werveke,  on  spilosites 206 

Variolite  in  metabasalt  described 108-111 

plate  of 110 

of  Point  Bonita ,  -  108 

Veins  of  iron  oxide  in  Groveland  formation 449 

of  pegmatite  in  greenstone 476 

of  quartz 14 

in  Groveland  formation 185,  449 

in  Rundville  foi  mation 435 

Volcanic  ashes  altering  to  greenstone 487 

Volcanic  breccias,  use  of  term 137 

Volcanic  elastics,  plate  of 284 

Volcanic  cones  iu  Hemlock  formation 78 

Volcanic  conglomerate  described ^ 143-145 

use  of  term 136 

Volcanic  rocks  of  Crystal  Falls  district xx,  95-148 

of  Hawaii 75 

of  Iceland 75 

of  Penokce  district xxi 

{Sec  Hemlock  formation.) 

Volcanic  band,  plate  of 290 

Volcanic  sedimentary  rocks  of  Hemlock  formation 

described 136-145 

Vulcan  iron  formation,  correlated xxv,  xxvi 

"W. 

Wadsworlh,  M.  E.,  ou  the  iron,  gold,  and  copper  dis- 
tricts of  Michigan 20 

on  dike  in  Paint  River  mine 183 

on  Felch  I\tountain  range 381 

on  Mesnard  series 452 

on  Upper  Huronian  conglomerate 166 

referred  to 95 

Washington,  H.  S.,  on  zonal  structure  in  olivine 258 

"Wauueta  mine.    (See  Hope  mine.) 

"Weathering  of  amygdules 125 

of  biotite 202 

of  Mansfield  slate 205 

of  metabasalt 134, 135 

of  metadolerite 199-200 

"Weidman,  S.,  referred  to 22 

"Wehrlite  described 253 

plate  of 320, 322 

analysis  of 259,263-264 

gradation  to  amphibole-peridotite  described 254-260 

(See  Peridotite.) 

We  we  slate,  correlation  of xxv,  xx  vi 


Page. 
Whin  Sill 211 

Whitney,  J.  D.     (See  Foster,  J".  W.) 

Wichmann,  Arthur,  on  iron-bearing  rocks  south  of 

Lake  Superior 21 

ou  serpentine 253 

on  Upper  Huronian  rocks 173, 174 

Williams,  G.  H.,  on  alteration  of  ilmenite 203 

on  Cortlandt  series 222 

on  Cortlandite 254 

on  ellipsoidal  structure ]18, 119 

on  gabbros 247 

on  micropoikilitic  texture 84,85,87 

on  production  of  schists  from  igneous  elastics..  152 

on  pyroclastics  of  Marquette  district 148 

on  pyroxene  zone  about  olivine 256 

on  ultrabasic  iutrusives 220 

referred  to 95 

Wincbell,  N,  H.,  on  ellipsoidal  structure 118-119 

Wolff,  J.  E.,  on  Hoosac  schists 394 

on  development  of  mica 130 

Wright,  C.E.,  on  Crystal  Falls  district 20 

on  garnetiferous  mica-schist 196 

on  granite  dikes  in  iron-bearing  formation 381 

on  Menominee  iron  region 21 

on  staurolitiferous  mica-schists 196 

on  Upper  Huronian 173-174 

referred  to 21 

Y. 

Toungstown  mine 182, 186 

location  of 178,  op.  186 

table  of  shipments  from op,  1S6 

Z. 

Zircon  included  in  biotite 234 

of  gabbro  and  norite 239 

of  rhyolite-porphyry 81 

Zirkel,  F.,  on  rock  nomenclature 97 

on  spilosites 206 

on  transfer  of  material  from  basic  intrusives 211 

Zoisite  from  feldspar 201 

plate  of 2R6 

included  in  epidote 445 

of  metabasalt 101, 117 

Zonal  structure  in  amygdules 124 

in  epidote-zoisite 101 

in  feldspar 193,194,197,233 

in  hornblende 226,  234,  235, 236, 256 

in  metabasalt 117 

in  olivine 259 

in  pyroclastics 146 

in  tuff 141,142 


[Monograph  XXXVI.] 


The  statute  approved  March  3,  1879,  establishing  the  United  States  Geological  Survey,  contains 
the  following  provisions : 

' '  The  publications  of  the  Geological  Survey  shall  consist  of  the  annual  report  of  operations,  geo- 
logical and  economic  maps  illustrating  the  resources  and  classiUcation  of  the  lands,  and  reports  upon 
general  and  economic  geology  and  paleontology.  The  annual  report  of  operations  of  the  Geological 
Survey  shall  accompany  the  annual  report  of  the  Secretary  of  the  Interior.  All  special  memoirs^and 
reports  of  said  Survey  shall  be  issued  in  uniform  quarto  series  if  deemed  necessary  by  the  Director,  but 
otherwise  in  ordinary  octavos.  Three  thousand  copies  of  each  shall  be  published  for  scientific  exchanges 
and  for  sale  at  the  price  of  publication ;  and  all  literary  and  cartographic  materials  received  in  exchange 
shall  be  the  property  of  the  United  States  and  form  a  part  of  the  library  of  the  organization :  And  the 
money  resulting  from  the  sale  of  such  publications  shall  be  covered  into  the  Treasury  of  the  United 
States." 

Except  in  those  cases  in  which  an  extra  number  of  any  special  memoir  or  report  has  been  sup- 
plied to  the  Survey  by  special  resolution  of  Congress  or  has  been  ordered  by  the  Secretary  of  the 
Interior,  this  office  has  no  copies  for  gratuitous  distribution. 

ANNUAL  REPORTS. 

I.  First  Annual  Report  of  the  United  States  Geological  Survey,  by  Clarence  King.  1880.  8'^.  79 
pp.     1  map.— A  preliminary  report  describing  jjlau  of  organization  and  publications. 

II.  Second  Annual  Report  of  the  United  States  Geological  Survey,  1880-81,  by  J.  W.  Powell. 

1882.  8°.     lv,588pp.     62  pi.    ,1  map. 

III.  Third  Annual  Report  of  the  United  States  Geological  Survey,  1881-'82,  by  J.  W.  Powell 

1883.  8°.     xviii,  564  pp.     67  pi.  and  maps. 

IV.  Fourth  Annual  Report  of  the  United  States  Geological  Survey,  1882-'83,  by  J.  W.  Powell 

1884.  8°.     xxxii,473pp.     85  pi.  and  maps. 

V.  Fifth  Annual  Report  of  the  United  States  Geological  Survey,  1883-'84,  by  J.  W.  Powell. 

1885.  8*^.     xxxvi,  469  pp.     58  pi.  and  maps. 

VI.  Sixth  Annual  Report  of  the  United  States  Geological  Survey,  1884-'85,  by  J.  W.  Powell 
1885.     8°.     xxix,  570  pp.     65  pi.  and  maps. 

VII.  Seventh  Annual  Report  of  the  United  States  Geological  Survey,  1885-'86,  by  J.  AV.  Powell. 

1888.  8^.     XX,  656  pp.     71  pi.  and  maps. 

VIII.  Eighth  Annual  Report  of  the  United  States  Geological  Survey,  1886-87,  by  J.  AV.  Powell. 

1889.  8°.     2  pt.     xix,  474,  xii  pp.,  53  pi.  and  maps ;  1  prel.  leaf,  475-1063  pp.,  54-76  pi.  and  maps. 

IX.  Ninth  Annual  Report  of  the  United  States  Geological  Survey,  1887-'88,  by  J.  W.  Powell 

1889.  8°.    xiii,717pp.     88  pi.  and  maps. 

X.  Tenth  Annual  Report  of  the  United  States  Geological   Survey,  1888-89,  by  J.  W.  Powell. 

1890.  8°.    2  pt.     XV,  774  pp.,  98  pi.  and  maps ;  viii.  123  pp. 

XI.  Eleventh  Annual  Report  of  the  United  States  Geological  Survey,  1889-90,  by  J.  "\V.  Powell. 

1891.  8^.     2  pt.     XV,  7.57  pp.,  66  pi.  and  maps;  ix,  351  pp.,  30  pi.  and  maps. 

XII.  Twelfth  Annual  Report  of  the  United  States  Geological  Survey,  1890-'91,  by  J.  W.  Powell. 
1891.     8°.    2  pt.,  xiii,  675  pp.,  53  pi.  and  maps ;  xviii,  576  pp.,  146  pi.  and  maps. 

XIII.  Thirteenth  Annual  Report  of  the  United  States  Geological  Survey,  1891-'92,  by  J.  AV. 
Powell.  1893.  8°.  3  pt.  vii,  240  pp.,  2  maps;  x,  372  pp.,  105  pi.  and  maps;  x'i,  486  pp.,  77 "pi.  and 
maps. 

XIV.  Fourteenth  Annual  Report  of  the  United  States  Geological  Survey,  1892-'93,  by  J.  W. 
Powell.    1893.     8°.     2  pt.     vi,  321  pp.,  1  pi. ;  xx,  597  pp.,  74  pi.  and  maps. 

XV.  Fifteenth  Annual  Report  of  the  United  States  Geological  Survey,  1893-'94,  T)y  J.  W.  Powell. 
1895.     8°.     xiv,  755  pp.,  48  pi.  and  maps. 

XVI.  Sixteenth  Annual  Report  of  the  United  States  Geological  Survey,  1894-'95,  Charles  D. 
Walcott,  Director.  1895.  (Part  I,  1896.)  S'^.  4  pt.  xxii,  910  pp.,  117  pi.  and  maps;  xix,  598  pp..  43 
pi.  and  maps;  xv,  646  pp.,  23  pi. ;  xix,  735  pp.,  6  pi. 

XVII.  Seventeenth  Annual  ]?eport  of  the  United  States  Geological  Survey,  1895-'96,  Charles 
D.  Walcott,  Director.  1896.  S-^.  3  pt.  in  4  vol.  xxii,  1076  pp.,  67  pi.  and  maps;  xxv,  864  pp.,  113  pi. 
and  maps;  xxiii,  542  pp.,  8  pi.  and  maps;  iii,  543-1058  pp.,  9-13  pi. 

XVIII.  Eighteenth  Annual  Report  of  the  United  States  Geological  Survey,  1896-'97,  Charles  D. 
Walcott,  Director.     1897.    (Parts  II  and  III,  1898. )     8^.    5  pt.  in  6  vol.     1-440  pp.,  4  pi.  and  maps ;  i-v, 

MON   XXXVI 33  1 


II  ADVERTISEMENT. 

1-653  pp.,  105  pi.  and  maps;  i-v,  1-861  pp.,  118  pi.  aud  maps;  i-x,  1-756  pp.,  102  pi.  and  maps;  i-xii, 
1-642  pp.,  1  pi. ;  643-1400  pp. 

XIX.  Nineteenth  Annual  Report  of  the  United  States  Geological  Survey,  1897-'98,  Charles  D. 
Walcott,  Director.     1898.     8".     6  pt.  in  7  vol. 

MONOGRAPHS. 

I.  Lake  Bonneville,  by  Grove  Karl  Gilbert.     1890.     4°.     xx,  438  pp.     51  pi.     1  map.     Price  $1.50. 

II.  TertiaryHistoryofthe  Grand  Canon  District,  with  Atlas,  by  Clarence  E.  Dutton,  Capt.,  U.  S.  A. 
1882.     4'=.     xiv,  264  pp.     42  pi.  aud  atlas  of  24  sheets  folio.     Price  $10.00. 

III.  Geology  of  the  Comstock  Lode  and  the  Washoe  District,  with  Atlas,  by  George  F.  Becker. 
1882.     4°.     XV,  422  pp.     7  pi.  and  atlas  of  21  sheets  folio.     Price  $11.00. 

IV.  Comstock  Mining  and  Miners,  by  Eliot  Lord.     1883.     4°.     xiv,  451  pp.     3  pi.     Price  $1.50. 

V.  The  Copper-Bearing  Rocks  of  Lake  Superior,  by  Roland  Duer  Irving.  1883.  4°.  xvi,  464 
pp.     15  1.     29  pi.  and  maps.     Price  $1.85. 

VI.  Contributions  to  the  Knowledge  of  the  Older  Mesozoic  Flora  of  Virginia,  by  William  Morris 
Fontaine.     1883.     4°.     xi,  144  pp.     54 1.     54  pi.     Price  $1.05. 

VII.  Silver-Lead  Deposits  of  Eureka,  Nevada,  by  Joseph  Story  Curtis.  1884.  4°.  xiii,  200  pp. 
16  pi.     Price  $1.20. 

VIII.  P.ileontology  of  the  Eureka  District,  by  Charles  Doolittle  Walcott.  1884.  4°.  xiii,  298 
pp.     241.     24  pi.     Price  $1.10, 

IX.  Brachiopoda  and  Lamellibranchiata  of  the  Raritan  Clays  aud  Greensand  Marls  of  New 
Jersey,  by  Robert  P.  Whitfield.     1885.     4^.     xx,338pp.     35  pi.     1  map.     Price  $1.15. 

X.  Dinooerata.  A  Monograph  of  an  Extinct  Order  of  Gigantic  Mammals,  by  Othniel  Charles 
Marsh.     1886.     4°.     xviii,  243  pp.     56 1.     56  pi.     Price  $2.70. 

XI.  Geological  History  of  Lake  Lahontan,  a  Quaternary  Lake  of  Northwestern  Nevada,  by 
Israel  Cook  Russell.     1885.     4^.     xiv,  288  pp.     46  pi.  and  maps.     Price  $1.75. 

XII.  Geology  and  Mining  Industry  of  Leadville,  Colorado,  with  Atlas,  by  Samuel  Franklin 
Emmons.     1886.     4°.     sxix,  770  pp.     45  pi.  aud  atlas  of  35  sheets  folio.     Price  $8.40. 

XIII.  Geology  of  the  Quicksilver  Deposits  of  the  Pacific  Slope,  with  Atlas,  by  George  F.  Becker. 
1888.     4°.     xix,  4fe"6  pp.     7  pi.  and  atlas  of  14  sheets  folio.     Price  $2.00. 

XIV.  Fossil  Fishes  and  Fossil  Plants  of  the  Triassic  Rocks  of  New  Jersey  and  the  Connecticut 
Valley,  by  John  S.  Newberry.     1888.     4°.     xiv,  152  pp.     26  pi.     Price  $1.00. 

XV.  The  Potomac  or  Younger  Mesozoic  Flora,  by  William  Morris  Fontaine.  1889.  4°,  xiv, 
377  pp.     180  pi.     Text  and  plates  bound  separately.     Price  $2.50. 

XVI.  The  Paleozoic  Fishes  of  North  America,  by  John  Strong  Newberry.  1889.  4^^.  340  pp. 
53  pi.     Price  $1.00. 

XVII.  The  Flora  of  the  Dakota  Group,  a  Posthumous  Work,  by  Leo  Lesquereux.  Edited  by 
F.  H.  Knowltou.     1891.     4^.     400  pp.     66  pi.     Price  $1.10. 

XVIII.  Gasteropoda  aud  Cephalopoda  of  the  Raritan  Clays  and  Greensand  Marls  of  New  Jersey, 
by  Robert  P.  Whitfield.     1891.     4^^.     402  pp.     50  pi.     Price  $1.00. 

XIX.  The  Penokee  Iron-Bearing  Series  of  Northern  Wisconsin  and  Michigan,  by  Roland  D. 
Irving  and  C.  R.  Van  Hise.     1892.    4^.    xix,  534  pp.    Price  $1.70. 

XX.  Geology  of  the  Eureka  District,  Nevada,  with  an  Atlas,  by  Arnold  Hague.  1892.  4".  xvii, 
419  pp.     8  pi.     Price  $5.25. 

XXI.  The  Tertiary  Ehynchophorous  Coleoptera  of  the  United  States,  by  Samuel  Hubbard  Scud- 
der.     1893.     4'=.     xi,  206  pp.     12  pi.     Price  90  cents. 

XXII.  A  Manual  of  Topographic  Methods,  by  Henry  Gannett,  Chief  Topographer.  1893.  4°. 
xiv,  300  pp.     18  pi.    Price  $1.00. 

XXIII.  Geology  of  the  Green  Mountains  in  Massachusetts,  by  Raphael  Pumpelly,  T.  Nelson  Dale, 
and  J.  E.  Wolif.     1894.    4°.     xiv,  206  pp.     23  pi.     Price  $1.30. 

XXIV.  Mollusca  and  Crustacea  of  the  Miocene  Formations  of  New  Jersey,  by  Robert  Parr  Whit- 
field.    1894.    4^.     193  pp.    24  pi.     Price  90  cents. 

XXV.  TheGlacialLakeAgassiz,  by  Warren Upham.   1895.   4°.  xxiv,  658  pp.   38  pi.   Price  $1.70. 

XXVI.  Flora  of  the  Amboy  Clavs,  by  John  Strong  Newberry;  a  Posthumous  Work,  edited  by 
Arthur  Hollick.     1895.     4°.    260  pp.     58  pi.     Price  $1.00. 

XXVII.  Geology  of  the  Denver  Basin  in  Colorado,  by  Samuel  Franklin  Emmons,  Whitman  Cross, 
and  George  Homans  Eldridge.     1896.    4*^.     556  pp.     31  pi.     Price  $1.50. 

XXVIII.  The  Marquette  Iron-Bearing  District  of  Michigan,  with  Atlas,  by  C.  R.  Van  Hise  and 
W.  S.  Bayley,  including  a  Chapter  on  the  Republic  Trough,  by  H.  L.  Smyth.  1895.  4'=.  608  pp.  35 
pi.  and  atlas  of  39  sheets  folio.     Price  $5.75.  . 

XXIX.  Geology  of  Old  Hampshire  County,  Massachusetts,  comprising  Franklin,  Hampshire,  and 
Hampden  Counties,  by  Benjamin  Kendall  Emerson.     1898.    4°.    xxi,  790  pp.     35  pi.     Price  $1.90. 

XXX.  Fossil  Medusa?,  by  Charles  Doolittle  Walcott.     1898.    4'^.     ix,201pp.    47  pi.     Price  $L 50. 

XXXI.  Geology  of  the  Aspen  Mining  District,  Colorado,  with  Atlas,  by  Josiah  Edward  Spurr. 
1898.     4°.     XXXV,  260  pp.     43  pi.  and  atlas  of  30  sheets  folio.     Price  $3.60. 

XXXII.  Geology  of  the  Yellowstone  National  Park,  Part  II,  Descriptive  Geology,  Petrography, 
and  Paleontology,  by  Arnold  Hague,  J.  P.  Iddings,  W.  Harvey  Weed,  Charles  D.  Walcott,  G.  H.  Girty, 
T.  W.  Stanton,  and  F.  H.  Knowltou.     1899.     4°.     xvii,  893  pp.     121  pi.     Price . 

XXXIII.  Geology  of  the  Narragansett  Basin,  by  N.  S.  Shaler,  J.  B.  Woodworth,  aud  August  F. 
Foerste.     1899.     4°.     xx,  402  pp.     31  pi.     Price . 


ADVERTISEMENT.  Ill 

XXXIV.  Tho  Glacial  Gravels  of  Maine  and  their  Associated  Deposits,  hj  George  H.  Stone.  1899. 
4^.     xiii,  499  pp.     52  pi.     Price . 

XXXV.  The  Later  Extinct  Floras  of  North  America,  by  John  Strong  Newberry;  edited  by 
Arthur  HoUiek.     1898.     4-.     xviii,  295  pp.     68  pi.     Price  $1.25. 

XXX\'I.  The  Crystal  Falls  Iron-Hearing  District  of  Michigan,  by  J.  Morgan  Clements  and 
Henry  Lloyd  Smyth ;  with  a  Chapter  on  the  Sturgeon  River  Tongue,  by  William  Shirley  Bayley,  and  an 

introduction  by  Charles  Richard  Van  Hise.    1899.4'^.     xxxvi, 51i!  pp.     53  pi.     Price . 

In  preparation: 

XXXVII.  Flora  of  the  Lower  Coal  Measures  of  Missouri,  by  David  White. 

XXXVIII.  The  Illinois  Glacial  Lobe,  by  Frank  Loverett. 

— Flora  of  the  Laramie  and  Allied  Formations,  by  Frank  Hall  Knowlton. 

BULLETINS. 

1.  On  Hypersthene-Audesite  and  on  Triclinic  Pyroxene  in  Augitio  Eocks,  by  Whitman  Cross, 
with  a  Geological  Sketcli  of  Buffalo  Peaks,  Colorado,  by  S.  F.  Emmons.  1883.  8°.  42  pp.  2  pi, 
Price  10  cents. 

2.  Gold  and  Silver  Conversion  Tables,  giving  the  Coining  Values  of  Troy  Ounces  of  Fine  Metal, 
etc.,  computed  by  Albert  Williams,  jr.     1883.    8°.     8  pp.     Price  5  cents. 

3.  On  the  Fossil  Faunas  of  the  Upper  Devonian,  along  the  Meridian  of  76°  30',  from  Tompkins 
County,  N.  Y.,  to  Bradford  County,  Pa.,  by  Henry  S.  Williams.     1884.     8".     36  pp.     Price  5  cents. 

4.  On  Mesozoic  Fossils,  by  Charles  A.  White.     1884.    8°.     36  pp.     9  pi.     Price  5  cents. 

5.  A  Dictionary  of  Altitudes  in  the  United  States,  compiled  by  Henry  Gannett.  1884.  8°.  325 
pp.     Price  20  cents. 

6.  Elevations  in  the  Dominion  of  Canada,  by  J.  W.  Spencer.     1884.     8°.     43  pp.     Price  5  cents. 

7.  Mapoteca  Geologica  Americana.  A  Catalogue  of  Geological  Maps  of  America  (North  and 
South),  1752-1881,  in  Geographic  and  Chronologic  Order,  by  Jules  Marcou  and  John  Belknap  Marcou. 

1884.  8°.     184  pp.     Price  10  cents. 

8.  On  Secondary  Enlargements  of  Mineral  Fragments  in  Certain  Rocks,  by  R.  D.  Irving  and 
C.  E.  A^in  Hise.     1884.     S^.    56  pp.     6  pi.     Price  10  cents. 

9.  A  Report  of  Work  done  in  the  Washington  Laboratory  during  the  Fiscal  Year  1883-84.  F.  W. 
Clarke,  Chief  Chemist ;  T.  M.  Chatard,  Assistant  Chemist.     1884.     8°.     40  pp.     Price  5  cents. 

10.  On  the  Cambrian  Faunas  of  North  America.  Preliminary  Studies,  by  Charles  Doolittle 
Walcott.     1884.     8°.     74  pp.     10  pi.     Price  5  cents. 

11.  On  the  Quaternary  and  Recent  MoUusca  of  the  Great  Basin;  with  Description  of  New 
Forms,  by  R.  Ellsworth  Call.  Introduced  by  a  Sketch  of  the  Quaternary  Lakes  of  the  Great  Basin, 
by  G.  K.  Gilbert.     1884.     8^^.     66  pp.     6  pi.     Price  5  cents. 

12.  A  Crystallograijhic  Study  of  the  Thinolite  of  Lake  Lahontan,  by  Edward  S.  Dana.  1884.  8°. 
34  pp.     3  pi.     Price  5  cents. 

13.  Boundaries  of  the  United  States  and  of  the  Several  States  and  Territories,  with  a  Historical 
Sketch  of  the  Territorial  Changes,  by  Henry  Gannett.     1885.    8°.     135  pp.     Price  10  cents. 

14.  The  Electrical  and  Magnetic  Properties  of  the  Iron-Carburets,  by  Carl  Barns  and  Vincent 
Strouhal.     1885.     8".     238  pp.     Price  15  cents. 

15.  On  the  Mesozoic  and  Cenozoic  Paleontology  of  California,  by  Charles  A.  White.  1885.  8°. 
33  pp.     Price  5  cents. 

16.  On  theHigherDevonianFaunasof  Ontario  County,  New  York,  by  John  M.Clarke.  1885.  8°. 
86  pp.     3  pi.     Price  5  cents. 

17.  On  the  Development  of  Crystallization  in  the  Igneous  Rooks  of  Washoe,  Nevada,  with  Notes 
on  the  Geology  of  the  District,  by  Arnold  Hague  and  Josejjh  P.  Iddings.  1885.  8°.  44  pp.  Price  5 
cents. 

18.  On  Marine  Eocene,  Fresh-Water  Miocene,  and  other  Fossil  Mollusca  of  Western  North 
America,  by  Charles  A.  White.     1885.    8^.     26  pp.     3  pi.     Price  5  cents. 

19.  Notes  on  the  Stratigraphy  of  California,  by  George  F.Becker.   1885.   8°.   28  pp.   Price  5  cents. 

20.  Contributions  to  the  Mineralogy  of  the  Rocky  Mountains,  by  Whitman  Cross  and  W.  F.  Hille- 
brand.     1885.     8>^.     114  pp.     1  pi.     Price"  10  cents. 

21.  The  Lignites  of  the  Great  Sioux  Reservation;  a  Report  on  the  Region  between  the  Grand 
and  Moreau  Rivers,  Dakota,  by  Bailey  Willis.     1885.     8"^.    .16  pp.     5  pi.     Price  5  cents. 

22.  On  New  Cretaceous  Fossils  from  California,  by  Charles  A.  White.  1885.  8°.  25  np.  5  pi. 
Price  5  cents. 

23.  Observations  on  the  Junction  between  the  Eastern  Sandstone  and  the  Keweenaw  Series  on 
Keweenaw  Point,  Lake  Superior,  by  R.  D.  Irving  and  T.  C.  Chamberlin.  1885.  8^.  124  pp.  17  pi. 
Price  15  cents. 

24.  List  of  Marine  Mollusca,  comprising  the  Quaternary  Fossils  and  Recent  Forms  from  American 
Localities  between  Cape  Hatteras  and  Cape  Roque,  including  the  Bermudas,  by  William  Healey  Dall. 

1885.  8*^.     336  pp.     Price  25  cents. 

25.  The  Present  Technical  Condition  of  the  Steel  Industry  of  the  United  States,  by  Phineas 
Barnes.     1885.     8^^.     85  pp.     Price  10  cents. 

26.  Copper  Smelting,  by  Henry  M.  Howe.     1885.     8°.     107  pp.     Price  10  cents. 

27.  Report  of  Work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  Fiscal  Year 
1884-'85.     1886.     8°.     80  pp.     Price  10  cents. 

28.  The  Gabbros  and  Associated  Hornblende  Rooks  occurring  in  the  Neighborhood  of  Baltimore, 
Maryland,  by  George  Huntington  Williams.     1886.     8°.     78  pp.     4  pi.    Price  10  cents. 


IV  ADVERTISEMENT. 

29.  On  the  Fresli-Water  Invertebrates  of  the  North  American  Jurassic,  by  Charles  A.  White.  1886. 
8*^.     41  pp.     4  pi.     Price  5  cents. 

30.  Second  Contribution  to  the  Studies  on  the  Cambrian  Faunas  of  North  America,  by  Charles 
Doolittle  Walcott.     1886.     8°.     369  pp.     33  pi.     Price  25  cents. 

31.  Systematic  Review  of  our  Present  Knowledge  of  Fossil  Insects,  including  Myriapods  and 
Arachnids,  by  Samuel  Hubbard  Scudder.     1886.     8°.     128  pp.     Price  15  cents. 

32.  Lists  and  Analyses  of  the  Mineral  Springs  of  the  United  States;  a  Preliminary  Study,  by 
Albert  CPeale.     1886.     8°.     235  pp.     Price  20  cents. 

33  NotesontheGeologyofNorthemCalifornia,  by  J.  S.  Diller.     1886.    8°.    23  pp.    Price  5  cents. 

34  On  the  Relation  of  the  Laramie  Molluscan  Fauna  to  that  of  the  Succeeding  Fresh-Water  Eocene 
and  Other  Groups,  by  Charles  A.  White.     1886.     8°.     54  pp.     5  pi.     Price  10  cents. 

35.  Physical  Properties  of  the  Iron-Carburets,  by  Carl  Barus  and  Vincent  Strouhal.  1886.  8°. 
62  pp.     Price  10  cents. 

36.  SubsideneeofFineSolidParticlesinLiquids,byCarlBarus.    1886.    8*^.    58pp.    Price  10 cents. 

37.  Types  of  the  Laramie  Flora,  liy  Lester  F.  Ward.     1887.     8°.     354  pp.     57  pi.     Price  25  cents. 

38.  PeridotiteofElliottCounty,Keutucky,byJ.S.Diller.     1887.     8='.    31pp.    Ipl.    PriceScents. 

39.  The  Upper  Beaches  and  Deltas  of  the  Glacial  Lake  Agassiz,  by  Warren  Uphani.  1887.  8"^. 
84  ijp.     1  pi.     Price  10  cents. 

40.  Changes  in  River  Courses  in  Washington  Territory  due  to  Glaciation,  by  Bailey  Willis.  1887. 
8°.     10  pp.     4  pi.     Price  5  cents. 

41.  On  the  Fossil  Faunas  of  the  Upper  Devonian — the  Genesee  Section,  New  York,  bv  Henry  S. 
Williams.     1887.     8°.     121pp.     4  pi.     Price  15  cents. 

42.  Report  of  Work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  Fiscal  Year 
1885-'86.     F  W.  Clarke,  Chief  Chemist.     1887.     8^.     152  pp.     Ipl.     Price  15  cents. 

43.  Tertiary  and  Cretaceous  Strata  of  the  Tuscaloosa,  Tombigbee,  and  AKabama  Rivers,  by  Eugene 
A.  Smith  and  Lawrence  C.  .Johnson.     1887.     8°.     189  pp.     21  pi.     Price  15  cents. 

44.  Bibliography  of  North  American  Geology  for  1886,  by  Nelson  H.  Darton.  1887.  8°.  35  pp. 
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45.  ThePresentConditionofKnowledgeoftheGeology  of  Texas,  by  Robert  T.Hill.  1887.  8°. 
94  pp.     Price  10  cents. 

46.  Nature  and  Origin  of  Deposits  of  Phosphate  of  Lime,  by  R.  A.  F.  Penrose,  jr.,  with  an  Intro- 
duction by  N.  S.  Shaler.     1888.     8°.     143  pp.     Price  15  cents. 

47.  Analyses  of  Waters  of  the  Yellowstone  National  Park,  with  an  Account  of  the  Methods  of 
Analysis  emjiloyed,  by  Frank  Austin  Gooch  and  James  Edward  Whitfield.  1888.  8°.  84  pp.  Price 
10  cents. 

48.  On  the  Form  and  Position  of  the  Sea  Level,  by  Robert  Simpson  Woodward.  1888.  8°.  88 
pi).     Price  10  cents. 

49.  Latitudes  aud  Longitudes  of  Certain  Points  in  Missouri,  Kansas,  and  New  Mexico,  by  Robert 
Simpson  Woodward.     1889.     8°.    133  pp.     Price  15  cents. 

50.  Formulas  and  Tables  to  Facilitate  the  Construction  and  Use  of  Maps,  by  Robert  Simpson 
Woodward.     1889.     8°.     124  pp.     Price  15  cents. 

51.  On  Invertebrate  Fossils  from  the  Pacific  Coast,  by  Charles  Abiathar  White.  1889.  8°.  102 
151>.     14  pi.     Price  15  cents. 

52.  Subaerial  Decay  of  Rocks  and  Origin  of  the  Red  Color  of  Certain  Formations,  by  Israel 
Cook  Russell.     1889.     8°.    65  pp.     5  pi.     Price  10  cents. 

53.  The  Geology  of  Nantucket,  by  Nathaniel  Southgate  Shaler.  1889.  8°.  55  pp.  10  pi.  Price 
10  cents. 

54.  On  the  Thermo-Electric  Measurement  of  High  Temperatures,  by  Carl  Barus.  1889.  8°. 
313  pp.,  incl.  1  pi.     11  pi.    Price  25  cents. 

55.  Report  of  Work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  Fiscal 
Year  1886-'87.     Frank  Wigglesworth  Clarke,  Chief  Chemist.     1889.     8°.     96  pp.     Price  10  cents. 

56.  Fossil  Wood  and  Lignite  of  the  Potomac  Formation,  by  Frank  Hall  Knowlton.  1889.  8^. 
72  pp.     7  pi.     Price  10  cents. 

.  57.  A  Geological  Reconnoissance  in  Southwestern  Kansas,  by  Robert  Hay.  1890.  8°.  49  pp. 
2  pi.     Price  5  cents. 

58.  The  Glacial  Boundary  in  Western  Pennsylvania,  Ohio,  Kentucky,  Indiana,  and  Illinois,  by 
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pp.,  incl.  1  pi.     8  pi.     Price  15  cents. 

59.  The  Gabln-os  and  Associated  Rocks  in  Delaware,  by  Frederick  D.  Chester.  1890.  8°.  45 
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60.  Report  of  AVork  done  in  the  Division  of  Chemistry  and  Physics,  mainly  during  the  Fiscal 
Year  1887-'88.     F.  W.  Clarke,  Chief  Chemist.     1890.     8-^.     174  pp.     Price  15  cents. 

61.  Contributions  to  the  Mineralogy  of  the  Pacific  Coast,  by  William  Harlow  Melville  and  Wal- 
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62.  The  Greenstone  Schist  Areas  of  the  Menominee  aud  Marquette  Regions  of  Michigan,  a  Con- 
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with  an  Introduction  by  Roland  Duer  Irving.     1890.     8'^.     241  pp.     16  pi.     Price  30  cents. 

63.  A  Bibliography  of  Paleozoic  Crustacea  from  1698  to  1889,  including  a  List  of  North  Amer- 
ican Species  and  a  Systematic  Arrangement  of  Genera,  by  Anthony  AV.  Vogdes.  1890.  8°.  177  pp. 
Price  15  cents. 

64.  A  Report  of  Work  done  in  the  Division  of  Chemistry  aud  Physics,  mainly  during  the  Fiscal 
Year  1888-'89.     F.  W.  Clarke,  Chief  Chemist.     1890.     8-^.     60  pp.     Price  10  cents. 


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T        ,  ',"?■  »-V'''^'*'''''V'.!!;^'  "i^^^'  l:'it"n"i"'"«  C(.al  FioUl  of  Pennsylvania,  Ohio,  and  West  Virginia,  bv 
Israel  C.  White.     IWIl.     y^.     212  p]..     U  jil.     Prico  20  cents.  '  i^mia,  oy 

66.  Oxi  a  Ui(mi)  of  Volcanic  Kuuk.s  irom  the  Tewau  Mountains,  New  Mexico,  and  on  the  Occur- 
rence of  Primary  Quartz  in  Certain  Basalts,  by  Joseph  Passon  Iddings.     1890.     8°.    34  pp.    Price  5 

T-T       .^'^'t^'^^"  ■'^'^''1.^?^°^''  "ft^e  Traps  of  the  Newark  System  in  the  New  Jersey  Region,  by  Nelson 
Horatio  Darton.     18fi0.     8^.     82  pp.     Price  10  cents.  h       ,     y  x^uibuu 

68.  Earthquakes  in  California  in  1889,  by  James  Edward  Keeler.     1890.     8^.     25  pp.     Price  5 
S'^      101 'i^  '^"price^S^tlnt^'^"^"*'"^  Biography  of  Fossil  Insects,  by  Samuel  Howard  Scudder.     1890. 

''\ 4  Repf  I't  oil  Astronomical  Work  of  1889  and  1890,  by  Robert  Simpson  Woodward.    1890     8° 
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:       '^^;,^?^'^^  ^l  "^«  ,^^'^°"^'S,J°^??^  Insects  of  the  World,  including  Myriapods  and  Arachnids,  by 

Samuel  Hubbard  Scudder.     1891.     8°.     744  pp.     Price  50  cents.  ='      -^      -^  "">  "y 

72.  Altitudes  lietween  Lake  Superior  and  the  Rocky  Mountains,  by  Warren  Unham     1891     8° 

J29  pp.     Price  20  cents.  ■    •.-  . 

If  -r?"  y,'.^^'"sity  of  Solids  by  Carl  Barns.     1891.     8-\     sii,  139  pp.     6  pi.     Price  15  cents. 
74.   The  Minerals  of  North  Carolina,  by  Frederick  Augustus  Genth.     1891.     8°      119  nn      Price 
15  cents.  ^  ^' 

iflQi     '^t  ^T'vT^  °^  ^^°-*^'  American  Geology  for  1887  to  1889,  inclusiTe,  by  Nelson  Horatio  Darton. 
1891.     8°.     173  xip-     Price  15  cents. 

ni,  •  f  'ip'  ^  Dictionary  of  Altitude^  in  the  United  States  (Second  Edition),  compiled  by  Henry  Gannett, 

Chief  lopographer.     1891.     8^.    393  pp.     Price  25  cents.  />         i  J  o-  f", 

77.  The  Texan  Permian  and  its  Mesozoic  Types  of  Fossils,  by  Charles  A.  White.     1891      8°      51 

pp.     4  pi.     Price  10  cents.  "  .         .     ^j. 

J?-^  A  Report  of  Work  done  in  the  Division  of  Chemistry  and  Physics,  mainly  durine  the  Fiscal 
learl889-'90.     F.  AV.  Clarke,  Chief  Chemist.     1891.     8".     ISi  pp.     Price  15  cents  ^ 

79.  A  Late  Volcanic  Eruption  in  Northern  California  and  its  Peculiar  Lava,  by  J.  S.  Diller 
o-rn  Correlation  Papers— Devonian  and  Carboniferous,  by  Henry  Shaler  Williams.     1891.     8° 

J79  i>p.     Price  20  cents. 

81.  Correlation  Papers— Cambrian,  by  Charles  Doolittle  Walcott.  1891.  8°'  547  ni)  3  nl 
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82.  Correlation  Papers— Cretaceous,  by  Charles  A.  White.     1891.     8°.     273  pp.     3  pi.     Price  20 

83.  Correlation  Papers— Eocene,  by  William  Bullock  Clark.  1891.  8°.  173  pp.  2  pi.  Price 
lo  cents. 

.      84.  Correlation  Papers— Neocene,  by  W.  H.  Dall  and  G.  D.  Harris.     1892.     8°.     349  pp      3  pi 
Price  25  cents.  1 1  •     "  i'*- 

85.  Correlation  Papers— The  Newark  System,  by  Israel  Cook  Russell.  1892.  8°.  344  up  13  pi 
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86.  Correlation  Papers— Archean  and  Algonkian,  by  C.E.  Van  Plise.  1892.  8".  549  pp  12  pi 
Price  2o  cents.  ^^'  ^ 

n\,     i^^Q^  Synopsis  of  American  Fossil.     Brachiopoda,  including  Bibliography  and  Synonymy,  by 
Charles  Schuchert.     1897.     8".     464  pp.     Price  30  cents.  s,    i^.i  y        ju^y,  "y 

a    T     ^-.TlieCretaceousForamimferaof  New  Jersey,  by  Rufus  Mather  Bagg,  Jr.     1898.     8°      89  pp 
b  pi.     Price  10  cents.  ^^" 

89.  Some  Lava  Flows  of  the  Western  Slope  of  the  Sierra  Nevada,  California,  by  F.  Leslie 
Kansome.     1898.     8^.     74  pp.     11  pi.     Price  15  cents. 

Year  1  sqn "^pf ''^?''' w^pT"  1 ""  'of  ^  IV^^^  ^V'^\ll?^  Chemistry  and  Physics,  mainly  diu-ing  the  Fiscal 

Year  1890- 91.     F.  W.  Clarke,  Chief  Chemist.     1892.     8^.     77  pp.     Price  10  cents. 

Price  10        i  ''^^  American  Geology  for  1890,  by  Nelson  Horatio  Darton.     1891.     8°.     88  pp. 

92.  The  Compressibility  of  Liquids,  by  Carl  Barns.  1892.  8°.  96  pp.  29  pi.  Price  10  cents. 
„f  n  1  fO'^e  Insects  of  Special  Interest  from  Florissant,  Colorado,  and  Other  Points  in  the  Tertiaries 
of  Colorado  and  Utah,  by  Samue  Hubbard  Scudder.     1892.     8°.     35  pp.     3  pi.     Price  5  cents. 

94.  The  Mechanism  of  Solid  Viscosity,  by  Carl  Barns.     1892.     8°.     138  pp.     Price  15  cents. 

PriceScehtT     ^^^         ™  ™''''''^^^^''^°'^^^^'^'^'^''^'^^'^'''^^™^^^*°'^'^°^^^^        ^^^^-    ^°-    ^^PP- 

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97.   IheMesozoicEchmodermataoftheUnitedStates,  by  W.B.Clark.    1893.    8°.    207  pp.    50pl. 

1BO..    ^lo  ^^i°on   "^  ttie  Outlying  Carboniferous  Basins  of  Southwestern  Missouri,  by  David  White. 
IbOo.     8*^.     139  pp.     o  pi.     Price  15  cents. 

99.  Record  of  North  American  Geology  for  1891,  by  Nelson  Horatio  Darton.  1892.  8°.  73  pp 
i^rice  10  cents.  .  ^  ^ ' 

Di,-i-  ■^S°'  I^iVl^^S^P^y  and  Index  of  the  Publications  of  the  U.  S.  Geological  Survey,  1879-1892,  by 
Phihp  Crevelmg  Warman.     1893.     8°.     495  pp.     Price  25  .cents.  "-^  .  oy 

101.  Insect  Fauna  of  the  Rhode  Island  Coal  Field,  by  Samuel  Hubbard  Scudder.  1893  8° 
^(  pp.     2  pi.     Price  o  cents. 

-Rv„  1  ^^^■■-.■^  Catalogue  and  Bibliography  of  North  American  Mesozoic  Invertebrata,  by  Cornelius 
Breckinridge  Boyle.     1892.     8°.     315  pp.     Price  25  cents. 


VI  ADVERTISEMENT. 

103.  Higli  Temperature  Work  in  Igneous  Fusion  and  Elmllition,  chiefly  in  Eelattou  to  Pressure, 
by  Carl  Barns.     1893.     8°.     57  pp.     9  pi.     Price  10  cents. 

104.  Glaciation  of  the  Yellowstone  Valley  north  of  the  Park,  hy  Walter  Harvey  Weed.    1893.    8°. 
41  pp.     4  pi.     Price  5  cents. 

105.  The  Laramie  and  the  Overlying  Livingstone  Formation  in  Montana,  by  Walter  Harvey 
Weed,  with  Report  on  Flora,  hy  Frank  Hall  Kno-witon.     1893.     8^.     68  pp.     6  pi.     Price  10  cents. 

106.  The  Colorado  Formation  and  its  Invertebrate  Fauna,  by  T.  W.  Stanton.     1893.     8==.     288 
pp.     45  pi.     Price  20  cents. 

107.  The  Trap  Dikes  of  the  Lake  Champlain  Eegion,  by   .Tames   Fnrman  Kemp  and  Vernon 
Freeman  Marsters.     1893.     S'^.     62  pp.     4  pi.     Price  10  cents. 

108.  A  Geological  Reconnoissauce  in  Central  Washington,  by  Israel  Cook  Russell.     1893.     8". 
108  pp.     12  pi.     Price  15  cents. 

109.  The  Eruptive  and  Sedimentarv  Rocks  on  Pigeon  Point,  Minnesota,  and  their  Contact  Phe- 
nomena, by  William  Shirley  Bay  ley.     1893.     8^'.     121  pp.     16  pi.     Price  15  cents. 

110.  The  Paleozoic  Section  in  the  Vicinity  of  Three  Forks,  Montana,  by  Albert  Charles  Peale. 
893.     8°.    56  pp.     6  pi.     Price  10  cents. 

111.  Geology  of  the  Big  Stone  Gap  Coal  Fields  of  Virginia  and  Kentucky,  by  Marius  E.  Camp- 
bell.    1893.     8^.''l06pp.     6pL     Price  15  cents. 

112.  Earthquakes  in  California  in  1892,  by  Charles  D.  Perrine.    1893.    8°.    57  pp.    Price  10  cents. 

113.  A  Report  of  Work  done  in  the  Division  of  Chemistrv  diiring  the  Fiscal  Years  1891-'92  and 
1892-'93.     F.  W.  Clarke,  Chief  Chemist.     1893.     8".     115  pp.     Price  15  cents. 

114.  Earthquakes  in  California  in  1893,  by  Charles  D.  Perrine.    1894.    8^.    23  pp.    Price  5  cents. 

115.  A  Geographic  Dietioiiary  of  Ehode  Island,  by  Henry  Gannett.     1894.     8°.     31  pp.     Price 
5  cents. 

-     116,  A  Geographic  Dictionary  of  Massachusetts,  by  Henry  Gannett.     1894.     8^.     126  pp.     Price 
15  cents. 

117.  A  Geographic  Dictionary  of  Connecticut,  by  Henry  Gannett.     1894.     8°.     67  pp.     Price  10 
cents. 

118.  A  Geographic  Dictionary  of  New  .lersey,  by  Henry  Gannett.     1894.     8^.     131  pp.     Price  15 
cents. 

119.  A  Geological  Reconnoissauce  in  Northwest  Wyoming,  by  George  Homans  Eldridge.     1894. 
8°.     72  pp.     Price  10  cents. 

120.  The  Devonian  System  of  Eastern  Penuyslvania  and  New  York,  by  Charles  S.  Prosser.     1894. 
8°.     81  pp.     2  pi.     Price  10  cents. 

121.  A  Bibliography  of  North  American  Paleontology,  by  Charles  Eolliu  Keyes.     1894.    8°.     251 
pp.     Price  20  cents. 

122.  Results  of  Primary  Triangulatiou,  by  Henry  Gannett.     1894.     8^.     412  pp.     17  pi.     Price 
25  cents. 

123.  A  Dictionary  of  Geographic  Positions,  by  Henry  Gannett.     1895.     8°.     183  pp.     1  pi.    Price 
15  cents. 

124.  Revision  of  North  American  Fossil  Cockroaches,  by  Samuel  Hubbard  Scndder.     1895.     8^. 
176  pp.     12  pi.     Price  15  cents. 

125.  The  Constitution  of   the   Silicates,  by  Frank  Wigglesworth  Clarke.     1895.     8'^.     109   pp. 
Price  15  cents. 

126.  A  Mineralogical  Lexicon  of  Franklin,  Hampshire,  and  Hampden  counties,  Massachusetts, 
by  Benjamin  Kendall  Emerson.     1895.     8'-.     180  pp.     1  pi.     Price  15  cents. 

127.  Catalogue  and  Index  of  Contributions  to  North  American  Geology,  1732-1891,  by  Nelson 
Horatio  Darton.     1896.     8-.     1045  pp.     Price  60  cents. 

128.  The  Bear  Eiver  Formation  and  its  Characteristic  Fauna,  by  Charles  A.  White.     1895.     S°. 
108  pp.     11  pi.     Price  15  cents. 

129.  Earthquakes  in  California  in  1894,  by  Charles  D.  Perrine.    1895.     8°.     25  pp.     Price  5  cents. 

130.  Bibliography  and  Index  of  North  American  Geology,  Paleontology,  Petrology,  and  Miner- 
alogy for  1892  and  1893,  by  Fred  Boughton  Weeks.     1896.     8°.     210  pp.     Price  20  cents. 

131.  Report  of  Progress  of  the'Division  of  Hydrography  for  the  Calendar  Y^ears  1893  and  1894, 
by  Frederick  Haynes  Newell,  Topographer  in  Charge.     1895.     8°.     126  pp.     Price  15  cents. 

132.  The  Disseminated  Lead  Ores  of  Southeastern  Missouri,  by  Arthur  Winslow.     1896.     8°. 
31  pp.     Price  5  cents. 

133.  Contributions  to  the  Cretaceous  Paleontology  of  the  Pacific  Coast:    The  Fauna  of  the 
Knoxville  Beds,  bvT.W.  Stanton.     1895.    8^.     132  pp.     20  ph     Price  15  cents. 

134.  The  Cambrian  Eocks  of  Pennsylvania,  by  Charles  Doolittle  Walcott.     1896.     8°.     43  pp. 
15  pi.     Price  5  cents. 

135.  Bibliography  and  Index  of  North  American  Geology,  Paleontology,  Petrology,  and  Miner- 
alogy for  the  Year  1894,  by  F.  B.  Weeks.     1896.     8°.     141  pp.     Price  15  cents. 

136.  Volcanic  Eocks  of  South  Mountain,  Pennsylvania,  by  Florence  Bascom.    1896.    8°.    124  pp. 
28  pi.     Price  15  cents. 

137.  The  Geology  of  the  Fort  Eiley  Military  Eeservation  and  Vicinity,  Kansas,  by  Robert  Hay. 
1896.     8°.     35  pp.     8  p'l.     Price  5  cents. 

138.  Artesian-Well  Prospects  in  the  A-tlantic  Coastal  Plain  Eegion,  by  N.  H.  Darton.     1896.     8°. 
228  pp.     19  pi.     Price  20  cents. 

139.  Geology  of  the  Castle  Mountain  Mining  District,  Montana,  by  W.  H.  Weed  and  L.  V.  Pirs- 
sou.     1896.     8-^.     164  pp.     17  pi.     Price  15  cents. 

140.  Report  of  Progress  of  the  Division  of  Hydrography  for  the  Calendar  Year  1895,  by  Frederick 
Haynes  Newell,  Hydrographer  in  Charge.     1896.     8*^.     356  pp.     Price  25  cents. 


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141.  The  Eocene  Peiiosits  of  the  Middle  Atlantic  Slope  iu  Deluwaro,  Maryland  and  Virsriui-i 
by  William  Bullock  Clark.     1896.    8'^.     167  pp.     40  pi.     Price  15  cents. 

112.  A  Brief  Contrilnition  to  the  Geolony  and  Paleontology  of  Northwestern  Louisiana  bv 
T.  Wayhind  Vanghan.     1896.     8'^.     6.5  jip.     4  pi.     Price  10  cents.  ' 

113.  A  Bibliography  of  Clays  and  the  Ceramic  Arts,  by  John  C.  Branner.  1896.  8°.  114  pp 
Price  15  cents.  '■  ^  ' 

144.  The  Moraines  of  the  Missouri  Coteau  and  their  Attendant  Deposits,  by  James  Edward  Todd 
1896.     8"^.     71  pp.     21  pi.     Price  10  cents. 

145.  The  Potomac  Formation  in  A^irginia,  by  W.  M.  Fontaine.  1896.  8^.  149  pp.  2  pi  Price 
15  cents.  ' 

146.  Bibliography  and  Index  of  North  American  Geology,  Paleontology,  Petrology,  and  Miner- 
alogy for  the  Year  1895,  by  F.  B.  Weeks.     1896.     8°.     130  pp.     Price  15  cents. 

147.  Earthquakes  iu  California  iu  1895,  by  Charles  D.  Perrine,  Assistant  Astronomer  iu  Charo-g 
of  Earthquake  Obseryations  at  the  Lick  Observatory.     1896.     8"-'.     23  pp.     Price  5  cents.  ° 

148.  Analyses  of  Rocks,  with  a  Chapter  on  Analytical  Methods,  Laljoratory  of  the  Uuited  States 
Geological  Suryey,  1880  to  1896,  by  F.  W.  Clarke  and  W.  F.  Hillebraud.  1897.  8'-\  306  pp  Price 
20  cents. 

149.  Bibliography  and  Index  of  North  American  Geology,  Paleontology,  Petrology,  and  Miner- 
alogy for  the  Year  1896,  by  Fred  Boughton  Weeks.     1897.     8°.     152  pp.     Price  15  cents 

150.  The  Educational  Series  of  Rock  Specimens  collected  and  distrilmted  by  the  United  States 
Geological  Survey,  by  Joseph  Silas  Diller.     1898.     8°.     398  pp.     47  pi.     Price  25  cents. 

151.  The  Lower  Cretaceous  Grypha^as  of  the  Texas  Region,  by  R.  T.  Hill  and  T.  Wayland 
Vaiighan.     1898.     8°.     139  pp.     25  pi.     Price  15  cents. 

152.  A  Catalogue  of  the  Cretaceous  and  Tertiary  Plants  of  North  America,  by  F  H  Knowlton 
1898.     8°.     247  pp.     Price  20  cents. 

^o^o    ^P'  ■*■  Bibliographic  Index  of  North  American  Carboniferous  Invertebrates,  by  Stuart  Weller 

1898.     8°.     653  pp.     Price  35  cents. 

154.  A  Gazetteer  of  Kansas,  by  Henry  Gannett.     1898.     8°.     246  pp.     6  pi.     Price  20  cents 

1.55.  Earthquakes  in  California  iu  1896  and  1897,  by  Charles  D.  Perrine,  Assistant  Astronomer 

m  Charge  ol  Earthquake  Obseryations  at  the  Lick  Observatory.     1898.     8^.     47  pp.     Price  5  cents 

156.  Bibliography  and  Index  of  North  American  Geology,  Paleontology,  Petrology,  and  Miner- 
alogy for  the  Year  1897,  by  Fred  Boughton  Weeks.     1898.     8".     130  pp.     Price  15  cents'! 

160.  A  Dictionary  of  Altitudes  in  the  United  States  (Third  Edition),  compiled  bv  Henry 
Gannett.     1899.     8^.     775  pp.     Price  40  cents.  '' 

^T,     x?-^'  Earthquakes  iu  California  in  1898,  by  Charles  D.  Perrine,  Assistant  Astronomer  in  Charo-e 
ot  Earthquake  Observations  at  the  Lick  Observatory.     1899.     8°.     31  pp.     1  pi.     Price  5  cents 
In  preparation: 

157.  The  Gneisses,  Gabbro-Schists,  and  Associated  Rocks  of  Southeastern  Minnesota,  by  C.  W. 

158.  The  Moraines  of  southeastern  South  Dakota  and  their  Attendant  Deposits,  by  J.  E   Todd 

159.  The  Geology  of  Eastern  Berkshire  County,  Massachusetts,  by  B.  K.  Emerson. 

WATER-SUPPLY  AND  IRRIGATION  PAPERS. 

By  act  of  Congress  approved  June  11,  1896,  the  following  provision  was  made  • 
"Provided,  That  hereafter  the  reports  of  the  Geological  Survey  in  relation  to  the  o-auo-ino-  of 
streams  and  to  the  methods  of  utilizing  the  water  resources  may  be  prin'ed  in  octavo  form^'no'tto 
exceed  one  hundred  pages  in  length  and  live  thousand  copies  in  number ;  one  thousand  copies  of  which 
shall  be  lor  the  official  use  of  the  Geological  Survey,  oue  thousand  five  hundred  copies  shall  be  deliv- 
ered to  the  Senate,  and  two  thousand  five  hundred  copies  shall  be  delivered  to  the  House  of  Renre- 
sentatives,  for  distribution."  ' 

Under  this  law  the  following  papers  have  been  issued: 

1.  Pumping  Water  for  Irrigation,  by  Herbert  M.  V'ilson.     1896.     8°.     57  pp.     9  pi 

2.  Irrigation  near  Phoenix,  Arizona,  by  Arthur  P.  Davis.     1897.     8°.    97  pp      31  pi 

3.  Sewage  Irrigation,  by  George  W.  Rafter.     1897.     8^.     100  pp.     4  pi. 

4.  AReconnoissanceinSoutheasternWashington,  by  Israel  Cook  Russeil.    1897.    8^^     96  pp     7  pi 

5.  Irrigation  Practice  on  the  Great  Plains,  by  Elias  Branson  Cowgill.     1897.     8°      39  pp      1''  pi' 

6.  Underground  Waters  of  Southwestern  Kansas,  by  Erasmus  Haworth.    1897.    8^     65  pp     12  pi' 

7.  Seepage  Waters  of  Northern  Utah,  by  Samuel  Fortier.     1897.     8°.    50  pp      3  pi 

8.  Windmills  for  Irrigation,  by  Edward  Charles  Murphy.     1897.     8°.     49  pp.    8  pi 

9.  Irrigation  near  Greeley,  Colorado,  by  David  Boyd.     1897.     8°.     90  pp     21  pi 

10.  Irrigation  in  Mesilla  Valley,  New  Mexico,  by  F.  C.  Barker.     1898.  8°.     51  pp.     11  pi 

11.  River  Heights  for  1896,  by  Arthur  P.  Davis.     1897.     8°.     100  pp. 

12.  Water  Resources  of  Southeastern  Nebraska,  by  Nelson  H.  Darton.  1898.     8°      55  pp      21  pi 
1?"  J'^"S'^*i°ii  Systems  iu  Texas,  by  William  Ferguson  Hutson.    1898.  8''.     67  pp.     10  pi. 

14.  New  Tests  of  Certain  Pumps  and  Water-Lifts  used  in  Irrigation,  by  Ozni  P.  Hood.    1889.     8^ 
al  pp.     1  pi. 

15.  Operations  at  River  Stations,  1897,  Part  I.     1898.     8'^.     100  pp. 

16.  Operations  at  River  Stations,  1897,  Part  II.     1898.    8°.    101-200  pp 

17.  Irrigation  near  Bakersfield,  California,  by  C.  E.  Grunsky.     1898.     8'=.     96  pp.     16  pi 

18.  Irrigation  near  Fresuo,  California,  by  C.  E.  Grunsky.     1898.     8°.    94  pp.     14  pi 

19.  Irrigation  near  Merced,  California,  by  C.  E.  Grunsky.     1899.     8°.     59  pp.     11  pL 

20.  Experiments  with  Windmills,  by  T.  O.  Perry.     1899.     8°.     97  pp      12  pi 


Hall. 


VlII 


ADVERTISEMENT. 


2  pi. 
7  pi. 


21.  Wells  of  Northern  Indiana,  by  Frank  Leverett.     1899.    8°.     82  pp. 

22.  Sewage  Irrigation,  Part  II,  by  George  AV.  Eaiter.     1899.     8°.     100  pp 

23.  Water-Right  Problems  of  Bighorn  Mountains,  by  Elwood  Mead.     1899. 

24.  Water  Resources   of  the   State  of  New  York,   Part  I,   bv   George  W. 
99  pp.     13  pi.  >  >      J  ^ 

25.  Water  Resources   of  the  State   of  New  York,   Part  II,  by  George  W.  Rafter. 
101-200  pp.     12  pi. 

26.  Wells  of  Southern  Indiana  (Continuation  of  No.  21),  by  Frank  Leverett.     1899. 

27.  Operations  at  River  Stations,  1898,  Part  I.     1899.     8°.     100  pp. 

28.  Operations  at  River  Stations,  1898,  Part  II.     1899.     8°.     101-200  pp. 

In  preparation: 

29.  Wells  and  Windmills  in  Nebraska,  by  Edwin  H.  Barbour. 

30.  Water  Resources  of  the  Lower  Peninsula  of  Michigan,  by  Alfred  C.  Lane. 


62  pp.     7  pi. 
Rafter.     1899.    S 


1899.     8°. 


8°.     64  pp. 


TOPOGRAPHIC  MAP  OF  THE  UNITED  STATES. 

When,  in  1882,  the  Geological  Survey  was  directed  by  law  to  make  a  geologic  map  of  the  United 
States  there  was  in  existence  no  suitable  topographic  map  to  serve  as  a  base  for  the  geologic  map. 
The  prepnration  of  such  a  topographic  map  was  therefore  immediately  begun.  About  one-fifth  of  the 
area  of  the  country,  excluding  Alaska,  has  now  been  thus  mapped.  The  map  is  published  in  atlas 
sheets,  each  sheet  representing  a  small  quadrangular  district,  as  explained  under  the  next  head- 
ing. The  separate  sheets  are  sold  at  5  cents  each  when  fewer  than  100  copies  are  purchased,  but  when 
they  are  ordered  in  lots  of  100  or  more  copies,  whether  of  the  same  sheet  or  of  different  sheets,  the 
price  is  2  cents  each.  The  mapped  areas  are  widely  scattered,  nearly  every  State  being  represented. 
About  900  sheets  have  been  engraved  and  printed;  they  are  tabulated  by  States  in  the  Survey's 
"List  of  Publications,"  a  pamphlet  which  may  be  had  on  application. 

The  map  sheets  represent  a  great  variety  of  topographic  features,  and  with  the  aid  of  descriptive 
text  they  can  be  used  to  illustrate  topographic  forms.  Tliis  has  led  to  the  projection  of  au  educational 
series  of  topographic  folios,  for  use  wherever  geography  is  taught  in  high  schools,  academies,  and 
colleges.     Of  this  series  the  first  folio  has  been  issued,  viz : 

1.  Physiographic  types,  by  Henry  Gannett,  1898,  folio,  consisting  of  the  following  sheets  and  4 
pages  of  descriptive  text:  Fargo  (N.  Dak. -Minn.),  a  region  in  youth;  Charleston  (W.Va.),a  region  in 
maturity;  Caldwell  (Ivans.),  aregion  in  old  age;  Palmyra  (Va!),  a  rejuvenated  region;  Mount  Shasta, 
(Gal.),  a  young  volcanic  mountain;  Eagle  (Wis.),  moraines;  Sun  Prairie  (Wis.),  drumlins;  Donald- 
souville  (La.),  river  flood  plains;  Boothbay  (Me.),  a  fiord  coast;  Atlantic  City  (N.  J.),  a  barrier-beach 
coast. 

GEOLOGIC  ATLAS  OF  THE  UNITED  STATES. 

The  Geologic  Atlas  of  the  United  States  is  the  final  form  of  publication  of  the  topographic  and 
geologic  maps.  The  atlas  is  issued  in  parts,  progressively  as  the  surveys  are  extended,  and  is  designed 
ultimately  to  cover  the  entire  country. 

Under  the  plan  adopted  the  entire  area  of  the  country  is  divided  into  small  rectangular  districts 
(designated  quadrangles),  bounded  by  certain  meridians  and  parallels.  The  unit  of  survey  is  also  the 
unit  of  publication,  and  the  maps  and  descriptions  of  each  rectangular  district  are  issued  as  a  folio  of 
the  Geologic  Atlas. 

Each  folio  contains  topographic,  geologic,  economic,  and  structural  maps,  together  with  textual 
descriptions  and  explanations,  and  is  designated  by  the  name  of  a  principal  town  or  of  a  prominent 
natural  feature  within  the  district. 

Two  forms  of  issue  have  been  adopted,  a  "library  edition"  and  a  "field  edition."  In  both  the 
sheets  are  bound  between  heavy  paper  covers,  but  the  library  copies  are  permanently  bound,  while 
the  sheets  and  covers  of  the  field  copies  are  only  temporarily  wired  together. 

Under  the  law  a  copy  of  each  folio  is  sent  to  certain  public  libraries  and  educational  institu- 
tions. The  remainder  are  sold  at  25  cents  each,  except  such  as  contain  au  unusual  amount  of  matter, 
which  are  priced  accordingly.  Prepayment  is  obligatory.  The  folios  ready  for  distribution  are  listed 
below. 


No. 

Name  of  slieet. 

State. 

Limiting  meridians. 

Limiting  parallels. 

Area,  in 
square 
miles. 

Price, 

in 
cents. 

^ 

Montana 

/Georgia 

110°-lllo 
j.                          85°-85o  30' 

120°  30'-121o 
840  30'-85o 

1210-121°  30' 
850-85°  30' 

1050-105°  30' 

85°  30'-80° 

106°  45'-107o  15' 

I                            77°  30'-78° 

450-46° 
34°  30'-35o 

38°  30'-39° 
35°  30'-36° 
38°  30'-59o 
350-350  30' 
380  30'-39o 
350-35°  30- 
380  45'-39o 

390-390  30' 

3,354 

980 

932 
969 
932 
975 
932 
975 
465 

925 

\Tennessee 

California 

Tennessee 

California 

Tennessee 

Colorado 

Tennessee 

Colorado 

Virginia 

mest  Virginia.. 

Maryland 

3 

25 

4 

25 

•i 

P, 

7 

R 

Pikes  Peak  (oat  of  stock) 

25 
25 

9 

in 

ADthracite-Crested  Butte 

50 

ADVERTISEMENT. 


IX 


No. 

11 

12 

13 

U 

15 

16 

17 
18 


30 


Name  of  sheet. 


State. 


Limiting  meridians. 


Limiting  parallels. 


Area,  in  Price, 
square       in 
miles. 


Jackson  ... 

Estillville  . 


Fredericksburg 

Staunton 

Laason  Peak... 
Knoxville 


Marys  ville. . 
Smartsville . 


Stevenson  . 


Cleveland 

Pikeville 

ilcMinn  ville. 

Noiiiini 


Three  Forks. 
Loudon 

Pocahontas .. 

MoiTistown.. 


Piedmont. 


Nevada  Citv. 


fXevada  City. 

<^  Grass  Valley. 

iBanuer  Hill  . 

rGallatin..] 

/Tellow.stoue    .Na.  JCauyon...[ 


California 

Virginia 

Kentucky 

.Tennessee 

fMaryland 

\Virgiuia 

/Virginia 

\*U^est  Virginia. 

California 

Teunessue 

North  Carolina 

California 

California 

{Alabama 
Georgia 
Tennessee 

Teuue.-^see 

Tennessee 

Tennessee 

fMaryland 

\  Virginia 

Montana 

Tennessee 

/Viiginia 

OVest  Virginia . 

Tennessee 

Virginia 

Maryland 

["West  Virginia. 

California 


121°  00' 
121°  01' 
120°  57' 


t    tional  Park. 

Pyramid  Peak  - 

Franklin 

Brice  ville 

Euckbannon... 

Gadsden 

Pueblo 

Do\\Tiieville  ... 
Butte  Special.. 

Truckee 

"Wartburg 

Sonera 

Nueces 

Bid^y6llBar  ... 

Tazewell 


I  Shoshone. 
[Lake 


Wyoming 


Boise 

Kichmond 

London 

Teumile  District  Special. 
Roseburg 

Hoi  yoke 


California 

/Virginia 

\West.  Virginia . 

Tennessee 

West  Virginia  . 

Alabama 

Colorado .... 

California 

Montana 

California 

Tennessee 

California 

'i'exas  

California 

/Virginia 

\West  Virginia. 

Idaho 

Kentucky 

Kentucky 

Cohii'ado 

( )regon 

/Hassacbuaetta 
\Connecticut  ... 


112°  29' 


120°  30'-121o 
82°  30'-83° 

770-77°  30' 

79°-79°  30' 

1210-122° 

830  30'-81° 

121°  30'-122o 
1210-121°  30' 

85°  30'-86° 

84°  30'-85° 
8o°-850  30' 
85°  30'-8S° 

76°  30'-77° 

lll°-112o 
84°-Sl°  30' 

81°-S1°  30' 

83°-S3o  30' 

79°-79°  30' 

25"-121o  03'  45" 
35"-121o  05'  04" 
05"-121°  00'  25" 


120O-120O  30' 

79°-79°  30' 

84°-84°  30' 

800-80°  30' 

86°-860  30' 

104°  30'-105° 

120°  30'-121° 

30"-112°  30'  42" 

120°-120°  30' 

840  30'-S5o 

1200-1200  30' 

100°-100o  30' 

121°-121°  30' 

81°  30'-82o 

1160-1160  30' 

840-840  30' 

840-84°  30' 

106°  8'-106°  16' 

1230-1230  30' 

72°  30'-73o 


38°-38o  30' 
360  30'-37o 

38°-38°  30' 

360-38°  30' 

400-41° 

350  30'-36° 

390-39°  30' 
390-390  30' 


350-350  30' 
350  30'-36o 
350  30'-36o 

380-380  30' 
450-460 
350  30'-36° 
370-370  30' 
360-360  30' 

390-390  30' 


39°  13'  50"-39°  17'  16" 
39°  10'  22"-390  13'  50" 
39°  13'  50"-39o  17'  16" 


380  30'-39° 

36°-36o  30' 
38°  30'-39° 
340-340  30' 

380-38°  30' 
39°  30'-40° 
450  59'  28"-46°  02'  54" 
390-390  30' 
360-360  30' 
37°  30'-38° 
290  30'-30° 
390  30'-40o 

370-370  30' 

430  30'-44° 
370  30'-38° 
370-370  30' 
390  22'  30"-39°  30'  30" 
43°-43°  30' 

420-12°  30' 


957 

938 

938 

3,634 

925 

925 
925 

980 

975 
969 
969 


3,354 
969 


11.65 
12.09 
11.  C5 

3,412 


963 
932 
986 
938 
919 

22.80 
925 
903 
944 

1,035 
918 

950 

864 

944 

950 

55 

871 


25 
25 
25 


25 
25 
25 


25 
25 
50 


25 
25 
25 


25 
25 
25 


STATISTICAL  PAPERS. 


Mineral  Resources  of  tlie  United  States  [1882],  by  Albert  Williams,  jr.  1883.  8^.  xvii,  813  pp. 
Price  50  cents. 

Mineral  Resources  of  tbe  United  States,  1883  and  1884,  by  Albert  Williams,  jr.  1885.  8°.  xiv, 
1016  pp.     Price  60  cents. 

Mineral  Resources  of  tlie  United  States,  1885.  Division  of  Mining  Statistics  and  Technology. 
1886.     8^^.     vii,  576  pp.     Price  40  cents. 

Mineral  Resources  of  the  United  States,  1886,  by  David  T.  Day.  1887.  8°.  viii,  813  pp.  Price 
(50  cents. 

Mineral  Resources  of  the  United  States,  1887,  by  David  T.  Day.  1888.  8°.  vii,  832  pp.  Price 
50  cents. 

Mineral  Resources  of  the  United  States,  1888,  by  David  T.  Day.     1890.     8°.     vii,  652  pp.     Price 

Mineral  Resources  of  the  United  States,  1889  and  1890,  by  David  T.  Day.     1892.     8^.     viii,  671pp. 

Mineral  Resources  of  the  United  States,  1891,  by  David  T.  Day.  1893.  8°.  vii,  630  pp.  Price 
50  cents. 


X  ADVERTISEMENT. 

Mineral  Resources  of  the  United  States,  1892,  by  David  T.  Day.  1893.  8'^.  vii,  850  pp.  Price 
50  cents. 

Mineral  Resources  of  the  United  States,  1893,  by  David  T.  Day.  1894.  8^.  viii,  810  pp.  Price 
50  cents. 

On  March  2, 1895,  the  following  provision  was  included  in  an  act  of  Congress : 

"Provided,  That  hereafter  the  report  of  the  mineral  resources  of  the  United  States  shall  be 
issued  as  a  part  of  the  report  of  the  Director  of  the  Geological  Survey." 

In  compliance  with  this  legislation  the  following  reports  have  been  published: 

Mineral  Resources  of  the  United  States,  1894,  David  T.  Day,  Chief  of  Division.  1895.  8'^.  xv, 
646  pp.,  23  pi. ;  xis,  735  pp.,  6  pi.     Being  Parts  III  and  IV  of  the  Sixteenth  Annual  Report. 

Mineral  Resources  of  the  United  States,  1895,  David  T.  Day,  Chief  of  Division.  1896.  9P. 
xxiii,  542  pp.,  8  pi.  and  maps ;  iii,  543-1058  pp.,  9-13  pi.  Being  Part  III  (in  2  vols.)  of  the  Seventeenth 
Annual  Report. 

Mineral  Resources  of  the  United  States,  1896,  David  T.  Day,  Chief  of  Division.  1897.  8°. 
xii,  642  pp.,  1  pi. ;  643-1400  pp.     Being  Part  V  (in  2  vols. )  of  the  Nineteenth  Annual  Eeport. 

Mineral  Resources  of  the  United  States,  1897,  David  T.  Day,  Chief  of  Division.  1898.  8°. 
viii,  651  pp.,  11  pi. ;  viii,  706  pp.     Being  Part  VI  (in  2  vols.)  of  the  Nineteenth  Annual  Eeport. 

The  money  received  from  the  sale  of  the  Survey  publications  is  deposited  in  the  Treasury,  and 
the  Secretary  of  that  Department  declines  to  receive  bank  checks,  drafts,  or  postage  stamps ;  all  remit- 
tances, therefore,  must  be  by  money  order,  made  payable  to  the  Director  of  the  United  States 
Geological  Survey,  or  in  currency' — the  exact  amount.  Correspondence  relating  to  the  publications 
of  the  Survey  should  be  addressed  to 

The  Director, 

United  States  Geological  Survey, 
Washington,  D.  C,  June,  1S99.  Washington,  D.  C. 


[Take  this  leaf  out  and  paste  the  separated  titles  upon  three  of  your  cata- 
logue cards.  The  hrst  and  second  titles  need  no  addition ;  over  the  third  write 
that  subject  under  "which  you  would  place  tho  book  in  your  library.] 


LIBRARY  CATALOGUE  SLIPS. 

United  States.     Department  of  the  interior.     {TJ.  S.  geological  survey.) 
Department  of  the  interior  |  —  |  Monographs  |  of  the  |  United 
States  geological  survey  |  Volume  XXXVI  |  [Seal  of  the  depart- 
ment] I 
Washington  |  government  prilitiug  office  |  1899 
Second    title:   United    States  geological   survey   |   Charles   D. 
Walcott,  director  |  —  |  The  |  Crystal  Falls  iron-bearing  district 
of  Michigan  |  by  |  .J.  Morgan  Clements  and  Henry  Lloyd  Smyth  | 
■with  I  a  chapter  on  the  Sturgeon  river  tongue  |  by  |  William 
Shirley  Bay  ley  |  and  |  an  introduction  |  by  |  Charles  Richard  Van 
Hise  I  [Vignette]  | 
Washington  |  government  printing  office  |  1899 
A°.    sssvi,  512  pp.    53  i3l. 


Clements  (J.  M.),  Smyth  (H.  L.),  Bayley  (W.  S.),  and  Van  Hise 
(C.  R.) 

United    States   geological   survey    |    Charles   D.  Walcott,   di- 
rector I  —  I  The  I  Crystal  Falls  iron-bearing  district  of  Michigan  | 
by  I  J.  Morgan  Clements  and  Henry  Lloyd  Smith  |  with  |  a  chap- 
ter on  the  Sturgeon  river  tongue  |  by  |  William  Shirley  Bayley  | 
and  I  an    introduction  |  by  |  Charles   Richard   Van    Hise   |  [Vig- 
nette] I 
Washington  |  government  piinting  office  |  1899 

4^.     xsxvi.  512  Jip.    53  pi. 

[U^'iTED  States.    Department  of   the  interior.     {U.   S.  geological  survey.) 
Monograph  XXXVl.] 


United   States   geological    survey    |    Charles   D.   Walcott,   di- 
rector I  —  I  The  I  Crystal  Falls  iron-bearing  district  of  Michigan  | 
by  I  J.  Morgan  Clements  and  Henry  Lloyd  Smyth  |  with  |  a  chap- 
ter on  the  Sturgeon  river  tongue  |  by  |  William  Shirley  Bayley  | 
and  I  an    introduction  |  by  |  Charles    Richard  Van    Hise  |  [Vig- 
nette] I 

Washington  |  government  printing  office  |  1899 

4°.    xxxvi,  512  pp.    53  pi. 

[TJxiTED  States.    Department  of  the  interior.     {U.   S.  geological  survey.) 
Monograph  XXSVI.] 


U  S  GEOUODICAL  SURVEV 


Topograprir  b,  G  E  h,Di 

H  L  Smiih.ana  from  «u 
Su'xyeilm  1090-90 


H 


TOI'OC.liAPIIK'AI.  MAP 

(  ItYSTAl.    r.VI.I.S    liisTUUT,  MM  iii(;ax 
MAHQUHTTK    lUHTHR'T.  MICIUCAN 


